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The title says it all. Why can't I use a projected level mark on a sextant and bring down the Sun/Star/Etc. to the level mark instead of a horizon? Assume that somehow I have figured out a way to hold the sextant level.

So what I am thinking is to project out where level is and use that instead of the horizon.

Wouldn't that actually be more accurate than a horizon or an artificial horizon? The reason I think it would be more accurate is that I would have removed the variables of the viewer's height and the height of the artificial horizon from the equation.

We are using the sextant to record in a sky-diary where we were looking when we saw an object.

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Your actions are laudable. Actually, these bubble jobbers have been around for a long time. You are probably using a pattern sextant and desire to affix a bubble horizon to it. Yes, you can do this. Nevertheless, there are bubble sextants for the same use; the principal advantage, of course, being that readings can be taken when the sky is visible but the horizon is not. The bubble horizon has been integrated into the sextant since before the Second World War. Apparently, they have been considered indispensable at times. An internet search reveals a variety of such sextants are currently made and available. One noted for use with any sextant is a bubble-horizon attachment that is available at quite reasonable cost.

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Well, you CAN do it, but it's not the best way to do those types of measurements on land. The advantage of a sextant on a boat is that you can account for the rocking of the boat by rhythmically rocking your body back and forth. If you take a sextant, add a bubble level, attach it to a tripod, and remove the mirrors used to line up with the horizon, what you have is a transit or theodolite.

That said, if you have a sextant, you can still use it on land. The best way to do that is by using an artificial horizon, which is simply a pan of water. You line up the reflection of the Sun in the water with the reflection of the Sun through the sextant mirrors. You have to divide the resulting angle by 2, and most professional so-called "sextants" actually go well past 90 degrees to make this type of measurement possible.

If you try to use a level attached to your sextant, you have to calibrate it to make sure it really lines up with the 0 point. With the pan of water, the water's surface levels itself. And if you try this method, you'll find it's actually easier than taking a measurement using the horizon, and you might wonder why not use this same method on a boat. But the rocking of the boat will put ripples in the water, making it tough to get a good reading.

An aircraft sextant, actually does work the way you're thinking. In an aircraft, you usually don't have access to a flat horizon, but still have to deal with the motion of the aircraft, so a bubble sextant actually makes sense in that case. I'm not sure if this method is more or less accurate than a regular sextant, but I can tell you the aircraft sextants are significantly more expensive, and likely require more care and calibration, which is probably why they're not generally used when a horizon line is available.

There's also a device called an Abney Level, which is mostly just a protractor, a short tube, a level and a mirror. And works pretty much like a bubble sextant would, but without the mirrors, so it's a much cheaper device. Calibration is still an issue though, and measuring large angles is difficult as you're looking at the reflection of the level at a steeper and steeper angle.

But, on land, a theodolite or transit are the preferred, and more accurate tools. They are less frequently used though, because once one person has done the measurement, it's not going to change very quickly, and you can just use their measurement.

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  • $\begingroup$ I tried using the "pan of water" at sea. It was a 320 foot ship so wave motion was not great. I filled the pan with heavy oil to try dampen machinery vibration, etc. It didn't work and made a mess. Not recommended. $\endgroup$
    – Woody
    Nov 26 '21 at 4:42

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