Відео

TRM, you missed it by about 10.3 nautical miles. There is a slight error in there somewhere.

@iviewthetube thanks for the quick response. I believe the small correction needed was in the issue of subtracting altitude correction first, then halved the sextant angles after. Still happy with 10 NM tho as I am an engineer and could hit an island if the deckies needed assistance 😂

Hello K. I'm not sure what went wrong. Your calculation is 21°, 580Nmi from my position. For this artificial horizon, to obtain the angle above the horizon, be sure to divide the measured angle by two. (1/2 of the reflected angle) Also, to get your angular distance from the Sun, use 90°- the angle above the horizon.

​@@iviewthetube Thanks so much for the notes, 14 years later. :) I made both of those adjustments in my original calculations. I'll review my work! I tried this method using a home-made sextant-like device, making my own sightings, and found myself off by only 1,000km! I assumed it was an instrumentation issue, but maybe there's a math error somewhere in here. Thanks so much!

Checking: the times are 1726h Feb 20 and 0016h Feb 21 UTC respectively, right? I found it hard to hear.

@@K-vp5sq I don't have my notes handy so I just watched the video again and came up with 2010, Feb 20, 17:26:26 UTC (upper limb) for the morning shot and 2010, Feb 21, 00:16:02 UTC (lower limb) for the afternoon shot.

Yes, they are very easy to use. Also, there is no need to consider height of eye (dip angle) as with the sea horizon.

Hello M R. The sextant is very good at measuring the angle between two objects. For example, if you turn the sextant on its side you can measure the angle between two towers. If you know the distance between those two towers, with trigonometry you can determine an arc of your possible positions. Regarding Artificial Horizons, there are other types other than the one I have shown.

It is difficult to use the Moon for navigation because of its parallax.

There are more corrections for the Moon, including parallax (as mentioned in the other reply) due to its closeness to Earth. Also, depending on the segment of the Moon visible, you might have to use the upper limb which can be confusing when using the artificial horizon! But the stars and planets are very difficult to see in the AH reflection so basically it is the Sun by day and the Moon by night when using the AH.

The artificial horizon produces a reflection off the water. The actual horizon is precisely half way between the direct image and its reflection. You must divide the measured angle by 2.

@@iviewthetube I think there is still a small altitude correction required which is caused by refraction. The almanac has tables for that divided into Oct-Mar and Apr-Sept and for angles less than 10º and angles greater than 10º. It is also divided into sun, stars and planets, on one table and the moon on another.

Yes, there is no dip correction with the AH. But there is still a small refraction in the atmosphere when looking up at high angles and it is not cancelled out by the short path of the same light reflected from the liquid in the tray. There is only one "ray" coming down from the body being measured. The general altitude correction accounts for parallax and this refraction. @@mikefochtman7164

​@@karhukiviIf you use the 'Altitude Correction" table for the sun, based on whether it is a lower limb, or upper limb sighting (be careful here, ivewthetube performed one of each in this video), that correction has refraction, parallax, and semi-diameter all combined. You can, of course, look up the refraction for the measured elevation (often found on the same page of the almanac) and the sun's semidiameter (found at the bottom of each GHA column in the 'daily pages' of the almanac) and apply these corrections separately.

Yes, I think most navigators use the general altitude correction for the Sun. @@mikefochtman7164

The horizon is precisely 1/2 between the Sun and its reflection.

Water would have frozen on the days that Lewis took his measurements in the middle of January 1805 at Fort Mandan. If I remember correctly, Lewis used a mirror leveled with a spirit level -- sort of like a surveyor's tribrach. I may be totally wrong so I would appreciate hearing back from you if you find out differently.

Regarding condensation, I've rarely have a problem with that. Have you tried very cold water? Seems to me that a non toxic anti-boil/anti-freeze would work fine too.

@@iviewthetube On the summer days when I was working with my sextant, the condensation made a "halo" around the Sun's image and it was difficult to see when the limbs touched. The PG seems to be an ideal liquid - I heard about it from another video on how to remove bubbles in a marine compass.

@@iviewthetube Good to hear from you - I read some historic accounts online and they seem to agree that it was water or a bubble level as you say. One book stated that they only made observations which had to be computed on their return, but I'm inclined to disagree and think they had some ability to compute their positions on the go, as they had a table of lunars in their kit. Many thanks for your insights!

I've used black coffee lol. I get a stronger reflection from the black liquid and it's not too hard to clean up. I let it cool thoroughly before hand to minimize condensation. Other bit of advice, don't put the glass shades on the artificial horizon until just before taking the sighting.

This is what inspired me to do this: lewis-clark.org/sciences/geography/celestial-data/

The birds were near 47°27'N, 122°14'W.

Yes, the Artificial Horizon works well. Just remember to divide the measured angle by two. Also with the Sun, limb shots are easier than trying to overlay one image with the other.

Another advantage to the Artificial Horizon is that dip can be ignored. With an Artificial Horizon, going up in elevation does not change the perceived horizon.

You can plot those two circles on Google Earth using the circle measuring tool. For greater accuracy and resolution, you can write your own kml file to draw those circles.

If you don't know math, you can string two circles on a globe or Google Earth. Your answer is within six miles.

I use spherical trigonometry which does not require dead reckoning. Here is the order I solve the angles and sides: ua-cam.com/video/uZk7W0i3jd8/v-deo.html

@@iviewthetube Thanks I got that. In effect your methods needs two measurements for calculation as opposed to the intercept method which needs (at least) two position lines calculated from two measurements. The "assumed position" required for the latter is just a convenient point nearby (can be DR or something else) so that the position lines are not too long (they are circles, in fact). In effect your "assumed latitude" is replaced by the zenith angles from the Pole, and the "assumed longitude" is replaced by the GHA from Greenwich. My interest in astro nav is also from reading the Lewis&Clark expedition and Sir Francis Chichester's round the world trip where he was relying on an ageing sextant with the silvering falling off and making mistakes due to fatigue. I'll get back to you with my results - thanks again!

@@karhukivi Admittedly, computers & scientific calculators make the spherical trig method a lot more possible.

@@iviewthetube The intercept method also uses spherical trig to calculate the Hc or calculated altitude and the azimuth. Then it reverts to a plotting sheet to locate the position. The spherical trig calculations were summarised in the sight reductions books before calculators were available. however not every position of latitude was compiled (the books would have been enormous) so only integral latitude values were shown. This accounts for the navigator having to round up or down the assumed latitude by shifting the assumed position (or DR) to an appropriate value that would cancel out the minutes and decimal minutes. I sometimes get schoolkids to make a rough sunsight at local noon using a simple clinometer and a compass and watch, and they are astonished how it can place them on a globe to within 60 nm. The clever ones are then shown the trig and the almanac, and if they are still interested, we have a go with a sextant!

Visually you can use Google earth using the circle measuring tool set for units of nautical miles. Center the mouse on the Sun's position and draw a circle which represents your angular distance from the sun (1 degree = 60 nautical miles) Do this twice and one of the intersections of the two circles represents your position. If you want it to be accurate then use Excel to write a kml file with more resolution.

The morning shot is an upper limb shot taken on February 20, 2010 UTC. The first beep is 1726 UTC and the second beep it 1727. The afternoon shot is a lower limb shot taken on February 21, 2010 UTC. The beep is at 0016 UTC.

The angle you want to measure is the angle between the celestial object and level. Dip corrections are not needed when using an artificial horizon such as a reflection off of water or a mirror leveled with a spirit level because they are level, no matter what elevation one is at. Conversely, dip corrections are only needed when you are measuring off of the physical horizon which lowers as you increase in elevation -- the horizon becomes no longer level as you rise in elevation.

iviewthetube Thank you ever so much for replying to my question so quickly. Now I have something to do, to make the time pass. I need a diversion to occupy my mind during this Flu virus. Taking Sun Shots should help the time pass. I guess Star Shots are out of the question on land when reflection off an Artificial Horizon is impossible.

No dip correction for the artificial horizon

I suppose you could superimpose but that would be very difficult; whereas, limb shots are very definite -- just touch the edge of the direct with the edge of the reflected image. In January the Earth is closest to the Sun and has an apparent diameter of 32′32″. In July the apparent diameter is 31′27″. The chart on the first page of The Nautical Almanac corrects for both refraction and the radius of the Sun for both an upper and lower limb shots depending on the time of year and the altitude of ones shot.

@@iviewthetube This guy seemed so sure of himself at 13:08 of this video: ua-cam.com/video/i9gUs3cLgx0/v-deo.html Thanks for the tip.

WWV transmits time signals on 2.5, 5, 10, 15, and 20 MHz.

iviewthetube 10-4 Good Buddy, I ended up spending the morning finding that out which lead me to another hour or so figuring out how they have a W call letter. I'm a nerd like that. It's great how it all goes together in some way shape or for.

@@TempoDrift1480 When I was a ham radio operator I had the call letters KE7I. I let my license lapse and those call letters have been snatched up by someone else.

I think this is referring to Empress Longyu. Honestly Consort Zhen (the prettier one) was not that beautiful either

This video illustrates the steps I use to calculate position using Spherical Trigonometry. ua-cam.com/video/uZk7W0i3jd8/v-deo.html

www.starpath.com/online/celestial/vernier.pdf

Sean, If you provide me a video and/or info with your measurements & time of your position then I will provide you with a video on how to do the calculations. If you do not have a sextant, then I can give you a rough estimate if you use a string and protractor. Keep in mind that I need a very good UTC time mark to get accurate longitude.

Thanks for the comment Drone Girl. I checked out your channel and saw some of your amazing drone footage. Nice job. On a humorous note: Those aren't starlings. Instead, I bought thousands of drones for this video and programmed them to fly in this pattern. Pretty impressive, eh?

I believe you are correct.