Why is there a difference between ephem positions and lookup table positions for moon

Question

I am trying to locate the moon positons (Altitude and Azimuth) using python's ephem module and comparing it to a lookup table for apparent moon positions for my location. I am noticing a significant difference in the values that i obtain for the angles. Here is my test code.

>>> o = ephem.Observer()
>>> o.lat = 39.2545
>>> o.lon = -76.7095
>>> o.elevation = 80
>>> o.date = datetime.datetime.utcnow()
>>> print o.date
2012/8/13 21:00:55
>>> m = ephem.Moon(o)
>>> import math
>>> azimuth = math.degrees(m.az)
>>> azimuth
286.2894281178355
>>> alt = math.degrees(m.alt)
>>> alt
19.35235063580148

Now, compare these angles to the lookup table values:

       Date/Time             Zenith         Azimuth
2012 Aug 13 21:00:00.0      88.45125       294.56966
2012 Aug 13 21:20:00.0      91.82583       297.59090

Note: Alt = 90 - Zenith. So our zenith value would be: 70.64764

My question is, why is there a difference? The lookup table gives the apparent angles. Does that have anything to do with it?

Answer 1

Your problem is that you are accidentally offering the lat and lon values in radians, because when PyEphem is given a floating-point number it always assumes that you are doing “raw math” and submitting values in radians:

>>> o.lat = 39.2545
>>> o.lon = -76.7095

If you instead want to provide degrees and have PyEphem translate, you can either provide strings:

>>> o.lat = '39.2545'
>>> o.lon = '-76.7095'

Or you can provide numbers that you compute using a math function, if that helps you keep everything straight, since the PyEphem practice of converting strings to numbers is not a standard Python idiom, whereas everyone will understand math.radians() when reading your code:

>>> o.lat = math.radians(39.2545)
>>> o.lon = math.radians(-76.7095)

The observer that you were actually asking about with those large radian values was at the location 89:07:01.8° N, 284:52:09.8° E which is extremely near the North Pole.