Sextants | TheaterFire

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demsocialist99@reddit

Maybe obvious, but do you live within driving distance of the ocean or Great Lakes? (Need to measure from a horizon). And for the purposes of learning the process and having fun a modestly priced instrument should work. You may end up with a fix that is, say, 20mi from your known position instead of a few, but just getting it anywhere close is an accomplishment.

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pcetcedce@reddit (OP)

Yeah as I said elsewhere I'm about an hour from the ocean in Maine and I go often.

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demsocialist99@reddit

Ha! A Maine beach is where I first learned sight reduction, 40+ yrs ago.

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pcetcedce@reddit (OP)

Nice!

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ccgarnaal@reddit

Thousands seems over the top. Merchant navy still has them. Modern ones are plastic / composite. And should cost 400-1000usd.

For example: https://www.svb24.com/en/davis-mark-25-deluxe-sextant.html

I got luck at a ship breaker yard. And got a 30 year old one new in the box from a merchant navy vessel.

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Zyj@reddit

That MK25 is very overpriced at $800

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SnooFoxes5258@reddit

If you wanted to go slightly upmarket a Davis Mk25 is my personal favourite

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pcetcedce@reddit (OP)

Thanks

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KeyGroundbreaking390@reddit

If you know the distance to the opposite shoreline (like on a lake or river) you can use "short dip correction".

For instance, I know at our lake the opposite shoreline is .7 NM away. The correction for that distance is 3 ' of arc (The true horizon would be 3 minutes of arc higher than the shoreline if the land wasn't blocking my view), so I just subtract 3' from my Ho and carry on with the reduction of sight. Here's a video explaining it - https://youtu.be/cjOSiE-MT_A?si=f8An7q23eYGSsoBq

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pcetcedce@reddit (OP)

That's great I'm going to try it out.

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KeyGroundbreaking390@reddit

Besides picking up a cheap sextant, consider installing a celestial navigation app to use as a quick way to check your accuracy. If the app uses the lat/long from the built in GPS in your phone, then any difference between your Ho (observed height) and your HC (computed height based on astronomical data stored in the app) is due to navigator error or error in the sextant ( which should always be checked by the navigator and entered in the reduction process, so it's all navigator error).

Eventually you want to do the reduction process without the aid of any electronics (including your phone), but initially that will add another whole layer of errors, so hone your skill with the sextant by using the app.

I practice my sextant shots whenever the sky allows. I force myself to do a paper and pencil reduction of sight (without the app) about once a month. Again, in the age of GPS, it's a backup skill.

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pcetcedce@reddit (OP)

I have a great GPS so I want to go backwards and try a sextant.

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mmaalex@reddit

Davis mark 3 does what it needs to do, and is shockingly accurate if you know how to read it. It sucks for doing star work thiugh because of zero magnification. Getting those weak stars can be a struggle withiut magnification.

I have successfully used one for a dozen or so nights of sights on my ship, but I had a lot easier time using a normal sextant with magnification, like a Davis MK 15 or Astra IIIb.

A sextant is just a precision optical angle measurement tool, nothing more nothing less.

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pcetcedce@reddit (OP)

Thanks for the information.

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Only_Razzmatazz_4498@reddit

There are some very good composite ones that people have pointed out. They are perfectly fine for learning and getting a decent fix. Working off of a small pitching boat makes the difference not that important anyway.

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KeyGroundbreaking390@reddit

Everyone thinks the pitching and rocking of a boat makes shooting a celestial object with a sextant very hard, but that is not the case. To prove it all you need to do is try taking a shot while doing deep knee bends. Due to the double mirrors in the sextant, the image remains very stable. The only problem with the irratic pitching and rocking on a boat is keeping yourself from being thrown about so hooking a leg or arm around a shroud or stanchion is key.

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pcetcedce@reddit (OP)

Thanks for the comment.

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SVAuspicious@reddit

I have a Celestaire Astra IIIB and the scars in my left elbow to show I've used it. I have a deviation chart for my mechanical watch. I'm pretty consistent at 1 nm triangle or less. I've found Faial without stress. GPS is better.

As others including u/youngrichyoung have pointed out you need an artificial horizon to use a sextant properly on land. You can make your own with a bowl of water but it's awkward.

You can generate your own tables using spherical trigonometry if you have ephemerides for stars of interest. Sun shots are easy.

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pcetcedce@reddit (OP)

Could I do it on the coast as the horizon?

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SVAuspicious@reddit

Some. The issue working on the beach is that stars over the land don't give you the real horizon. Artificial horizons are cheap.

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pcetcedce@reddit (OP)

Actually my wife's family have a tidal Island so I think that will work.

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SVAuspicious@reddit

Tidal island is just a boat very very hard aground. Should be fine. You need a horizon.

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pcetcedce@reddit (OP)

Thanks definitely doable.

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Older_cyclist@reddit

The cheaper, the less accurate. Think about it.

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pcetcedce@reddit (OP)

I understand that but I'm not using it for true navigation.

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Dudarro@reddit

I’m a low end not very good dinghy sailor who’s also in the us navy. (but stuck as a shore guy).

USNI had an article in January 2025 arguing that we should re-establish the skillset for use in low-emission environments.

free USNI link here

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pcetcedce@reddit (OP)

That is a great article thanks.

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BarnOwl-9024@reddit

The Davis Mark 3 is the cheapest that is considered to be accurate, but are built… different from a “standard” sextant. The other Davis plastic ones are excellent for learning with, have all the knobs and lenses, and are “shaped” like the expensive ones, but way cheaper than the master-class ones (are still considered to be very accurate, though). Although at a couple hundred bucks, not exactly cheap. Celestaire sells a wide variety and has a lot of reference materials and books for learning. The Power Squadrons (now Americas Boating Club) and ASA have classes you can take to learn how to use them, as well as some online seminars.

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pcetcedce@reddit (OP)

Thanks for the recommendation.

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duckwood98@reddit

the davis mark iii is a great trainer. surprisingly accurate with the vernier. it is great for shooting the sun. the davis mark 25 is the next step up. i have used both. i also own an expensive one. a russian cho-t. i love it. the one thing to remember is that no matter how accurate the sextant is, it is very hard to get better than 40 to 60 miles on a small bouncing boat. another thing the mark iii does well is to use it horizontally to take relative bearings. it can lay flat. most others cannot. btw i was able to use the mark 25 to get “lunars” to within 15 minutes or so. about same with the expensive one. also you would be surprised at how close you can get with a straw taped to a plastic protractor.

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pcetcedce@reddit (OP)

Thanks for the info.

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Rino-feroce@reddit

The Astra model from Celestiaire is considered excellent value for money. The plastic full frame from David tend to work very well initially, but with time the frame and screws tend to warp (with use, change of temperature…). The cheap one from Davis (mark 3 I think) is simpler but more resilient (different frame, fewer screws) but quite accurate

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pcetcedce@reddit (OP)

Thank you.

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One_Loquat_3737@reddit

I got an extremely accurate former ship's sextant from ebay a few years back for something in the $500 dollar range, including calibration chart. Using a mirror leveled with a builder's spirit level in my back yard I could get fixes accurate to 2-3 miles (you halve the measured angle). It sold later for about what I paid for it.

It's obviously not as accurate as GPS but if you can get consistently within 2-3 miles that's enough for a lot of real-live navigational tasks, allowing you to refer to a chart and then switch to visual navigation near land.

I'm told that a real expert can get to about a quarter of a mile with one, and I take my hat off to them as your time source needs to be very accurate too if you want to achieve that.

I learned a lot, part of which was that I hope never to have to rely on a sextant :)

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millijuna@reddit

I crossed the Atlantic a few years ago on a Navy Auxiliary ship. Asking for the ride was a Navy navigation officer and several junior officers. It was just like the old days, she (the NavO) was teaching the junior officers the ins and outs of stellar navigation. They’d take their sights, do the reductions, and compare the results to the ECDIS. By the end of the trip, they were within a mile of so of reality.

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pcetcedce@reddit (OP)

That's great Happy to hear about that.

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Dioscouri@reddit

Like you I hope I never have to rely on a sextant, but I'd be foolish to exclusively rely on GPS when technology exists that means I'll be able to get around with a complete power loss.

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One_Loquat_3737@reddit

Agreed, you want backups for your backups. A lot of people have sailed around the world with just a sextant and it worked well for them, but you have to keep up with it and ensure your tables are up to date, your time accurate.

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texasrigger@reddit

If you are handy at all, you might enjoy the book "Latitude Hooks and Azimuth Rings" by Dennis Fisher. It details the construction and use of a bunch of historical navigational instruments, including the octant, which was the immediate precursor to the sextant.

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pcetcedce@reddit (OP)

Thanks I will check that out.

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youngrichyoung@reddit

Another commenter alluded to this, but you'll need a wet horizon to really use any sextant. If you don't live near water, you can use an "artificial horizon" in place of a proper sea view ;-)

Landfall Navigation has a pretty good self-instruction course, and they sell sextants as well. The Davis Mk 15 ($400) is a little nicer and more "real" than the Mk 3 everyone is recommending. You could also do their $10 cardboard sextant with bubble horizon, if you want a really affordable option.

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pcetcedce@reddit (OP)

I'm good I'm about 45 minutes from the coast of Maine. And I spend a lot of the summer there. Thanks for the recommendations.

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robsea69@reddit

Look on Craigslist. They have older professional grade sextants for under $500. Note: there is a way to search All of Craigslist for such an item.

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pcetcedce@reddit (OP)

Thanks very much.

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AnchorManSailing@reddit

I'd like to get my hands on a Bris Sextant. World's smallest.

https://www.yrvind.com/bris-mini-sextant/

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pcetcedce@reddit (OP)

Wow.

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KK7ORD@reddit

If you have a carpenters square and a plumb bob, you can slap together the precursor of the sextant, the "quadrant"

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pcetcedce@reddit (OP)

That's interesting I was just reading about that In the book I referenced.

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FarAwaySailor@reddit

Unless you're going offshore on multi-day passages, it's pointless

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pcetcedce@reddit (OP)

I am not trying to navigate it I just want to stand on land and have fun figuring out the latitude.

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FarAwaySailor@reddit

You'll need to be able to see the (sea) horizon (or some fake version of it) and be standing on something not more than a few m above sea-level.

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Effective_Corner694@reddit

My grandfather had one from the USS Saratoga when he was in WW2. He taught me how to use it when I was a kid. Unfortunately, after he passed away his wife (3rd one) who’d did not care for his kids or grandchildren sold most of his belongings and cut off contact with us. She basically liquidated everything and when she died 2 years later, gave everything to a family none of us knew anything about. The only reason we found out was because her sister was pissed she didn’t get anything from her. There was a lot of drama going on and by that point I had already moved on.

Of all the grandchildren, I was 1 of 2 who took to the water and went into the navy. That sextant was the only possession of his that I ever wanted to have and pass down.

Anyway, using a sextant is a lost art nowadays and I commend you for wanting one

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pcetcedce@reddit (OP)

I'm sorry about your story. My grandfather also was in the Navy in world war II.

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betelgeuse63110@reddit

Try the EZ Sextant app. That would be a good cheap place to start.

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pcetcedce@reddit (OP)

That's sounds cool thanks very much

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Cole_Slawter@reddit

Davis instruments mark 3. West Marine has them for about $100 right now.

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LegitMeatPuppet@reddit

100% agree. They have a nice carrying case. It’s easy to find online classes covering the basics.

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oudcedar@reddit

Just make sure you know your height

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LameBMX@reddit

I upvoted, but you can scrap like half or all the corrections and still be close enough to know when you need to scan horizon and switch to charts. I can admire the goal of near GPS precision, but it's functionally overkill.

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T1ck-T0ck@reddit

I have never used a sextent but you seem to have been down voted?

Did you mean elevation or was mean your physical height as a joke or something?

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oudcedar@reddit

No they are right as I’ve just checked. When using a sextant at sea in clear visibility (which is all I’ve ever done) you need to know your height above the horizon, so I wrongly assumed that this would be necessary on land too even using the artificial horizon on a sextant. I have to admit I haven’t got my head round what I have just looked up which is that your height above the actual sea level horizon is irrelevant.

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MissingGravitas@reddit

I'll take a stab... when sighting against the "real" horizon, the sextant is not held perfectly level (i.e. tangent to the planet's surface); it's tilted slightly downward and thus the angle measured is just slightly too large.

If you used a bubble level with your sextant instead of a horizon, you similarly wouldn't account for dip because the sextant would be truly level. Similarly an artificial horizon is providing a truly level surface.

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IceTech59@reddit

There are correction factors (dip table) that consider 'height of eye' above sea level when using the visible horizon at sea. (Source: good friend who was a Navy Quartermaster in the days before GPS).

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T1ck-T0ck@reddit

First of I admit i am ignorant in this. :-) ? So they can only be used at sea with the sea horizon?

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oudcedar@reddit

No because the artificial horizon is not something you need to use on a clear day because you have a perfect sea level horizon you can use and you know the height of the platform (usually the cockpit floor) and your own height to “dip” if trying to be extremely accurate. As as you need a clear view of the sun and/or other celestial objects then the conditions usually mean the horizon is visible except on the darkest nights.

But my mistake not realising that (somehow?) height is not needed for the artificial horizon which is what you would need to use on land.

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2airishuman@reddit

The Astra IIIB at around $1000 is the cheapest reliable, precise alternative.

https://www.landfallnavigation.com/astra-iiib-deluxe-sextant.html

The plastic/composite ones have poor stability due to the thermal expansion properties of the materials from which they are made. That's fine for inexpensive, less precise "lifeboat" sextants like the Davis Mark 3 but it is unwise to get expensive plastic ones.

Good used sextants that need minor repairs will end up costing more than an Astra IIIB when the repair costs are considered.

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LameBMX@reddit

you also don't need to get gps accuracy out of a sextant. Just close enough to know when to watch out for land and gets the charts out.

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caeru1ean@reddit

Another vote for the plastic Davis mark 3. Good value and works well