PyEphem

Comet sun_distance = 0.0?

Asked by Jure

I'm trying to comupte the heliocentric and geocentric distance for a comet C/2010 X1 (Elenin), which i need do calculate the standard photometric lightcurve of a comet.
Im using the orbital elements generated by minorplanet epehemeris generator

C/2010 X1 (Elenin),e,1.8393,323.2273,343.8064,11358,0.0000008,0.999958,360.0000,08/27.0/2011,2000,g 8.0,4.0

When i compute the comet position for a date 2011/03/07 21:00 and print the sun_distance the result is always 0.0 no mater what date i put in.

Any ideas why i always get the result 0.0 ?

Thanks
Jure

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Launchpad Janitor (janitor) said : 		#1

This question was expired because it remained in the 'Open' state without activity for the last 15 days.

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Jure (jure-zakrajsek2) said : 		#2

The above error happens also for some other comets.
For example C/1999 H1
The orbital elements from MPC are:
C/1999 H1 (Lee),e,149.3529,162.6506,40.7012,2758,0.0000068,0.999743,359.9999,07/01.0/1999,2000,g 9.0,4.0

Thank you for any ideas how to solve this problem

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Brandon Rhodes (brandon-rhodes) said : 		#3

I did not write the comet-part of PyEphem, so I am not familiar with the actual mathematics. When I try to do the computation you suggest, I get:

>>> o.sun_distance

Near-parabolic orbit: inaccurate result.
  e = 0.999958, lambda = 0.000021, w = 365.846867
Near-parabolic orbit: inaccurate result.
  e = 0.999958, lambda = 0.000021, w = 365.8468670.0

Which makes it sound like these particular orbital parameters are outside of what "libastro" is prepared to compute. For a comet like this with a 600,000-year orbit, it would probably be better for PyEphem if you could get hold of some orbital elements that stopped trying to be elliptical and just gave up and pretended that this was a one-visit parabolic comet. :)

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