Check position against horizon about stardroid HOT 2 OPEN

Given RA & Dec, time, and latitude/longitude, the calculation is pretty 
straightforward. Essentially, you want to calculate altitude and azimuth. I 
found this site that has the formulae:
http://www.stargazing.net/kepler/altaz.html

Then, and object is above the horizon if altitude > 0.

In the Sky Map code, you can get the RA & Dec using the RaDec class 
(trunk/app/src/com/google/android/stardroid/units/RaDec.java). We don't 
explicitly calculate Alt/Az, which is why you have to do the calculation by 
hand.

Let me know if that makes sense.

It does make a lot of sense :) Thanks so much.
I have already found some javascript code that calculates the sun's horizontal 
altitude directly and i successfully ported it to Java.

See the result here: 
https://market.android.com/details?id=de.goddchen.android.solarcollector :)