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Yesterday night I have tried find comet 45P, that should be still easily visible with my equipment at hand, however I was unable to find it. I used data provided by this website.

Today I've checked the website's data against the Minor Planets Center's data loaded into Stellarium (the program) and found the predicted positions of the comet to differ by more than 7°. That is totally unacceptable when trying to find the comet with a FOV of a few tens of arcminutes (the finder presumably is too light-weak).

So does anybody know a definite, reliable source for the position data of 45P? I'm gonna try again today anyway with the positions provided by Stellarium, but it might be good to have another alternative.

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The best way to get ephemeris of solar system bodies is to use JPL HORIZONS:

http://ssd.jpl.nasa.gov/horizons.cgi

You put "Target Body" as 45P, your "Location", "Time", and click "Generate Ephemeris". This will give the RA/DEC information for time by default.

If you want some advanced data other than the listed, e.g., phase angle, V-magnitude, RA/DEC change rate, etc, you can tune "Table Settings". I usually skip daytime and airmass < 2.

There can be some mismatch between the ephemeris and real observation. This is not because JPL HORIZONS gives wrong calculation, but because the reported observation data is not perfect. It calculates the orbit of the target from previously reported observational data to IAU MPC. IAU MPC provides the orbital elements inferred from that reported data, and these elements are used to generate ephemeris from JPL HORIZONS. So if the report is slightly imperfect and thus the inferred orbital element differs from the true value by a slight amount, it may cause the mismatch at a time far from the original observation. This is the reason we need more and more data continuously and update the catalog.

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  • $\begingroup$ Welcome to Astronomy. Thanks for your answer. Please take the time to take the tour. Note, there is no need to sign your answers, thanks. $\endgroup$ – James K Feb 15 '17 at 18:06
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You can try this site
http://www.heavens-above.com/comet.aspx?cid=45P&lat=0&lng=0&loc=Unspecified&alt=0&tz=UCT which agrees with the planetarium program I use. I think Stellarium is reliable, and there is Cartes du Ciel, which is also free.

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  • $\begingroup$ Hm I've tried now 4 different sets of positions - still nothing. I got a very diffuse cloud cover, but I can see stars of 9th magnitude... so I don't understand where the comet went :( $\endgroup$ – AtmosphericPrisonEscape Feb 13 '17 at 23:50
  • $\begingroup$ Comets typically have low surface brightness (ie the light they emit is spread over a reasonable area) and I think that the combination of diffuse cloud and moonlight would mean that the light from 45P is effectively drowned out. $\endgroup$ – Dr Chuck Feb 14 '17 at 8:50

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