Question

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?

Answer 1

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°.