Artificial Horizon and Altitude

[Jason]

New to celestial here.

I have a Davis Mk 25 and a Davis artificial horizon. I am having trouble wrapping my head around how to correlate altitude with my readings. I get to take readings at sea level relatively often when I am on the coast (MS Gulf of Mexico), but I most often tinker when inland at home. I am at an average of 400′ ASL.

I have found plenty of information on how to use my artificial horizon, but none of the information mentions how to take altitude into consideration. Thanks in advance.

Jason

[Iman]

From my understanding, you do not need to worry how high above sea level you are when dealing with an artificial horizon. There’s no DIP correction to take into account when using one because your horizon is essentially directly in front of you instead of miles away and you’re looking essentially into a mirror and not a horizon. Aside from remembering to divide your Hs by two because your horizon is a reflection, the only thing to keep in mind is the altitude correction for the celestial body you are considering. If you are shooting the sun or moon, for example, you have a choice to overlap the celestial image with the reflection, which erases the semidiameter correction (exactly like taking a star sight), or stack them on top of each other, which mimics an upper/lower limb sighting, in which case, you would take all celestial body corrections into account. Hope that makes sense!

[Bill]

Iman is correct. Think of a bubble sextant intended for use in aircraft. Altitude isn’t factored into that sight either.
When using an artificial horizon, it is at the same altitude as you are.

[Jason]

Maybe my question should be “How does altitude not matter?” The part I cannot get my mind wrapped around is that it seems like if the sun is at 20° at 100′ ASL and someone is on a 500′ tower (total 600′ ASL) in the same spot, the angle is not going to be 20° when the sight is taken from the tower at the same instant.

[Philippe]

The altitude of the sun is the angle between the horizontal line passing at yours eyes level and the light ray line from the sun.
At sea, this horizontal line can be shown by the horizon line, but in this case and because you are upper the sea level, this “horizontal” line is not really horizontal and you must to take in account the distance between the sea level and your eyes. This is the “depression” error.
When you use an artificial horizon, the real horizontal line pass exactly at yours eyes level, and you have no depression error to take in account.
Remember one thing : all the rays from the celestial body are parallels, and all the horizontal lines are parallels too. So the angle between these parallels is always the same, no matter your own altitude above the ground.

[Jason]

Posth, Thank you. It is all coming together now.

I did some calculations that helped, too, to figure out that the difference in the angle of the sun at sea level and at 20,000 ft is less the 2 millionths of a degree. Me and my Davis Mk25 are not quite that accurate, yet, so we’ll roll with what we have for now!!!

Jason

[Joe Langone]

I have a plastic Davis Mk25 as well and my sights were always off from the metal sextants, but an old salt came by one day and corrected my mirrors and it was accurate within 2-3 degrees.

[Author]

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