Expert: Jayendra Upadhye - 9/16/2004

Question
Why do I obtain different rates of change of altitude for different stars?

Answer
Hi,
Laura, If i understand you right, you most probably find stars that are spread around +/- 45 deg the celestial equator show a greater "movement" (rate of change of altitude) than their brothren situated closer to the poles.
And that by Altitude, you mean their displacement with respect to the celestial equator, in degrees, when each star is as directly over head as it can get.(zenith).

If this is correct, the culprit for the observed variation is the earth's atmospgere.
That is also the reason why the apparent motion of the sun is distorted when it is at low altitudes from the eastern and western horizons.
The atmosphere acts like a prism / as well as a lens, magnifying the apparent diameter of the sun, and also "playing mischief" with is apparent motion.
it is all very nicely explained at this site:-
http://www.eps.nagoya-u.ac.jp/examin/981d/981dej.html

The important thing is to understand that what happens to the sun in its daily journey, also happens to the stars in their nightly traverse across the sky.

Thus those northern and southern stars that have their altitudes (anlgle between celestial equator and their hieght when they are at thier zenith, and this varies per seasons) low, placing them almost near the northern and southern horizons, display a slower rate as compared to stars with higher altitudess.

In fact on planets like mars, that have very thin atmospheres, the apprent motion of the sun and the rate of change of altitude of all stars would be unsullied and uniform.
Jayen.