Can a $20 sextant tell me where I am ?

I head up into the mountains and and use a cheap ornamental sextant to take a noon sight. Can this cheap gimmick replica give me an accurate position ?

[@givenfirstnamefamilyfirstn3935]

Measuring the calibration error is really easy, point it at a distant object and see how far off zero it is.

[@robertgoss4842]

This video, combined with the lensatic compass video, could make the amateur quite skilled. Packed with information, these two programs are very satisfying. Thanks a million.

[@MCToon]

Great video! A proper sextant is ridiculously expensive. This is a great demonstration of the usefulness of a small one.

[@skybugvideos]

Glad you enjoyed it!

[@fakiirification]

you can get a decent emergency backup sextant for around 200 bucks. it'll get you within a couple miles of where you need to be if you know how to use it. a perfect one that you can rely on if your going electronics free sailing or just really want to test your abilities will set you back a few thousand.

[@MCToon]

@@fakiirification I have considered purchasing on if the less expansive ones. Living in minnesota I would get few chances to use it. I did put in some money to purchase one for a friend in South America. He had great success using it.

[@marcg1686]

@@MCToon Hi Mike, you could surely find a used Davis Mk15. You could then go down to Alameda and challenge 10thman to show you how to use one. 🤣 My Mk15 is not noticably less accurate than my Cassens & Plath (I have four). It just needs more frequent adjustment

[@canberradogfarts]

In the hands of a true experts, sextants provide sub arc second accuracy in less than a minute. And if you are shooting stars at night, you can pinpoint your location to a couple of feet.

[@johnhawkswell202]

Longitude error does not depend on sextant error. The error is down to time keeping. You need to make sure watch is set to GMT to nearest second, then read the time in seconds as soon as you have brought the 2 "suns" together, then read the sun's altitude after recording the time. Each second spent talking is an error of 0.25nM.

[@mikefochtman7164]

Exactly right. Back in the days of sail, time-keeping was a much bigger problem (hence the 'longitude prize'). But with modern GPS time-based phones and such, getting accurate time is so much easier. And finding the time of maximum elevation is tricky, so his noting two times, on each side of local noon works much better.

[@geraldheinig1473]

Agreed. I was very surprised that his longitude was so accurate, given that he's using a mechanical watch (big thumbs-up from me, an automatic - mechanical - watch fan 😁). I would use a digital watch for that sort of thing and set it just prior to leaving on the walk. As you said, every second will cost you a quarter of a mile. I really liked his idea of using the reflection on the liquid - very clever. I'm going to try this myself: I bought an antique sextant very similar to his a few years ago and I've been meaning to try getting a fix with it for quite a while, but never got round to it, since some of the parts need fixing.

[@ianb4801]

One of the risks with these old sextants is that the filters are not great and you can damage your eye if you're not careful

[@protothad837]

An excellent video. The fact that your longitude was more accurate than your latitude is not too surprising... that could just means your sextant had a large index error (somewhat expected for an ornamental unit). You could check that by setting the arm to zero, then sight on the horizon and adjust until everything lines up. The amount that you adjusted away from zero is your index error that you should add or subtract from any reading. If you are looking for a decent 'real' sextant that is not too expensive, I recommend the Davis MK15. I recently did a review of it, comparing it to my expensive C.Plath, and it performed pretty well.

[@skybugvideos]

Thanks for the comment. I've actually just bought a Davis Mk15, I'm going to make a follow up video where I return to the same spot and repeat the exercise and I reckon I'll get to within a mile or two.

[@mikefochtman7164]

I've got a couple 'ornamental' as well as a 'certified' sextant. Even after adjustment to get the index error to very nearly zero, it still wasn't very good. After a lot of experiments and comparison, I found that the SCALE itself wasn't correct. The larger the angle, the further off it is. So even if you correct for the index error at zero, it was still off by almost a degree by the time I swung it out to 60. They are cheap for a reason. :(

[@protothad837]

​@@mikefochtman7164 Interesting. If the scale error is consistent across the entire arc, you might be able to correct for it by deriving the appropriate multiplier. Take readings of some nearby structures using both the decorative unit and your 'good' unit and chart the difference, then derive a 'scale correction'. It creates an extra step in the process, but it would be interesting to see if it works. Just a thought.

[@mikefochtman7164]

Nice video, explaining the principles and the use of artificial horizon. Enjoyed it, even though your sextant seems to be off by several miles. Your watch (for longitude) seems quite accurate though. :)

[@donlindell1994]

I really enjoyed this. Have watched a dozen videos on sextant usage and the equation of time but somehow I really found this the most enjoyable. I suppose it was the investment of a beautiful day's journey that made the difference. What a lovely challenge. So glad you brought us all along for the adventure and I hope you borrow someone's higher quality device for a follow-up.

[@skybugvideos]

Thanks for your comment Don. I'd love to get hold of a decent sextant and get up there again and have another go. I'd have to get a good measurement though, I can't blame the sextant next time round !

[@dominictarrsailing]

my understanding is that just eyeballing isn't a really accurate way to get the time of the zenith, since the sun seems to "hang there" for a while. So you got lucky there. I have read that the simple way to do it accurately is sight the sun before zenith, record the time, then wait until it's exactly the same, but on the other side of the zenith. average those times and you now have an accurate zenith time! If the ornamental sextant is consistent with it self, and measures the same angle at the same mark, then it might even be accurate with an ornamental sextant

[@dominictarrsailing]

Thanks for making this video! I have also wondered how accurate those ornamental sextants are! I think you could improve on this using the same sextant if you calibrated it. check the zero is actually zero, and check that the index mirror is parrallel to the horizon mirror by setting it to zero and holding it sideways, then rock it side to side. if it's not parallel you'll see the two parts of the image sliding past each other

[@HexterTheJexter]

Excellent! Thanks for the explanation.

[@Dreancaidi]

I'm not a sextant genius but I would think the distance and angle you're away from the cup makes a huge difference, more than the sextant inaccuracy maybe? I'd love to see you do this at sea on a calm day.

[@shaninapier6135]

Hey Steve - great info!

[@leythecg]

Great video! Very interesting content!👍👍👍

[@pokrec]

Longitude determining did not involve the scale. You just set the same reading after culmination. The actual elevation of the Sun was irrelevant, you just used your watch. For longitude determining you do not need the scale, you can use even the sextant stuck in some intermediate position. Such instrument is called Danjon astrolabe and it is basically a prismatic sextant stuck at 60 degrees of height. You measure moments of transition before culmination and after and you have your longitude (if you know the Greenwich times of both measurements). Latitude requires precision scale, so you should have first to check the scale and find scale error by matching the Sun observed through horizon mirror and it's reflection in the moving mirror. So, you have the "zero" of your device (because the Sun and the Sun are 0 degrees apart ;-) ). Measuring objects, that you know angular distances between (like bright stars on the night sky) would allow to check the linear coefficient of your scale and maybe to find possible nonlinear corrections (I know, maybe not relevant in this particular instrument). When you have your scale rectified, I bet your 20 buck toy sextant would give you quite surprising results, probably sufficiently accurate to find not only Hawaii, but even much smaller islands on the sea.

[@cartographicprojection]

Very well done video and extremely informative. Thanks Sky Bug, from someone who is just getting started in sextant use.

[@skybugvideos]

Glad it was helpful!

[@stuartmccloud307]

Very interesting video, thanks for sharing 😊 Ive got a similar one and was wondering how accurate it could be in an emergency

[@francisbryan4485]

Thanks for sharing your experiences. There may be adjustments you can make to your ornament. I have a cheap one and it can be corrected.

[@franklinblunt69]

The sextant accuracy probably within horizon anyway, such navigation relies upon chronometry & Sun beside the convenient cup of coffee😁Even desert scenario may involve shifting horizon, dunes influenced from wind.

[@marcg1686]

25 nautical miles is not bad considering that you didn't apply any corrections to the measured elevation angles and the fact that the index arm has a short radius. That your longitude determination was accurate is not surprising. Your methodology was sound, hence the good result.

[@protothad837]

Absolutely. Since getting the longitude depends on finding the accurate time between matching altitude readings, that can be accurate even when the angle reading has a large (but consistent) error. Thus it is not too surprising that his decorative sextant gave good longitude but poor latitude. This tells me his technique was good and the tool was largely to blame.

[@RodCornholio]

Thank you.

[@alastairnewman1233]

You've made a magic video here. If I may make one comment, a little more detail on the coffee cup levelling technique and you would have knocked this one out the park. But that is a small detail. The fascination you have created with your presentation is more than enough to awaken the imagination of the curious. I'm looking forward to the next one.

[@skybugvideos]

Thanks for your comment and the feedback. I did consider adding a side-on view of me aiming the sextant at the coffee cup, with lines showing the various angles. In the end I decided I'd spent enough time on the video, it's always tough to know when to keep adding stuff and when to release the video and move on to the next one. I also wanted to point the camera through the sextant so you could see what I was seeing but I couldn't get it to work. If I ever make the follow-up video with a proper sextant, I'll make sure to do that.

[@sailingmrnice]

Correct the error!!!

[@Arafury57]

Throw in a heaving deck on a small boat and +/- 5 miles looks pretty good.

[@bigred5287]

For your latitude calculations, I find it works better if instead of subtracting 90 degrees, you subtract 89 degrees and 60 seconds instead. When I ran your numbers using this trick, I got 51.25N latitude, which is 1.3nm off your actual latitude. That is more than accurate enough. Hope this helps.

[@n.mariner5610]

I guess, even far out in the wilderness the coffee cup trick will not work so well, because the natural trembling of the earth will not give you a good reflection to achieve a clear measurement. You better use the last oil change of your car instead . It even is more black than the coffee, so the reflection will be much better. I tried this to train myself, and it works even close to a busy road . Besides this, are you really sure to place picture and reflection on top of each other? The tables want you to place the picture not into the horizon, but touching the horizon. So the angle depends on the size of the star, which is of great difference in case of the sun or the moon. Maybe it is better, to make picture and reflection just touch each other.

[@franklinblunt69]

Aw man, ruined it for me when showed that map😁expected fundamental presentation the rudiments in use but kinda jumped a century (or so?) of navigation history. Don't mind, appreciate the topic.🌌The sextant accuracy probably within horizon anyway, such navigation relies upon chronometry & Sun beside performance skill & the convenient cup of coffee😁Even desert scenario may involve shifting horizon, dunes influenced from wind.

[@svgalene465]

I picked up a Davis plastic sextant on eBay for $40. It’s not a professional level instrument but I can use it to get within five NM of my GPS position. That’s good enough for ocean crossings.

[@skybugvideos]

I've just got a Davis Mk15 sextant myself and I hope to make a follow-up video on Tuesday, if Britain's endless cloudy weather ever ends.

[@svgalene465]

@@skybugvideos That’s the one I have. They work well enough for learning how to use a sextant, which is what I bought it for. I may be crewing on a sailing voyage from California to Hawaii next spring and if I do, I'll bring it along just for the fun of it.

[@fakiirification]

IF you have good current charts of your route. yes. go for it.

[@CharlesMead]

Nice video, although I think you'd need to account for your height above sea level to increase latitude precision. It's certainly easier to get longitude, because you based the readings from the same (possibly flawed) instrument/scale, and then derived time, and assuming your time piece wasn't also ornamental, it should be pretty accurate. Enjoyed this though.

[@skybugvideos]

Glad you liked the video. You don't need to account for height above sea level when doing the reflection-on-liquid trick. Since I'm not using the horizon, there's no horizon dip due to height. I realised after the video that the accurate longitude wasn't such a surprise since it doesn't matter if there's a sextant error. I've got a Davis Mk15 sextant now and might make a follow-up video where I (hopefully) get a really accurate fix.

[@johnhawkswell202]

Latitude error does depend on the sextant. Did you check index error? Set sextant to 0', look at a distant object and bring the 2 views of it together, and read off the index error of sextant. You could do this now and apply the error to your readings on the video, you might be closer than you think if you did not do this on the day. Nautical almanac lists other corrections to apply to account for you altitude above sea level and refraction etc. Otherwise very good. I note you took into account the equation of time, and the sun's declination.

[@skybugvideos]

I checked the index error at 0 degrees and accounted for it but I think the error occurs higher up the angle scale (it really is a bad sextant). I'm happy to leave this bad result as it is, the point was to see how good / bad the sextant was. I've now bought a Davis Mk15 sextant so I'm going to make a follow up video where I expect to get a much better position, hopefully within 2 miles. I didn't bother adjusting for refraction because, at the sun's height, it was less than a minute of arc. Also there's no need to adjust for height above sea-level when using a reflection on the surface of a liquid. I'll use oil though next time instead of water. I had to put cling-film over the cup to stop the wind blowing the liquid and I got a lot of condensation so hopefully using oil will fix that. Anyway, follow-up video to come soon, probably next month

[@johnhawkswell202]

Wow thats a bit better than the Davis Mk 3. The other option, more expensive still is a theodolite. Thats the way the arctic explorers navigated. With a theodolite you measure the angle between the heavenly body and your vertex (90' above your head) rather than the altitude, this gives you the angular distance between your self and the global position of the heavenly body. One other source of error when bringing the sun down to the horison is that you measure the angle to the lower limb of the sun (usually, can also measure the upper limb) but all calculations are based on the angle to the center of the sun, which you cant measure with enough accuracy, so you have to add or subtract 1/2 the angular diameter of the sun.

[@davidgillingham9446]

When you determined your latitude and found it off by 25 miles, did you adjust for elevation?

[@skybugvideos]

There's no need to take elevation into account when using the liquid surface reflection method. If I was using the traditional horizon method then I'd need to use my elevation to adjust for horizon dip.

[@2011Matz]

The short answer is "No." You may be able to establish a line of position, but without a clear sea horizon the tables consulted will give inaccurate calculations. Just because you plot a possible line of position, does not guarantee anything.

[@mymumbakescakes]

Excellent video on how to use a Sextant. Flerfs (Flat Earthers) should view this. However, you did mention that the surface of the liquid is always going to be flat and level. A flerf could quote this and try to justify the point. Flerfs like to lie.

[@Mr22brian22]

He said that the surface of liquid is always flat and level because ( wait for it ) it is ???? We are not the liars here buddy, you can’t do dip correction to get to the point of using curved radii ( oxymoron ) and then measuring arc lengths that are in reality horizontal distances, and this is after ye have hijacked the celestial dome model, something that’s based upon perspective, you know the thing that doesn’t exist in your model ? So there’s no need for damage control on videos like this, as the man is not even involved in the argument, just leave his video alone….

[@mymumbakescakes]

@@Mr22brian22 level adjective having no part higher than another : conforming to the curvature of the liquid parts of the earth's surface. Why not ask the EXPERTS, the People that actually Sail the oceans. As in Captains. Or find out how to ACTUALLY use a Sextant. And ACTUALLY use one.

[@primonomeultimonome]

​@@Mr22brian22The Earth is measurably curved. Get an education.

[@Mr22brian22]

@@primonomeultimonome Really did they use the process of Rise over Run lol…. Get an Education Bonehead ?

[@dipokroy5427]

Plz sell me

[@MrPilgrimuk]

Excellent video. Genuine offer let me know if you want to try out the old British plastic EBBCO sextant. 😉 Proper marine sextant that people have crossed Oceans with. 👍

[@skybugvideos]

I've just got myself a Davis Mk15 sextant to do a follow-up video. Thanks for the offer though. I'll go back to the Cambrian mountains and repeat the last attempt but I should hope to get within a mile of my true position. That's if I continue with the Sky Bug channel, it's just not taking off how I hoped so I'll make a decision soon about whether I carry on making videos.

[@fakiirification]

your latitude was off because of the artificial horizon and the rough accuracy of the decorative sextant. i bet if you tried this again down on the coast with a view of a true horizon you would get a much closer latitude. But you will still be a few miles off because your device cant measure seconds and can barely manage to measure minutes. 1 degree of latitude is 60 nautical miles. each minute of latitude is 1 nautical mile. so if the scale gives you 2 decimal places of precision you should be able to get within 1 nautical mile (6076 feet vs 5280 for standard mile) of your latitude. good enough for ploting a ship, since all you need to do is get in sight of shore then start dead reckoning off of landmarks. and you could be ok on land in this day and age, we are rarely more than a mile away from civilization anywhere on the planet.

[@marcg1686]

I use a Davis artificial horizon. I get intercepts of 5nm. You will rarely get intercepts of 1nm. Any single digit intercepts are good enough.

[@user-yr5bk2cx4k]

using a body of water will not work!! take your measurements to a level then add the distance from your position to sea level. it should work.