@@chrimony Wow, you must have been some hype wealthy, privileged school or something. Our paper maps were always crinkled and lumpy and .... quite representative of the actual earth, so maybe I was the privileged one 🤣
A quick note on the former Paris meridian: France accepted the swap to the Greenwich meridian on condition that the UK would go full metric. Which is still not completed.
The future past thing pleases my brain. I think because it's taking the piss out of (or at least deconstructing) such a mainstay of mainstream factual program making.
From that part, my mind was blown by Matt's pronunciation of "premise". First time I remember hearing it like that! I've always heard and/or assumed that the I was the same as in the plural version "premises" /ˈpɹɛmɪsɪz/, so /ˈpɹɛm.ɪs/.
Something that blew my mind years ago was when a miner explained to me that a long mine he had worked in was curved to match the curvature of the earth so the mine would be level. They could have made it perfectly straight with a laser but you would go from walking slightly downhill to walking slightly uphill by walking in a perfectly straight line. 🤯
When the Humber Bridge was built few people other than engineers realised that the two suspension towers would have to be a fraction of a degree off parallel to allow for the curvature of the Earth. When it first opened in 1981 I walked across it, unfortunately at a time when there was a 30mph crosswind and everything was swaying so much that you had to wonder if that fraction of a degree really mattered. Fortunately everything was designed and built to accommodate that motion and greater.
Yes, today lasers are often used for such things and if you want a long run to be level you would have to account for the curvature of the Earth but even to this day, usually something that determines level based on local gravity (bubble levels, plumb bobs, etc.) are still used and those will account for the curvature for you so your straight and level building or mining project will naturally curve
Punta de la Orchilla on the south-western side of El Hierro, is a significant location in terms of the Canaries, as it is one of the most westerly points in the archipelago. A meridian memorial close to the lighthouse, is a reminder that historically it was considered to be a prime meridian for early map makers, and was known as the Ferro Meridian, at the western extremity of the known world
The did an experiment with a teddy bear weighing it at the bottom of a mine, and at the top of a mountain to show the difference between mass and weight. Mass stayed the same, weight changed because of the mass of earth under it. But that is why we still use a hose with water to find level. Quick and easy no matter what the ground is doing. Then you can measure off of reference points.
Well the 4 above replies show the lifetime of programming has worked. Very sad. The weak find contentment in the consensus of ignorance. Trust your own senses, not the nonsense given to us from fake science.
12 year old me dragging my mom all the way to Greenwich on our trip to London in 2010 just so I could have a leg on each side of the line would be devastated by this news.
@@IamGrimalkin it is a Royal Park so presumably King Charles would have to do it. But then it would be free and they would lose money. After all few people use the line on the path below the observatory.
@@georgesibley7152 Do you have to pay to visit that line in grenwich observatory? I haven't been since I was little, but I don't remember having to pay to get in.
Having read “Longitude” by Dava Sobel, I was perpetually spinning along the floor in anguish at the lack of import. Parker will rue the day he fecklessly slighted John Harrison’s great deed.
To add to the fun, plate techtonics are moving the Greenwich line to the North East, so the 100 m difference is reducing at the moment by 2.5 cm (1 inch) per year.
@@AxR558 If it's any consolation there's also a temporary 30cm lurch every time the moon passes overhead. That'll be mostly upwards and slightly westwards though, before sinking slightly eastwards. Oh well. As you were.
@ethelmini True, the meridian can't move N. On the other hand, the telescope defining it can, and so can the line drawn on the ground that visitors photograph. And yes, it's something that some people get wrong : it's still the Greenwich Meridian. It's just no longer the 0 degree lline, the International Reference Meridian.
It came off to people that way but it is the fact that when shooting one has to imagine the other scene and Steve is not a performer or perhaps has no imagination except scientific.
I was fortunate enough to get to visit the International Date Line marker in Fiji. As a true nerd would do, I took along my handheld GPS to check it out. I could not measure the deviation between the marker and the GPS reading since, when I walked toward the line, the device froze up completely. Eventually I got it running again by removing the batteries. Some software designer probably forgot to catch the divide by zero exceptions. I was pleased that I wasn't trying to navigate a boat or plane with that version of the software.
Yikes... This is why everything is done in Cartesian coordinates inside navigation software, and only converted to Lat-Lon for the output... The same thing happens at the north pole, you would get a singularity. You will be pleased to know that these things are checked during release testing now due to Simulators 👍
@@johnperkin3029 (you don't happen to know what chipset is in your GPS device, do you? Just asking for, erm, a friend and definitely not because I might need to check some code on Monday morning 😂)
The 1/3 of a second out = 100m wrong has made me finally realise why the chronograph problem for navigation was so important. A clock that lost 5min a day doesn't sound a lot and I've always thought "how can that matter" but from this video, that's 9km out a day for navigation! Now i can see how hard it was trying to find somewhere like Barbados (23km wide) when it takes two weeks to get there and your navigator is wrong by 9km every day at sea 🤯
The solution was to simply get to the latitude desired and head east or west to the desired destination or waypoint. Say in the case of Barbados from Europe, if the destination is West by Southwest from the point of origin, a ship could follow the coast South or head Southwest to the latitude of Barbados reasonably confident of not over shooting and simply head West. This had the advantage of being correctable, Sextant like devices and magnetic compasses were reasonably mature technologies even hundreds of years ago.
@@corkjaguar The problem with that is that increasing your margin of error increases your travel time. At best, wasting time and money, at worst, life threatening.
I am so bad at math (I dunno why UA-cam thought I would be a good fit for the channel), but here I am, and am very entertained and actually learning so much! These guys are fantastic educators!
Steve is being such a good sport pretending that most of this is new to him to help the narrative of the video. Matt giving him credit for previous related work. This is friendly UA-cam collaboration at its best.
8:00 "We'll assume they're infinitely far away." Early astronomers actually tried to determine the movement of the stars (Tycho Brahe comes to mind), but their equipment was not sufficiently precise to measure any change at all, so they concluded that the stars were in fact effectively infinitely far away.
Tycho Brahe (applying the scientific method) concluded for geocentrism. Brahe could have measured stellar aberration, a parallax on speed instead of displacement
They couldn't be infinitely far away, because they were visibly large in the sky. Astronomers of the day attempted to estimate the angular diameter of the dots they were seeing, and some claimed that the brightest stars were a few _minutes_ of arc in diameter (the true size is less than 0.06 _seconds_ of arc, typically _far_ less). The fact is that the human eye simply can't tell how small these microscopic but bright points of light are. But based on this false assumption, they concluded that in the heliocentric model, stars would have to be not only extremely distant but also ridiculously large (much bigger than the orbit of Saturn). So they took this as evidence in favor of geocentrism.
@@EebstertheGreat Galileo wanted to prove heliocentrism with the diameter of Venus, goes in theory from 1 to 3. Couldn’t measure that difference and said, forget the scientific method, it’s nevertheless heliocentric
@@2adamast The angular diameter of Venus varies from 9.7 seconds at apogee to 66 seconds at perigee. It's right at the limit of being observable even with no telescope, as are its sequence of phases. Galileo observed both of these with an early version of his telescope. He was also obviously able to constrain the angular diameter of stars by using his telescope. His measurements were still extremely optimistic, e.g. measuring Vega at 5 seconds (true size 0.003 seconds), but they were much more realistic than earlier guesses. Compare it to Brahe's estimate that magnitude 1 stars like Sirius had a diameter of 2 minutes, i.e. 120 seconds.
I love how much of a kick you get out of doing the future/past bit and how its like a running joke on the channel. You need to somehow include a future/past Matt across two different videos, something that doesn't fully make sense in the first video but the joke is finished in the next video.
To know the latitude is pretty easy: just looking at Polaris you can determine with about 1 degree error where you are since Polaris is at less than 1 degree from the center of rotation (true north). If you are in the Southern Hemisphere things get a bit more complicated since there's no Polaris, so you'll have to take the Crux and Octans constellations to determine it.
When I was at Greenwich I asked several of the tourist guides in the observatory why the zero meridian in the pavement is not exactly zero, and none of them gave me a satisfactory answer. Thanks guys for your explanation, this astronomy graduate did learn something from your video. I for one did not know they used a mercury surface as a mirror that is automatically at right angles to local gravity.
Yes, and why local down at Greenwich does not pass through the centre of the Earth is that the the Observatory is on the edge of Blackheath Common and the ground slopes down towards the Thames from the Observatory in a direction about 25 degrees west of north. So there is more local mass south and east of the Observatory than there is north and west and that is what tilts the mercury mirror surface. I am surprised that they didn't explain this as it is very easy to do if you are at Greenwich.
What answer did they give? "It was correct until 1984 it was moved" or "Nah. That's just the GPS line, but this is the astronomical line" would technically be correct. (And that's the best kind of correct... :-P )
The Greeniwich meridian is correct, it is what it is. The GPS system is a different system and is what it is. No harm in having different systems but it is more practical to use one system.
@@beargillium2369 The liquid mirror was only used to set the local vertical at the telescope, rather than all the time, and only needed to be a few inches across. Think of the tides; there is a negligible change over a distance of a few inches. So, in theory yes; in practice it is not measurable.
The need to define local down reminds me of my personal favourite scientific experiment, the Schiehallion experiment of 1774. It was an attempt to measure the Earth's average density. The idea was to calculate the mass of a mountain, Schiehallion in Scotland, and the location of its centre of mass. Then to see how much the gravitational attraction of the mountain moved a pendulum. In order to achieve this absolute down relative to the rotation of the earth had to be determined. To this end two obsevatories were built on opposite sides of the mountain. It is one of the first times stellar aboration had to be used to account for the rotation of the earth and the speed of light. It is also where contour lines were first used in cartography. It could make a great stand up maths video.
@@QuantumHistorian Light is coming in at the speed of light. The earth is also moving in both its orbit and its rotation. This makes the light appear to be coming in at a slight angle. It's similar to having rain that is falling vertically. If you walk into this rain the raindrops will hit you at a slight angle. The effect on starlight is very slight. If I remember correctly it was in the order of around 10 arcseconds for the schiehallion experiment. This was very significant as the amount the gravity of the mountain moved the pendulum from true vertical was less than this.
@@nathanevans6277 Thanks! It's unbelievable that corrections on the order of v/c (speed of earth relative to stars / speed of light) come into play here. That's normally when you have to start worrying about relativity and the like. Mind boggling precision for the 18th century, I didn't even know they'd mapped out stellar proper motion with any accuracy by then. Just goes to show that optics has been the high precision way of doing things for a _very_ long time (VIRGO / LIGO show what it can do now).
@@donperegrine922 Could I suggest the book 'Weighing the world: the quest to measure the Earth' by Edwin Danson (2006) - it would have much more detail than a YT video would, and a story such as the Schiehallion experiment deserves so much more than a five minute video.
There's a survey marker in Wisconsin at 90 deg west, 45 deg north. Except...it's just a tourist thing in completely the wrong place. The actual confluence is a few hundred meters away in the middle of a farmer's field. (Edit: the survey point was moved to the correct location years after I visited.)
You have to realize the actual lat/long of any particular point in a field is not fixed, but is constantly changing because the entire continent is floating on the mantle and slowly moving with time. It also makes a big difference which datum is being referenced if you're using a different one.
@@stargazer7644 The US is only drifting 1-2 inches per year. The survey point is hundreds of feet off. The farmer refused to sell the land to the state, so the state just plopped down a phony survey point as a tourist point of interest.
5 місяців тому+4
@@stargazer7644 It's changing very slowly. So if the plaque is, say, 10 cm high and 10 cm wide, the actual 90 deg west / 45 deg north would stay within the plaque for a long, long time.
So the Prime Meridian is a great example of doing the best they could with what was known or repeatable at the time. I'm still happy that by being cheap and by wandering about outside of the Greenwich observatory because I didn't want to pay to get in meant that I actually crossed the real Prime Meridian in October of 2022, just to cross the equator about 2 months later in December of 2022 when Visiting Kenya with my Dad (who's Kenyan and went to a boarding school in Nanyuki, Kenya)
The equatorial line star has the caveat that it would have to be measured at the highest point it reaches within a day, since the sky will rotate and so when you measure matters. That's the beauty of the north star for latitude because while it does rotate it rotates in a perfect circle that maintains the same angle relative to directly overhead and so you can measure that angle at any time and not only at its highest point.
What I found most impressive about Greenwich wasn’t the line, but they also have the first ever pocket watch, that was invented with the express purpose to be able to use the line.
@@Gawdssakes I am sorry for mixing up the numbering. I wrote H3/H4, it should have been H4/H5. As for the size of this pocket watch: Measurements: Dial diameter: 102 mm;Overall: 165 mm x 124 mm x 28 mm x 1.45 kg I think my clame stands: diameter 165 mm weigth 1,45 kg, for sure you need a large pocket for this ACCURATE watch.
I was waiting for you to point out that the WGS84 datum as used by GPS is stationary with respect to the average motion of the Earth's crustal plates. This means that if you mark zero GPS longitude on the ground and then come back later it will no longer be at zero due to the motion of the Eurasian Plate relative to the average. Admittedly it is a small effect.
Thank you for your effort to simplify this issue 🙂 How small is this effect? Continental drift is of the same order as nail growth. The difference between the Eurasion movement and the average movement shouldn't be much larger, I guess? So we are talking centimeters per year?
I ran into some GPS surveyors out in the Mojave who were measuring the motion of a survey marker with respect to the satellite system. They would come back at fixed intervals and measure the position of a mark on a brass cap with a very long integration. They can measure the position to a mm in 3 dimensions. It was one of many. I think there's still a lot of it being done around Parkfield to measure the movement of the two sides of the San Andreas Fault.
A good resource to go along with this video, is the wikipedia page on the IERS Reference Meridian. IERS stands for the International Earth Rotation and Reference Systems Service. It explains that the reason for the difference between the current merdian and the transit circle ( I think the first telescope they showed) is that the plane of the meridian contains the center of mass of the earth ( this was referenced in the video when they talked about down being the center of the earth.) GPS uses this "new" meridian.
The English meridian has moved quite a few times but its most significant move was its first. John Flamsteed was the first Astronomer Royal & proposed the first meridian through his observatory (& home) in Derby. The navy insisted the meridian pass through a port and so the Meridian Derbensis became the Greenwich Meridian. Flamsteed's house still stands, just, on Queen Street in Derby, it has had a storied history with rumours of visits by Benjamin Franklyn. Yes, that one. Most recently, from the mid 19th century up to the first decade of the 21st century it was home to clock makers Smith of Derby. Known by some as the creators of the Swiss Clock in Leicester Square.
@@ericerickson21 It's called 'A Podcast Of Unnecessary Detail' They also do live events on tour in "Festival of the Spoken Nerd". And by tour, I believe it's just New York and London. But more than one location is a tour, isn't it?
This is a real problem - particularly in Australia and particularly for self-driving cars. Australia is moving north really quite quickly so the physical roads are not mapped accurately enough (from year to year) to allow for perfect lane control.
Thought this was a joke at first, but then looked up tectonic plate movement and it seems its just fast enough to matter over the course of decades. eg. NA moving away from Europe at 2.5cm/year.
My parents gave me a copy of Dava Sobel's book "Longitude" as a child, and I loved it. When I visited Greenwich as a young adult, and finally saw Flamsteed House for myself (and all the gloriously intricate chronometers that John Harrison build by hand), it all made total sense.
I met a sailor who explained how they did all this at sea, and when I realized it depended on having a chronometer, I was blown away. This was 60 years ago, so pre-GPS.
I think it's important for navigators to still know how to do celestial navigation. GPS was created by and for the military. The first thing that might happen in a serious war between superpowers is to try to take out the satellites. The various national GPS systems will be blocked to the other countries and the public. Many of the Loran stations are now defunct. So, if a navigator doesn't have a decent chronometer, a sextant, an almanac, and sight reduction tables, they're screwed. Of course the latter two can be done on a phone now using an app.
@@stevenvarner9806 Oh you can bank that they're still teaching it old school in the military. Things like knowing where your targets are is sort of critical lol.
@@homeonegreen9 I know what you mean. It's so easy to depend on newer technology, but I would hope those in our military should definitely learn how to use a chronograph. Are you in the Navy?
Having recently paid a brief visit to Greenwich Park to see the prime meridian in mid July on a trip to Europe, I found this episode incredibly fascinating and... timely.
When you started talking about the earth's rotation relative to the stars vs relative to the sun I got very excited. Not because of how interesting I think the topic is (although I do think it's interesting) but because I was finally going to learn how "sidereal" was pronounced.
@@wolfgangmcq only for the weirdos who read encyclopedias, nobody went on wiki walks back then! But seriously, while it could happen before, we do have a lot more opportunity to go half our life without ever hearing a word spoken that we read fairly regularly with the internet now.
another fun thing to add about why the timing part is an issue, we actually have micro influences on the rotational V of Earth as well. Dams elevate largeeeeeeee masses essentially bringing them from lower points to further out points from the axis. which slows rotation which absolutely effects the time of our rotation.
I had read that your GPS wouldn't show exactly 0 before visiting the UK and London in 2019 and the explanation mostly made sense but the way you guys did made complete sense. Next time I'm on that side of the pond I'm going to find the GPS 0 line in the park. I was bummed that they weren't shining the laser down the line when I was there but I think they recently started doing it again. The camera obscura there was really cool too.
How can I have got to my late sixties and never really got my head around this, and yet so easily explained (the looking up bit from equator vs elsewhere etc). Great explanations and getting Mr Mould involved was as genius as the content. Loved this.
Nine years ago Tom Scott did a greatly simplified and somewhat different explanation of why the Prime Meridian isn't in the right place. His video is "Why The Prime Meridian Isn't At 0º" Either way, I was thrilled to actually have my chance to straddle "the line" when I visited the Greenwich Royal Observatory!
Really interesting topic. They don't specifically say it, but in 1984 they realign the prime meridian for the WGS84 reference frame, for use with the shiny new GPS that has recently been launched. I always assumed that the WGS84 meridian was chosen to minimise errors over the continental USA to minimise the frame transformations needed when moving old US maps to new ones, but since its a military system, they just picked a prime meridian that minimised the errors over the great circle. You learn something new every day!
@@Muzikman127 The issue is that anyone can create a Meridian locally (they even did it in this video) and in theory. The question is how to realise it on larger scales. If you get the vector to the center of the Earth wrong, then the Ellipse that goes "north-south" around the Earth will vary eastwards in some parts and westwards in other parts, with respect to an idealised "true" Great Ellipse. When WGS84 was introduced, we had satellite orbits to help determine the center of mass of the Earth more accurately, and so had a much better Great Ellipse estimate. So what I mean is that the DMA created WGS84 and aligned the Prime Meridian so that there were equal parts of the errors "eastwards" and "westwards" from the old Meridian, so that there wasn't an overall net bias. It just happened that in Greenwich, there was an error of a few hundred meters...
(at least, this is what I think happened - this video is a bit vague at that, and I have never found the original DMA documentation on WGS84, only the various NIMA / NGA updates to it. It may be of interest to know that WGS84 gets updated every so often to align with the ITRF frame, which is the main international frame used for high precision orbits, scientific missions etc. These updates shift things now by a few mm, because the center of the Earth position is known so well after decades of satellite orbits)
Punta de la Orchilla on the south-western side of El Hierro island, is a significant location in terms of the Canaries , as it is one of the most westerly points in the archipelago. A meridian memorial close to the lighthouse, is a reminder that historically it was the prime meridian for early map makers, and was known as the Ferro Meridian, at the western extremity of the known world
I love your enthusiasm. Your videos really set you apart - this could be two blokes in a pub having a laugh but it's two brilliant teachers teaching us another awesome thing. Love learning from you guys. Thanks Past Matt and Steve, and Past Future Matt and Steve, for some more fabulous information about the world around us and how we came to be here.
Wife and I were in the UK last month and visited the PM as it was a bucket list place for me. Was so proud of us and shared the photos with many friends. Now I find out that I have to return and find some spot in the grass. Thanks a ton guys. Thanks a ton.... ;o)
I'm gobsmacked! I thought one person made an observation and from that point onward we had "0" Longitude. Who knew there were many, many versions of "up" ---- or "down"
That might have been the greatest future/past Parker interaction of all time, Steve as witness made it amazing. "Just go with it mate." So in 1984 we stopped looking at the stars and started watching ourselves... that lines up.
Awwww! As a Brit who faithfully bought the hardback paper version of your book, I am bitterly disappointed that we don’t get our own electronic bookmark………🙄
The tools used are called an astrolabe and a sextant. The astrolabe helps you identify the star along with it's angle to which you compare the observed angle. The sextant is a telescope with a protractor attached and it gives you the angles.
This one was really nerd heaven! Since 24 hours is defined with respect to a solar day, a sidereal day turns out to be 23 hours, 56 minutes, 4.1 seconds.
Fascinating video. I did learn a new term, "sidereal time" and now I want to read up more on that. I do understand that in concept. Always tried to understand how science defined places in space for astrometry where some one finds something and then can send out the coordinates for other's anywhere to find it. I'm retired and always like to learn something new, to me.
"Longitude" by Dava Sobel is an excellent account of the efforts to develop highly reliable clocks so that seafaring people could keep track of their longitudinal position on earth.
John Harrison was a carpenter who spent 31 years developing and refining marine chronometers to be able to precisely calculate longitude. The books title is Longitude , also a PBS documentary.
You missed (at least) one other factor: the Earth wobbles. The location of the poles is not quite static so the precise alignment of a meridian will depend on just when you measured it.
Also, the earth is presently tilting (not good) it's caused by extracting too much water from under the ground and building too many large buildings in certain parts of the world. Don't believe it? Do a Google!
In addition to Earth wobbling, I wonder how much plate tectonics plays into moving the line - at least, the physical representations of them on the ground.
Yes it wobbles a bit, but in relation to human lives that is an infinite time line. The wobble to return to any pole location is 10's of thousands of years.
@@TheFrewah Gotta start somewhere. My Minesweeper code is now mostly functional, after many many hours, I just want to add a few more features that I like and wish any other minesweeper version had, and make it look a little snazzier.
@@TheFrewah That's actually something I had to do at work a few months back. We use GNSS modules and we have to do a test where we log all the data for a few hours then perform analysis of the satellites that were visible, which of course requires reading in all the NMEA sentences. You never know when these things will be useful and it all builds skills. A bit of code like that almost certainly got me my first job out of uni - it was a telephone call timer that knew all the call rates and told you how much your call was costing in real time. I just wrote it for fun and a few weeks later it got me a paying job!
@@timlong7289 A funny program that I made was a fake installer in a professional looking folder which looked like it would install something to gain access to a bank account. There was a ”password” file there as well. A scammer using rat to control my computer saw it. I gave hom quality time to download it by claiming i had to do something. I left for one hour and when i came back he was still there. I tell you, he was not happy. Alas, these people don’t call me anymore.
Great to see you two bouncing thoughts and ideas off each other. Refreshing to watch such an enjoyable and informative video. Thanks for making and posting
Went on a trip to London today and I took two pictures: one with the people taking pictures of the observatory meridian line and another on the actual line in the park. This made my day :))) Thanks for the heads-up
I love that something as simple as "where is up" can prompt two very different "well, obviously it's this" responses that are equally valid. And even the "passes through the center" definition is complicated by how you define the center, because I assume that the center of rotation is not exactly the same as the geometric midpoint of the spheroid.
It's like the question 'what is the tallest mountain in the world?' which yields three different answers depending on how you define 'tallest' (i.e., highest from mean sea level, highest from base, or closest to the stars).
It's the center of a spheroid that is connected to the positions of ground stations all over the world but it isn't connected to the spin axis anymore. For a vertical, I use the normal to the surface of the spheroid that passes through your position on the surface of the earth. It makes the math easier.
@@KaitouKaiju Well, the center that the earth spins around *is* the center of mass. The center of gravity is _mostly_ identical to the center of mass, except for when the thing that is gravitationally pulling on earth is close enough that it's gravitational field is non-uniform. You know, like, close enough that it generates tides. Second fun fact: the center of gravity, in relation to the "spin/mass" center earth is always towards the thing that's pulling on earth. Which means that it keeps spinning around (in an earth-stationary reference frame) as the earth rotates. In contrast, the midpoint of the spheroid, or the center of the volume or whatever else you may want to define stay in the same position relative to each other and the center of mass (if you ignore tides and similar deformations).
@@Pystro The problem for Greenwich is that the local center of gravity is different than the average center of gravity. There's ever so slightly more mass in one direction than the other.
@@neilreynolds3858 Sure enough - but when you want an app to work off-line and all you have is a screen shot of an OS map then fixing your location on the map in real time is quite a challenge.
If it had happened in The States people would have complained about having to travel further. You think I'm making a cruel joke, but I'm being serious.
I just want to brag about it, On my birthday in 2008 I crossed the prime meridian and the equator, I took a photo of myself holding a 3kg GPS receiver as we called it at the time (I'm a ship captain).
I someone who is watching this video directly after finally binging all 3 original Star Wars films for the first time, I’m very pleased to know what that is
tbh it looked like an antique leather beach ball from 1700s or something. I kept getting distracted trying to think about how to protect the leather from salt water, sand abrasion etc
In my hometown we have a 10 degrees East line segment which at one point in time was supposed to be correct but now has the same issue when measured in WGS84 format. Thanks for an excellent video about the measurement of the zero line!
The Canadian / US border was originally to be defined as the 49 degrees North Latitude from the Western most point on the Great Lakes to the Pacific Ocean. Teams of surveyors went along the border area and placed markers, that are now big concrete carins, along the border. The border is now defined as the lines between these carins. When North America redefined the datum in the mid 1980s from North American Datum 1927 (NAD27) to NAD83 the 49th North Latitude moved just over 100m North. But the border did not move. .
@@MaxTSanches It’s actually relatively common when looking at US state boundaries (and presumably other boundaries around the world) that the boundaries would first be defined by things like lat/long coordinates, then surveyors would go mark them and invariably be off a bit. In almost every case, the borders were later redefined to be what the surveyors marked, not the original definition. You can find videos on UA-cam claiming the four-corners monument is in the wrong spot. But like the Greenwich Meridian, it is - by definition - in exactly the right spot, no matter what your GPS might say.
11:45 actually the rest acceleration vector of the mirror is not exactly the local gravity acceleration vector, because of the rotation of the Earth. The normal vector is going to be a little tilted to the East.
Very enlightening! I was actually at Greenwich recently (possibly even around the time you were there, as I got rained on too) doing some tests for creating Augmented Reality landmarks and I was rather surprised to find that the Royal Observatory wasn’t at 0 degrees longitude. Now you have explained it perfectly though and cleared up the mystery for me.thank you.
I stopped by the Observatory when I visited London just a few weeks ago! The public exhibits and signage felt a little passive-aggressively bitter about WGS84 (the GPS datum), essentially saying “your GPS says it’s a couple hundred meters over there but actually *this* is zero and *that* is wrong.” It’s still well worth a visit! It’s neat to see on video but much cooler in person, particularly the timekeeping devices.
GPS zero was figured out, they said, as a least square from that projected from other worldwide measurements, which would have presumably been with their own level mirrors and star observations and clocks. Why don't they have at least a small monument to the GPS zero in the adjoining park? "It's complicated."
@@PaulFisher I agree with the Observatory. Well, sort of. I wouldn’t eat WGS84 is wrong. I would just say it is its own thing that has no bearing on the Greenwich meridian.
@@georgesibley7152 yep. to quote Wikipedia: the IERS Reference Meridian “is the reference meridian of the Global Positioning System (GPS) operated by the United States Space Force, and of WGS 84…”
@@katrinabryceThe Earth has a lot of great circles that aren't the equator or pass through the poles, and I'm sure you can find several that are exactly 40,000 km
As they started talking about sidereal time, I got excited because I knew I was about to hear the word pronounced for the first time...and then they dissected it's pronunciation like I hoped they would. Beautiful.
The more I think about stuff like this, the more my brain hurts. Like everything is dynamic in its own way, and no matter how hard you try you can never be the most precise. The tectonic plate movement, the Earth's wobble, the magnetic field moving, the changes in density underneath us causign gravity to slightly change direction and strength, our movement through space. It's all crazy and mind boggling to think about.
A geologist would mention that the continental shelf and tectonic plates are also in constant motion, in some places by several centimeters a year, so wherever the line was relative to the axis of rotation of the earth after a few hundred years it would have shifted as well.
It’s ok. In Ecuador the French calculated the equator a few hundred meters off. In their defense they calculated it in like 1751 when their gps ran out of batteries.
Future Steve is still cold.
He should've visited the equator instead of the prime meridian
did you cut the cold in half, put a layer of acrylic over it and fill it with blue liquid to figure out why?
That's how you know you're in the right country
* is still cool 😎
@@ezgarrth4555 Instructions unclear, wound up in Antarctica.
Can't wait for Steve's explanation using a 2D model
Chapeau
Fr
@@chrimony Wow, you must have been some hype wealthy, privileged school or something. Our paper maps were always crinkled and lumpy and .... quite representative of the actual earth, so maybe I was the privileged one 🤣
Yeah if it hasn't been recreated in a flat Perspex representation then has it even been explained at all?
A 2d model that involves hydrodynamics in some way
A quick note on the former Paris meridian: France accepted the swap to the Greenwich meridian on condition that the UK would go full metric. Which is still not completed.
is that a surprise? we are talking about the british here... ;)
Which metric? The UK has metrics on a full array of things.
I mean, if there's one thing we learned from Brexit, it's that the Brits don't like to rush things ;)
@@TheDuckofDoom. Metric is basically just latin for: to measure. Like of coarse they measure things with a system.
Let's swap to a nother meridian, but be more accurate, and close enough to the current system.
Finally, subtitles on your videos!
Thank you!
As my Comms prof used to answer whenever asked what's up: "A direction away from the geoid center of mass, normal to the local vertical."
If it’s normal to the local vertical, isn’t it then horizontal and therefore not up?
@@emma.j.nationthey did say it was there Comms professor, not math or geography or astronomy or physics or engineering professor 😉😁
Moon has entered the chat.
if i could leave it zero stars, i would not. i would leave it -0.0014333 stars.
???
@@fipachu because that's how far of gps zero the Greenwich meridian was shown to be.
(edited because I apparently can't spell Greenwich)
Remember when you could leave stars to youtube videos?
@@falsemcnuggethope Pepperidge Farms remembers.
Plus the UK is moving south about 13 mm per year!
Steve finding the future-past-thing silly, and Matt not being bothered by Steve finding it silly, is just a joy to see
The future past thing pleases my brain. I think because it's taking the piss out of (or at least deconstructing) such a mainstay of mainstream factual program making.
Lol Steve really sold that too, loved it
Past Steve had no idea how cold he was going to be in 15 minutes of the good English weather...yes, it can always get worse.
Steve exists outside of time, it's the same instance of him simultaneously, that's why he's confused and cold in both
From that part, my mind was blown by Matt's pronunciation of "premise". First time I remember hearing it like that! I've always heard and/or assumed that the I was the same as in the plural version "premises" /ˈpɹɛmɪsɪz/, so /ˈpɹɛm.ɪs/.
A Parker Meridian
💀
Damn you beat me to it.
@@OverkillSD Damn you beat me to saying you beat me to it.
in the park t was meant to be
Noooooooo
Eye piece is a point. It is the angle of swing of the transit scope that defines the line.
Something that blew my mind years ago was when a miner explained to me that a long mine he had worked in was curved to match the curvature of the earth so the mine would be level. They could have made it perfectly straight with a laser but you would go from walking slightly downhill to walking slightly uphill by walking in a perfectly straight line. 🤯
When the Humber Bridge was built few people other than engineers realised that the two suspension towers would have to be a fraction of a degree off parallel to allow for the curvature of the Earth. When it first opened in 1981 I walked across it, unfortunately at a time when there was a 30mph crosswind and everything was swaying so much that you had to wonder if that fraction of a degree really mattered. Fortunately everything was designed and built to accommodate that motion and greater.
Yes, today lasers are often used for such things and if you want a long run to be level you would have to account for the curvature of the Earth but even to this day, usually something that determines level based on local gravity (bubble levels, plumb bobs, etc.) are still used and those will account for the curvature for you so your straight and level building or mining project will naturally curve
Punta de la Orchilla on the south-western side of El Hierro, is a significant location in terms of the Canaries, as it is one of the most westerly points in the archipelago. A meridian memorial close to the lighthouse, is a reminder that historically it was considered to be a prime meridian for early map makers, and was known as the Ferro Meridian, at the western extremity of the known world
The did an experiment with a teddy bear weighing it at the bottom of a mine, and at the top of a mountain to show the difference between mass and weight. Mass stayed the same, weight changed because of the mass of earth under it.
But that is why we still use a hose with water to find level. Quick and easy no matter what the ground is doing. Then you can measure off of reference points.
Well the 4 above replies show the lifetime of programming has worked.
Very sad.
The weak find contentment in the consensus of ignorance.
Trust your own senses, not the nonsense given to us from fake science.
12 year old me dragging my mom all the way to Greenwich on our trip to London in 2010 just so I could have a leg on each side of the line would be devastated by this news.
If you've ever crossed the line, there was at least one moment when precisely half of you was on each side, so you're all good 👍
To be honest they should just make a new line in that park.
Why not?
It's not private property, the council can just do it.
@@IamGrimalkin it is a Royal Park so presumably King Charles would have to do it. But then it would be free
and they would lose money. After all few people use the line on the path below the observatory.
@@georgesibley7152
Do you have to pay to visit that line in grenwich observatory?
I haven't been since I was little, but I don't remember having to pay to get in.
@@IamGrimalkin no idea as it is behind railings, I assumed that you did, I always used the line lower.
"Harrison made some clocks" is one of the greatest understatments ever made scientifically, artistically, and engineering-ly. Well done Matt. 15:40
Famously _The Lesser Watch_ was used in a major plot point in the British comedy, _Only Fools And Horses_
Some might call it a Parker Statement
Having read “Longitude” by Dava Sobel, I was perpetually spinning along the floor in anguish at the lack of import. Parker will rue the day he fecklessly slighted John Harrison’s great deed.
Then a few seconds later "this was pre radio waves". Er what?!?
@@CharlieQuartz Dava Sobel
To add to the fun, plate techtonics are moving the Greenwich line to the North East, so the 100 m difference is reducing at the moment by 2.5 cm (1 inch) per year.
Only 4,000 years to go then!
@@AxR558 If it's any consolation there's also a temporary 30cm lurch every time the moon passes overhead. That'll be mostly upwards and slightly westwards though, before sinking slightly eastwards. Oh well. As you were.
To the East, it can't move N or S without still being on the line.
As a datum it doesn't move at all: everything else does.
@ethelmini True, the meridian can't move N. On the other hand, the telescope defining it can, and so can the line drawn on the ground that visitors photograph.
And yes, it's something that some people get wrong : it's still the Greenwich Meridian. It's just no longer the 0 degree lline, the International Reference Meridian.
So nature heals brexit
Steve is such a good guy, you know bc he never speaks out of step nor does he interrupt i any way. A nice, calm person.
I actually visited the Royal Observatory in Greenwich during my mini-vacation in London last year. The view from up there is quite spectacular. 🤩
I'm cry-laughing at Steve not being enthused by Matt's past and future skit. Hilarious.
Indoor UA-cam vs. Outdoors UA-cam, right there.
It came off to people that way but it is the fact that when shooting one has to imagine the other scene and Steve is not a performer or perhaps has no imagination except scientific.
I was fortunate enough to get to visit the International Date Line marker in Fiji. As a true nerd would do, I took along my handheld GPS to check it out. I could not measure the deviation between the marker and the GPS reading since, when I walked toward the line, the device froze up completely. Eventually I got it running again by removing the batteries. Some software designer probably forgot to catch the divide by zero exceptions. I was pleased that I wasn't trying to navigate a boat or plane with that version of the software.
Yikes... This is why everything is done in Cartesian coordinates inside navigation software, and only converted to Lat-Lon for the output... The same thing happens at the north pole, you would get a singularity. You will be pleased to know that these things are checked during release testing now due to Simulators 👍
@@johnperkin3029 (you don't happen to know what chipset is in your GPS device, do you? Just asking for, erm, a friend and definitely not because I might need to check some code on Monday morning 😂)
Happened with the new US fighter F35 that had its computer hang while crossing the date line !
Crossed the line back when loran was still in use , back and forth a couple times, still not sure if I made it now ? Hahahaha thx. 👍
@@peterwstacey I can remember trying to use the GPS in my phone while at the South Pole. Poor thing got confused.
The 1/3 of a second out = 100m wrong has made me finally realise why the chronograph problem for navigation was so important. A clock that lost 5min a day doesn't sound a lot and I've always thought "how can that matter" but from this video, that's 9km out a day for navigation!
Now i can see how hard it was trying to find somewhere like Barbados (23km wide) when it takes two weeks to get there and your navigator is wrong by 9km every day at sea 🤯
Which makes what The Polynesians did all the more extraordinary.
The solution was to simply get to the latitude desired and head east or west to the desired destination or waypoint. Say in the case of Barbados from Europe, if the destination is West by Southwest from the point of origin, a ship could follow the coast South or head Southwest to the latitude of Barbados reasonably confident of not over shooting and simply head West. This had the advantage of being correctable, Sextant like devices and magnetic compasses were reasonably mature technologies even hundreds of years ago.
@@corkjaguar The problem with that is that increasing your margin of error increases your travel time. At best, wasting time and money, at worst, life threatening.
900 x 100m = 90km
@@d_andrews my bad, but this makes it even worse!
4:25 "The eyepiece is shaped like a zero." Thanks, I never thought of it that way!
I am so bad at math (I dunno why UA-cam thought I would be a good fit for the channel), but here I am, and am very entertained and actually learning so much!
These guys are fantastic educators!
"You can contribute to the problem by preordering" is now my favourite Matt's quote.
I do love contributing to problems...
As a mathematician, he's in the business of making problems.
gaming this past decade be like
I will NEVER get tired of the past-Matt/future-Matt schtick. Well done for dragging Steve into it too
Both Matts do the timing of the interactions so well. It's almost like he scripts these videos or something. Brilliant!
@@Sembazuru I figure that for the second one at least, he's actually watching the video of the first one, so that he can match up the timings
Matt: What's up, Steve? Steve: It's complicated...
the direction opposing gravity... in a gravityless situation the direction of forward movement 😮
Spectacular video. I would LOVE to see more collabs between you two.
Excellent Presentation. Bravo.
Josh Gates has an episode where he visits the equator line, and he shows how the line is misplaced.
~3K
Steve is being such a good sport pretending that most of this is new to him to help the narrative of the video. Matt giving him credit for previous related work. This is friendly UA-cam collaboration at its best.
They're way more than just friendly UA-cam collaborators.
8:00 "We'll assume they're infinitely far away." Early astronomers actually tried to determine the movement of the stars (Tycho Brahe comes to mind), but their equipment was not sufficiently precise to measure any change at all, so they concluded that the stars were in fact effectively infinitely far away.
Tycho Brahe (applying the scientific method) concluded for geocentrism. Brahe could have measured stellar aberration, a parallax on speed instead of displacement
So very much in the development of science has come about because of the need for (and development of) better measurements!
They couldn't be infinitely far away, because they were visibly large in the sky. Astronomers of the day attempted to estimate the angular diameter of the dots they were seeing, and some claimed that the brightest stars were a few _minutes_ of arc in diameter (the true size is less than 0.06 _seconds_ of arc, typically _far_ less). The fact is that the human eye simply can't tell how small these microscopic but bright points of light are. But based on this false assumption, they concluded that in the heliocentric model, stars would have to be not only extremely distant but also ridiculously large (much bigger than the orbit of Saturn). So they took this as evidence in favor of geocentrism.
@@EebstertheGreat Galileo wanted to prove heliocentrism with the diameter of Venus, goes in theory from 1 to 3. Couldn’t measure that difference and said, forget the scientific method, it’s nevertheless heliocentric
@@2adamast The angular diameter of Venus varies from 9.7 seconds at apogee to 66 seconds at perigee. It's right at the limit of being observable even with no telescope, as are its sequence of phases. Galileo observed both of these with an early version of his telescope.
He was also obviously able to constrain the angular diameter of stars by using his telescope. His measurements were still extremely optimistic, e.g. measuring Vega at 5 seconds (true size 0.003 seconds), but they were much more realistic than earlier guesses. Compare it to Brahe's estimate that magnitude 1 stars like Sirius had a diameter of 2 minutes, i.e. 120 seconds.
I love how much of a kick you get out of doing the future/past bit and how its like a running joke on the channel. You need to somehow include a future/past Matt across two different videos, something that doesn't fully make sense in the first video but the joke is finished in the next video.
@TheRenegade... r/whoosh
Also, can we appreciate how good the timing is? the videos line up very well in the beginning and the end, not easy to do at all. Nice1!
@@barongerhardtA joke isn't finished when you're only told part of it
No the joke is finished in the first video and started in the second.
@@barongerhardtTHIS! Of course why limit it to starting in the second video? So much better to leave viewers shivering with antici
To know the latitude is pretty easy: just looking at Polaris you can determine with about 1 degree error where you are since Polaris is at less than 1 degree from the center of rotation (true north).
If you are in the Southern Hemisphere things get a bit more complicated since there's no Polaris, so you'll have to take the Crux and Octans constellations to determine it.
Thanks to you two I've found the exact spot you taped today.
When I was at Greenwich I asked several of the tourist guides in the observatory why the zero meridian in the pavement is not exactly zero, and none of them gave me a satisfactory answer. Thanks guys for your explanation, this astronomy graduate did learn something from your video. I for one did not know they used a mercury surface as a mirror that is automatically at right angles to local gravity.
Yes, and why local down at Greenwich does not pass through the centre of the Earth is that the the Observatory is on the edge of Blackheath Common and the ground slopes down towards the Thames from the Observatory in a direction about 25 degrees west of north. So there is more local mass south and east of the Observatory than there is north and west and that is what tilts the mercury mirror surface. I am surprised that they didn't explain this as it is very easy to do if you are at Greenwich.
What answer did they give? "It was correct until 1984 it was moved" or "Nah. That's just the GPS line, but this is the astronomical line" would technically be correct. (And that's the best kind of correct... :-P )
The Greeniwich meridian is correct, it is what it is. The GPS system is a different system and is what it is. No harm in having different systems but it is more practical to use one system.
would the moon not affect such a mirror?
@@beargillium2369 The liquid mirror was only used to set the local vertical at the telescope, rather than all the time, and only needed to be a few inches across. Think of the tides; there is a negligible change over a distance of a few inches. So, in theory yes; in practice it is not measurable.
The need to define local down reminds me of my personal favourite scientific experiment, the Schiehallion experiment of 1774. It was an attempt to measure the Earth's average density.
The idea was to calculate the mass of a mountain, Schiehallion in Scotland, and the location of its centre of mass. Then to see how much the gravitational attraction of the mountain moved a pendulum. In order to achieve this absolute down relative to the rotation of the earth had to be determined. To this end two obsevatories were built on opposite sides of the mountain.
It is one of the first times stellar aboration had to be used to account for the rotation of the earth and the speed of light.
It is also where contour lines were first used in cartography.
It could make a great stand up maths video.
That's an interesting story! I would love to see that video, too.
How does the speed of light come into it?
@@QuantumHistorian Light is coming in at the speed of light. The earth is also moving in both its orbit and its rotation. This makes the light appear to be coming in at a slight angle.
It's similar to having rain that is falling vertically. If you walk into this rain the raindrops will hit you at a slight angle. The effect on starlight is very slight. If I remember correctly it was in the order of around 10 arcseconds for the schiehallion experiment. This was very significant as the amount the gravity of the mountain moved the pendulum from true vertical was less than this.
@@nathanevans6277 Thanks! It's unbelievable that corrections on the order of v/c (speed of earth relative to stars / speed of light) come into play here. That's normally when you have to start worrying about relativity and the like. Mind boggling precision for the 18th century, I didn't even know they'd mapped out stellar proper motion with any accuracy by then.
Just goes to show that optics has been the high precision way of doing things for a _very_ long time (VIRGO / LIGO show what it can do now).
@@donperegrine922 Could I suggest the book 'Weighing the world: the quest to measure the Earth' by Edwin Danson (2006) - it would have much more detail than a YT video would, and a story such as the Schiehallion experiment deserves so much more than a five minute video.
There's a survey marker in Wisconsin at 90 deg west, 45 deg north. Except...it's just a tourist thing in completely the wrong place. The actual confluence is a few hundred meters away in the middle of a farmer's field. (Edit: the survey point was moved to the correct location years after I visited.)
You have to realize the actual lat/long of any particular point in a field is not fixed, but is constantly changing because the entire continent is floating on the mantle and slowly moving with time. It also makes a big difference which datum is being referenced if you're using a different one.
Accurate to 2sf...
@@stargazer7644 Not in Wisconsin. Things never change there.
@@stargazer7644 The US is only drifting 1-2 inches per year. The survey point is hundreds of feet off. The farmer refused to sell the land to the state, so the state just plopped down a phony survey point as a tourist point of interest.
@@stargazer7644 It's changing very slowly. So if the plaque is, say, 10 cm high and 10 cm wide, the actual 90 deg west / 45 deg north would stay within the plaque for a long, long time.
So the Prime Meridian is a great example of doing the best they could with what was known or repeatable at the time.
I'm still happy that by being cheap and by wandering about outside of the Greenwich observatory because I didn't want to pay to get in meant that I actually crossed the real Prime Meridian in October of 2022, just to cross the equator about 2 months later in December of 2022 when Visiting Kenya with my Dad (who's Kenyan and went to a boarding school in Nanyuki, Kenya)
That is correct
The equatorial line star has the caveat that it would have to be measured at the highest point it reaches within a day, since the sky will rotate and so when you measure matters. That's the beauty of the north star for latitude because while it does rotate it rotates in a perfect circle that maintains the same angle relative to directly overhead and so you can measure that angle at any time and not only at its highest point.
What I found most impressive about Greenwich wasn’t the line, but they also have the first ever pocket watch, that was invented with the express purpose to be able to use the line.
But you need pretty large pocket to carry a Harrison H3/H4 ;).
Urmmmmm the first you did not pay much attention did you ! It was the first ACCURATE pocket watch
@@Gawdssakes I am sorry for mixing up the numbering. I wrote H3/H4, it should have been H4/H5.
As for the size of this pocket watch:
Measurements: Dial diameter: 102 mm;Overall: 165 mm x 124 mm x 28 mm x 1.45 kg
I think my clame stands: diameter 165 mm weigth 1,45 kg, for sure you need a large pocket for this ACCURATE watch.
I mean I’m sure I could make it work if they allowed me to carry it around.
Also they invented peanut butter & mayonnaise iced t.
I was waiting for you to point out that the WGS84 datum as used by GPS is stationary with respect to the average motion of the Earth's crustal plates. This means that if you mark zero GPS longitude on the ground and then come back later it will no longer be at zero due to the motion of the Eurasian Plate relative to the average. Admittedly it is a small effect.
I was also wondering how much was due to continental drift
Thank you for your effort to simplify this issue 🙂
How small is this effect? Continental drift is of the same order as nail growth. The difference between the Eurasion movement and the average movement shouldn't be much larger, I guess? So we are talking centimeters per year?
The good news is that the UK is moving north east, so the error is getting smaller all the time.
I ran into some GPS surveyors out in the Mojave who were measuring the motion of a survey marker with respect to the satellite system. They would come back at fixed intervals and measure the position of a mark on a brass cap with a very long integration. They can measure the position to a mm in 3 dimensions. It was one of many. I think there's still a lot of it being done around Parkfield to measure the movement of the two sides of the San Andreas Fault.
Matt (or one of his descendants) should come back and make another video when the line on the ground has drifted to the right place.
Matt pointing at the camera and saying "A star" should be a motivational meme
A good resource to go along with this video, is the wikipedia page on the IERS Reference Meridian. IERS stands for the International Earth Rotation and Reference Systems Service. It explains that the reason for the difference between the current merdian and the transit circle ( I think the first telescope they showed) is that the plane of the meridian contains the center of mass of the earth ( this was referenced in the video when they talked about down being the center of the earth.) GPS uses this "new" meridian.
The English meridian has moved quite a few times but its most significant move was its first. John Flamsteed was the first Astronomer Royal & proposed the first meridian through his observatory (& home) in Derby. The navy insisted the meridian pass through a port and so the Meridian Derbensis became the Greenwich Meridian. Flamsteed's house still stands, just, on Queen Street in Derby, it has had a storied history with rumours of visits by Benjamin Franklyn. Yes, that one. Most recently, from the mid 19th century up to the first decade of the 21st century it was home to clock makers Smith of Derby. Known by some as the creators of the Swiss Clock in Leicester Square.
so nice for Steve to join smaller channels for a collab.
uuh shots fired :D
Given that Steve and Matt host a podcast together, I'm surprised they don't do more collabs.
@ptrinch thanks for the info.
@ptrinch oh, yeah, what's it called?
@@ericerickson21 It's called 'A Podcast Of Unnecessary Detail'
They also do live events on tour in "Festival of the Spoken Nerd". And by tour, I believe it's just New York and London. But more than one location is a tour, isn't it?
For extra fun, add in continental drift to compensate for the motion of tectonic plates.
Came here just for this comment.
It baffeled me when i took gps measurements for work and it asked when i took them in order to match different maps
This is a real problem - particularly in Australia and particularly for self-driving cars. Australia is moving north really quite quickly so the physical roads are not mapped accurately enough (from year to year) to allow for perfect lane control.
Thought this was a joke at first, but then looked up tectonic plate movement and it seems its just fast enough to matter over the course of decades. eg. NA moving away from Europe at 2.5cm/year.
@@NJ-uh6hz I'm imagining someone maintaining those undersea cables and being briefly confused why its getting pulled tighter and tighter.
Underrated comment
My parents gave me a copy of Dava Sobel's book "Longitude" as a child, and I loved it. When I visited Greenwich as a young adult, and finally saw Flamsteed House for myself (and all the gloriously intricate chronometers that John Harrison build by hand), it all made total sense.
Amazing how Stonehenge doesn't have the same problem of the star moving,
I met a sailor who explained how they did all this at sea, and when I realized it depended on having a chronometer, I was blown away. This was 60 years ago, so pre-GPS.
I think it's important for navigators to still know how to do celestial navigation. GPS was created by and for the military. The first thing that might happen in a serious war between superpowers is to try to take out the satellites. The various national GPS systems will be blocked to the other countries and the public. Many of the Loran stations are now defunct. So, if a navigator doesn't have a decent chronometer, a sextant, an almanac, and sight reduction tables, they're screwed. Of course the latter two can be done on a phone now using an app.
@@stevenvarner9806 Certainly an essential skill for anyone serious about yachting.
@@stevenvarner9806
Oh you can bank that they're still teaching it old school in the military. Things like knowing where your targets are is sort of critical lol.
@@GingerNinja1Navy is teaching it still... Officers and enlisted navigators remembering it long after that training is a different question.
@@homeonegreen9
I know what you mean. It's so easy to depend on newer technology, but I would hope those in our military should definitely learn how to use a chronograph. Are you in the Navy?
Having recently paid a brief visit to Greenwich Park to see the prime meridian in mid July on a trip to Europe, I found this episode incredibly fascinating and... timely.
Jay Foreman made a great video on the longitude problem!
The clocks mentioned in said video are housed in the greenwich observatory too!
I was wondering whose video about this I watched and that would match up with the style my subconscious remembers
I then watched the series linked called the longitude problem and found it quite fascinating. It's right here on UA-cam BTW
Dava Sobel wrote an excellent book on the subject.
Math men, math men, math math math men men
When you started talking about the earth's rotation relative to the stars vs relative to the sun I got very excited.
Not because of how interesting I think the topic is (although I do think it's interesting) but because I was finally going to learn how "sidereal" was pronounced.
Problems of the Wikipedia generation
@@SimonBuchanNzIt is written in the wikipedia article on sidereal time how to pronounce it.
@@SimonBuchanNz The encyclopedia generations had the same issue TBF
@@wolfgangmcq only for the weirdos who read encyclopedias, nobody went on wiki walks back then! But seriously, while it could happen before, we do have a lot more opportunity to go half our life without ever hearing a word spoken that we read fairly regularly with the internet now.
another fun thing to add about why the timing part is an issue, we actually have micro influences on the rotational V of Earth as well. Dams elevate largeeeeeeee masses essentially bringing them from lower points to further out points from the axis. which slows rotation which absolutely effects the time of our rotation.
I had read that your GPS wouldn't show exactly 0 before visiting the UK and London in 2019 and the explanation mostly made sense but the way you guys did made complete sense. Next time I'm on that side of the pond I'm going to find the GPS 0 line in the park. I was bummed that they weren't shining the laser down the line when I was there but I think they recently started doing it again. The camera obscura there was really cool too.
How can I have got to my late sixties and never really got my head around this, and yet so easily explained (the looking up bit from equator vs elsewhere etc). Great explanations and getting Mr Mould involved was as genius as the content. Loved this.
Nine years ago Tom Scott did a greatly simplified and somewhat different explanation of why the Prime Meridian isn't in the right place. His video is "Why The Prime Meridian Isn't At 0º" Either way, I was thrilled to actually have my chance to straddle "the line" when I visited the Greenwich Royal Observatory!
Thanks for the tip; Tom Scott is actually more accurate and to the point in a fraction of the time.
Thanks, I’ll go and watch that instead, this one is annoying me 😑
Agreed, too much beating about the bush and being silly instead of getting to the point. Steve's irritation at various points is obvious.
You are coming out with some of the most interesting content on UA-cam, keep it up!
Really interesting topic. They don't specifically say it, but in 1984 they realign the prime meridian for the WGS84 reference frame, for use with the shiny new GPS that has recently been launched. I always assumed that the WGS84 meridian was chosen to minimise errors over the continental USA to minimise the frame transformations needed when moving old US maps to new ones, but since its a military system, they just picked a prime meridian that minimised the errors over the great circle. You learn something new every day!
Actually, my first guess was that the US got it wrong when they created GPS! 😆
my assumption too, and now I also learned something!
When you say minimised the errors "over the great circle" what do you mean precisely?
@@Muzikman127 The issue is that anyone can create a Meridian locally (they even did it in this video) and in theory. The question is how to realise it on larger scales. If you get the vector to the center of the Earth wrong, then the Ellipse that goes "north-south" around the Earth will vary eastwards in some parts and westwards in other parts, with respect to an idealised "true" Great Ellipse. When WGS84 was introduced, we had satellite orbits to help determine the center of mass of the Earth more accurately, and so had a much better Great Ellipse estimate. So what I mean is that the DMA created WGS84 and aligned the Prime Meridian so that there were equal parts of the errors "eastwards" and "westwards" from the old Meridian, so that there wasn't an overall net bias. It just happened that in Greenwich, there was an error of a few hundred meters...
(at least, this is what I think happened - this video is a bit vague at that, and I have never found the original DMA documentation on WGS84, only the various NIMA / NGA updates to it. It may be of interest to know that WGS84 gets updated every so often to align with the ITRF frame, which is the main international frame used for high precision orbits, scientific missions etc. These updates shift things now by a few mm, because the center of the Earth position is known so well after decades of satellite orbits)
Punta de la Orchilla on the south-western side of El Hierro island, is a significant location in terms of the Canaries , as it is one of the most westerly points in the archipelago. A meridian memorial close to the lighthouse, is a reminder that historically it was the prime meridian for early map makers, and was known as the Ferro Meridian, at the western extremity of the known world
I love your enthusiasm. Your videos really set you apart - this could be two blokes in a pub having a laugh but it's two brilliant teachers teaching us another awesome thing. Love learning from you guys. Thanks Past Matt and Steve, and Past Future Matt and Steve, for some more fabulous information about the world around us and how we came to be here.
We need a petition to call it the Parker Meridian
Or as I ve said before, a Parkeridian.😊
@@georgesos That's very Parkeridereal.
Wow, this is the first time I finally understood exactly why knowing the time while was so important while sailing!
You might enjoy the book Longitude by Dave Sobel about the history of "the longitude problem" and the development of the chronometer.
@@scottlasley8896 That would be Dava Sobel, damn autocorrect. And I whole-heartedly agree with the recommendation.
@@scottlasley8896I have read it and it’s really good. My mom had not heard about this and was fascinated.
Sun over the yardarm?
Thank you, slightly-less-past Steve for putting up with slightly-more-past-Matt's shenanigans.
Wife and I were in the UK last month and visited the PM as it was a bucket list place for me. Was so proud of us and shared the photos with many friends. Now I find out that I have to return and find some spot in the grass. Thanks a ton guys. Thanks a ton.... ;o)
I'm gobsmacked!
I thought one person made an observation and from that point onward we had "0" Longitude. Who knew there were many, many versions of "up" ---- or "down"
I cant believe you've done this video, this is something I teach regularly at the Museum. Its a facinating topic (with a great view).
The growing intricacy of the future/past versions bit brings me SO MUCH JOY
That might have been the greatest future/past Parker interaction of all time, Steve as witness made it amazing. "Just go with it mate." So in 1984 we stopped looking at the stars and started watching ourselves... that lines up.
The shirt with a line running through a zero is a nice touch.
Awwww! As a Brit who faithfully bought the hardback paper version of your book, I am bitterly disappointed that we don’t get our own electronic bookmark………🙄
Modern methods also avoid issues with Continental drift, the changing rate of Earth's rotation, subsidence, and the like.
The tools used are called an astrolabe and a sextant. The astrolabe helps you identify the star along with it's angle to which you compare the observed angle. The sextant is a telescope with a protractor attached and it gives you the angles.
*its
This one was really nerd heaven!
Since 24 hours is defined with respect to a solar day, a sidereal day turns out to be 23 hours, 56 minutes, 4.1 seconds.
I am starting a petition to rename the Prime Meridian into the Parker Meridian. Because it's good enough.
Fascinating video. I did learn a new term, "sidereal time" and now I want to read up more on that. I do understand that in concept. Always tried to understand how science defined places in space for astrometry where some one finds something and then can send out the coordinates for other's anywhere to find it. I'm retired and always like to learn something new, to me.
"Longitude" by Dava Sobel is an excellent account of the efforts to develop highly reliable clocks so that seafaring people could keep track of their longitudinal position on earth.
You Are Here by Haiwatha Bray is (in my opinion) a better book on the same material.
And you can see the Harrison sea clocks at the Greenwich observatory. They are amazing.
John Harrison was a carpenter who spent 31 years developing and refining marine chronometers to be able to precisely calculate longitude. The books title is Longitude , also a PBS documentary.
@@WesB1972 *book's
You missed (at least) one other factor: the Earth wobbles. The location of the poles is not quite static so the precise alignment of a meridian will depend on just when you measured it.
Also, the earth is presently tilting (not good) it's caused by extracting too much water from under the ground and building too many large buildings in certain parts of the world. Don't believe it? Do a Google!
In addition to Earth wobbling, I wonder how much plate tectonics plays into moving the line - at least, the physical representations of them on the ground.
Yes it wobbles a bit, but in relation to human lives that is an infinite time line. The wobble to return to any pole location is 10's of thousands of years.
@@enforcer-e1sit’s because all those lazy Africans never built anything. The US and Europe are weighing up the northern hemisphere.
Don't forget plate shifts.
Fun fact - my software is used to drive the Great Equatorial Telescope in the "Onion Dome". I feel like I'm right up there with Flamsteed and Halley!
That is great. I wrote some code that could read nmea sentences but did nothing.
@@TheFrewah Gotta start somewhere. My Minesweeper code is now mostly functional, after many many hours, I just want to add a few more features that I like and wish any other minesweeper version had, and make it look a little snazzier.
@@TheFrewah That's actually something I had to do at work a few months back. We use GNSS modules and we have to do a test where we log all the data for a few hours then perform analysis of the satellites that were visible, which of course requires reading in all the NMEA sentences. You never know when these things will be useful and it all builds skills. A bit of code like that almost certainly got me my first job out of uni - it was a telephone call timer that knew all the call rates and told you how much your call was costing in real time. I just wrote it for fun and a few weeks later it got me a paying job!
@@timlong7289 A funny program that I made was a fake installer in a professional looking folder which looked like it would install something to gain access to a bank account. There was a ”password” file there as well. A scammer using rat to control my computer saw it. I gave hom quality time to download it by claiming i had to do something. I left for one hour and when i came back he was still there. I tell you, he was not happy. Alas, these people don’t call me anymore.
Great to see you two bouncing thoughts and ideas off each other. Refreshing to watch such an enjoyable and informative video. Thanks for making and posting
Went on a trip to London today and I took two pictures: one with the people taking pictures of the observatory meridian line and another on the actual line in the park. This made my day :)))
Thanks for the heads-up
I love that something as simple as "where is up" can prompt two very different "well, obviously it's this" responses that are equally valid. And even the "passes through the center" definition is complicated by how you define the center, because I assume that the center of rotation is not exactly the same as the geometric midpoint of the spheroid.
Right and the funny thing is neither of those centers are the center of gravity
It's like the question 'what is the tallest mountain in the world?' which yields three different answers depending on how you define 'tallest' (i.e., highest from mean sea level, highest from base, or closest to the stars).
It's the center of a spheroid that is connected to the positions of ground stations all over the world but it isn't connected to the spin axis anymore. For a vertical, I use the normal to the surface of the spheroid that passes through your position on the surface of the earth. It makes the math easier.
@@KaitouKaiju Well, the center that the earth spins around *is* the center of mass. The center of gravity is _mostly_ identical to the center of mass, except for when the thing that is gravitationally pulling on earth is close enough that it's gravitational field is non-uniform. You know, like, close enough that it generates tides.
Second fun fact: the center of gravity, in relation to the "spin/mass" center earth is always towards the thing that's pulling on earth. Which means that it keeps spinning around (in an earth-stationary reference frame) as the earth rotates. In contrast, the midpoint of the spheroid, or the center of the volume or whatever else you may want to define stay in the same position relative to each other and the center of mass (if you ignore tides and similar deformations).
@@Pystro The problem for Greenwich is that the local center of gravity is different than the average center of gravity. There's ever so slightly more mass in one direction than the other.
12:54 - that Bradley line explains why it's *so* complicated to convert OS northings and eastings to GPS lat/long (and vice versa)!
You can get it done on a OS website.
@@neilreynolds3858 Sure enough - but when you want an app to work off-line and all you have is a screen shot of an OS map then fixing your location on the map in real time is quite a challenge.
I used to cross the meridian every day, my school was on it. It was big local news when it suddenly moved
Where did they move the school to?
@@barongerhardt about 100m that way 👉
If it had happened in The States people would have complained about having to travel further. You think I'm making a cruel joke, but I'm being serious.
was there a woosh sound or anything when it moved?
@@BluishNomad it was more of a twang, nearly took out the English department
I just want to brag about it, On my birthday in 2008 I crossed the prime meridian and the equator, I took a photo of myself holding a 3kg GPS receiver as we called it at the time (I'm a ship captain).
When I visited Quito, Ecuador, they told us there was a difference between the actual equator and the commercial equator defined by gps
My answer to the question of "What's up?" is "Up is generally defined as the direction directly opposite the prevailing force of gravity."
I usually answer with "the ceiling" but this would be much funnier
…at a point in space, relative to the observer. (Adjusting for Newton and Einstein)
Brings back memories - 5 years ago I was in the the park searching for ZERO. Hope to come back to glorious England soon
This is the earth... Brings out the Death Star...
@@evaDrepuS 🤭🤭🤭🤭🤭
I someone who is watching this video directly after finally binging all 3 original Star Wars films for the first time, I’m very pleased to know what that is
That’s no Earth that’s a space station.
tbh it looked like an antique leather beach ball from 1700s or something. I kept getting distracted trying to think about how to protect the leather from salt water, sand abrasion etc
Vogon Constructor Fleet
In my hometown we have a 10 degrees East line segment which at one point in time was supposed to be correct but now has the same issue when measured in WGS84 format. Thanks for an excellent video about the measurement of the zero line!
"Every triangle is a love triangle if you love triangles enough."
6:51 Matt was so pissed off by the weather at Greenwich that he seems to have relocated it to northern Africa.
The Canadian / US border was originally to be defined as the 49 degrees North Latitude from the Western most point on the Great Lakes to the Pacific Ocean. Teams of surveyors went along the border area and placed markers, that are now big concrete carins, along the border. The border is now defined as the lines between these carins. When North America redefined the datum in the mid 1980s from North American Datum 1927 (NAD27) to NAD83 the 49th North Latitude moved just over 100m North. But the border did not move.
.
@@MaxTSanches It’s actually relatively common when looking at US state boundaries (and presumably other boundaries around the world) that the boundaries would first be defined by things like lat/long coordinates, then surveyors would go mark them and invariably be off a bit. In almost every case, the borders were later redefined to be what the surveyors marked, not the original definition. You can find videos on UA-cam claiming the four-corners monument is in the wrong spot. But like the Greenwich Meridian, it is - by definition - in exactly the right spot, no matter what your GPS might say.
11:45 actually the rest acceleration vector of the mirror is not exactly the local gravity acceleration vector, because of the rotation of the Earth. The normal vector is going to be a little tilted to the East.
I'm happy to see Matt help get small creators like Steve Mould get noticed.
Very enlightening! I was actually at Greenwich recently (possibly even around the time you were there, as I got rained on too) doing some tests for creating Augmented Reality landmarks and I was rather surprised to find that the Royal Observatory wasn’t at 0 degrees longitude. Now you have explained it perfectly though and cleared up the mystery for me.thank you.
I stopped by the Observatory when I visited London just a few weeks ago! The public exhibits and signage felt a little passive-aggressively bitter about WGS84 (the GPS datum), essentially saying “your GPS says it’s a couple hundred meters over there but actually *this* is zero and *that* is wrong.” It’s still well worth a visit! It’s neat to see on video but much cooler in person, particularly the timekeeping devices.
GPS zero was figured out, they said, as a least square from that projected from other worldwide measurements, which would have presumably been with their own level mirrors and star observations and clocks. Why don't they have at least a small monument to the GPS zero in the adjoining park? "It's complicated."
@@PaulFisher I agree with the Observatory. Well, sort of. I wouldn’t eat WGS84 is wrong. I would just say it is its own thing that has no bearing on the Greenwich meridian.
Well they are right.
I assume the WGS84 is the IERS Reference Meridian.
@@georgesibley7152 yep. to quote Wikipedia: the IERS Reference Meridian “is the reference meridian of the Global Positioning System (GPS) operated by the United States Space Force, and of WGS 84…”
I love using masking tape since it automatically follows geodesics on curved surfaces(if you have a thin enough strip)
I can't wait for the next video, where they talk about the point where Earth's circumference is exactly 40000 km
Only if meausred via the poles. If you go round the equator it is 40,075 km, as the earth isn't a perfect sphere.
@@katrinabryceOblate spheroid!
@@katrinabryceThe Earth has a lot of great circles that aren't the equator or pass through the poles, and I'm sure you can find several that are exactly 40,000 km
As they started talking about sidereal time, I got excited because I knew I was about to hear the word pronounced for the first time...and then they dissected it's pronunciation like I hoped they would. Beautiful.
The more I think about stuff like this, the more my brain hurts. Like everything is dynamic in its own way, and no matter how hard you try you can never be the most precise. The tectonic plate movement, the Earth's wobble, the magnetic field moving, the changes in density underneath us causign gravity to slightly change direction and strength, our movement through space. It's all crazy and mind boggling to think about.
Setting something in stone is never as permanent as people think when they do it.
Thanks for clarifying it was british spring, because it looked like british summer to me
Britan doesn't have a summer or winter we go straight from Spring to Autumn.
Summer is when the rain is least cold
@@davidhumble1679 lmao
@@lexman7179 Hey now, just because you didn't go out during those 3 days it doesn't mean they didn't happen.
A geologist would mention that the continental shelf and tectonic plates are also in constant motion, in some places by several centimeters a year, so wherever the line was relative to the axis of rotation of the earth after a few hundred years it would have shifted as well.
It’s ok. In Ecuador the French calculated the equator a few hundred meters off. In their defense they calculated it in like 1751 when their gps ran out of batteries.
One is the Analogue line and one is the Digital line.