I'm using ephem
for the first time, and having trouble understanding the output of oberver.sidereal_time()
I've written a couple scripts to determine solar time from hour angle. The first one uses ephem to compute right ascension and a formula from Meeus' Astronomical Algorithms to get Greenwich mean sidereal time, which can be converted to local mean sidereal with the longitude.
import sys
from datetime import datetime, time, timedelta
import ephem
def hour_angle(dt, longit, latit, elev):
obs = ephem.Observer()
obs.date = dt.strftime('%Y/%m/%d %H:%M:%S')
obs.lon = longit
obs.lat = latit
obs.elevation = elev
sun = ephem.Sun()
sun.compute(obs)
# get right ascention
ra = ephem.degrees(sun.g_ra)
# get sidereal time at greenwich (AA ch12)
jd = ephem.julian_date(dt)
t = (jd - 2451545.0) / 36525
theta = 280.46061837 + 360.98564736629 * (jd - 2451545) \
+ .000387933 * t**2 - t**3 / 38710000
# hour angle (AA ch13)
ha = (theta + longit - ra * 180 / ephem.pi) % 360
return ha
def main():
if len(sys.argv) != 6:
print 'Usage: hour_angle.py [YYYY/MM/DD] [HH:MM:SS] [longitude] [latitude] [elev]'
sys.exit()
else:
dt = datetime.strptime(sys.argv[1] + ' ' + sys.argv[2], '%Y/%m/%d %H:%M:%S')
longit = float(sys.argv[3])
latit = float(sys.argv[4])
elev = float(sys.argv[5])
# get hour angle
ha = hour_angle(dt, longit, latit, elev)
# convert hour angle to timedelta from noon
days = ha / 360
if days > 0.5:
days -= 0.5
td = timedelta(days=days)
# make solar time
solar_time = datetime.combine(dt.date(), time(12)) + td
print solar_time
if __name__ == '__main__':
main()
This gives output I would expect when I plug in some data:
> python hour_angle_ephem.py 2012/11/16 20:34:56 -122.2697 37.8044 3.0
2012-11-16 12:40:54.697115
The second script I wrote calculates right ascension the same way, but uses ephem's sidereal_time() to get the local apparent sidereal time.
import sys
from datetime import datetime, time, timedelta
import math
import ephem
def solartime(observer, sun=ephem.Sun()):
sun.compute(observer)
# sidereal time == ra (right ascension) is the highest point (noon)
t = observer.sidereal_time() - sun.ra
return ephem.hours(t + ephem.hours('12:00')).norm # .norm for 0..24
def main():
if len(sys.argv) != 6:
print 'Usage: hour_angle.py [YYYY/MM/DD] [HH:MM:SS] [longitude] [latitude] [elev]'
sys.exit()
else:
dt = datetime.strptime(sys.argv[1] + ' ' + sys.argv[2], '%Y/%m/%d %H:%M:%S')
longit = float(sys.argv[3])
latit = float(sys.argv[4])
elev = float(sys.argv[5])
obs = ephem.Observer()
obs.date = dt.strftime('%Y/%m/%d %H:%M:%S')
obs.lon = longit
obs.lat = latit
obs.elevation = elev
solar_time = solartime(obs)
print solar_time
if __name__ == '__main__':
main()
This does not get me the output I would expect.
python hour_angle_ephem2.py 2012/11/16 20:34:56 -122.2697 37.8044 3.0
9:47:50.83
AFAIK, the only difference between the two scripts is that the first bases hour angle on local mean sidereal time, while the second bases hour angle on local apparent sidereal time, which takes into account the nutation of the earth, which I think should be a very small factor. Instead I'm seeing a difference of about three hours. Can anyone explain to me what is going on?