I'm experimenting a bit with PyEphem and I found out that the magnitudes of comets calculated with PyEphem are a bit off when you compare them with other (online) resources.
Here's the code I'm using:
import ephem
comet = ephem.readdb("C/2017 O1 (ASASSN),e,39.8496,25.8112,20.9067,454.6043,0.0001017,0.99670320,0.0000,10/14.7834/2017,2000,g 13.0,4.0")
date = ephem.now()
# compute on date
comet.compute(date)
# define an observer
obs = ephem.city('Utrecht')
print "%s is in constellation %s with magnitude %s" % (comet.name,
ephem.constellation(comet)[1], comet.mag)
This is the output (on October 3rd, 2017) of this script:
C/2017 O1 (ASASSN) is in constellation Perseus with magnitude 14.17
These are the results of other (online) resources on October 3rd, 2017:
The Sky Live: magnitude 11.17 (https://theskylive.com/c2017o1-info)
Heavens Above: magnitude 5.5 (IMHO a bit too much off...)
In the Sky: magnitude 7.8.
What causes PyEphem to calculate such a high magnitude? Are more parameters needed in the observer configuration to get a better calculation?
Thanks for any help/pointers.