A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Jun 5, 11:44 -0700
If we suppose we have two observers at the same latitude and longitude at the same instant of UT both observing the Sun's altitude above the sea horizon, and one of those observers has a height of 0m above the sea (an idealized observer) while the other is 1000m above the sea, the observer at 1000m will measure a larger altitude for two reasons:
- Greater dip. You can look up the value in a table or calculate it from dip (in minutes of arc) = 0.97·sqrt[3.3·(ht in meters)]. This is a large correction.
- Slightly reduced refraction. Take the sea level refraction value and multiply it by a factor of exp[-(ht in meters)/9500m]. The difference between the adjusted refraction and the sea level refraction should be added on to the altitude. This is a small correction.
Note: increased dip only applies when using the sea horizon (or a lake horizon, if the lake is large enough), but that's not necessarily reasonable. If the observer is using a bubble sextant or an artificial horizon (a mirror), then there is no dip correction regardless of altitude.
This looks like it might be a test question of some sort, and if that's the case then the "correct" answer is not necessarily "right" and instead you should try to guess what the test creator is looking for. In that case, the answer is probably just the change in dip, and you can ignore the change in refraction and other details.