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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.

astroland
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