For a given observer (lon, lat, time) on Earth and a given Galactic coordinate (GLON, GLAT), how can I compute the corresponding (alt, az) point in the sky with PyEphem?
Given the way that PyEphem currently works, there are two steps to answering your question. First, you have to convert a pair of galactic coordinates into an equatorial RA/dec pair of coordinates instead.
import ephem
# Convert a Galactic coordinate to RA and dec
galactic_center = ephem.Galactic(0, 0)
eq = ephem.Equatorial(galactic_center)
print 'RA:', eq.ra, 'dec:', eq.dec
→ RA: 17:45:37.20 dec: -28:56:10.2
Those coordinates are pretty close to the ones that the Wikipedia gives for the galactic center.
Now that we have a normal RA/dec coordinate, we just need to figure out where it is in the sky right now. Since PyEphem is built atop a library that knows only about celestial “bodies” like stars and planets, we simply need to create a fake “star” at that location and ask its azimuth and altitude.
# So where is that RA and dec above Boston?
# Pretend that a star or other fixed body is there.
body = ephem.FixedBody()
body._ra = eq.ra
body._dec = eq.dec
body._epoch = eq.epoch
obs = ephem.city('Boston')
obs.date = '2012/6/24 02:00' # 10pm EDT
body.compute(obs)
print 'Az:', body.az, 'Alt:', body.alt
→ Az: 149:07:25.6 Alt: 11:48:43.0
And we can check that this answer is reasonable by looking at a sky chart for Boston late that evening: Sagittarius — the location of the Galactic Center — is just rising over the southeastern rim of the sky, which makes perfect sense of a southeast azimuth like 149°, and for an as-yet low altitude like 11°.