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Cometary Magnitude discrepency


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I would like to know which ephemerides source you trust in relation to cometary magnitudes.

I write a wee newsletter piece for my society and refer to http://www.aerith.net/comet/future-s.html to give me an idea of what may be visible. I have a limit of 12th magnitude.

I then download a fresh ephemerides from the MPC in CdC and search for those comets which aerith.net suggests are there. Now I have a problem.

More often than not, the two cometary magnitude sources are way off each other, sometimes around 6 mags, sometimes 1-2. Which source would be the more accurate?

I'm picking the MPC download but why the discrepancy? Is aerith based on predicted light curves rather than actual visual observation?

Stu 

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It's only an estimate which will vary with whereabouts the observer is on Earth and the seeing conditions, even observing 21PGZ from home recently the magnitude appears to vary from night to night, seems somewhere around 7 ATM bearing in mind it's an integrated brightness a bit like galaxies.

Not only do the magnitude estimates vary so do the positions for some reason, I can't see any excuse for this but a couple of nights ago Stellarium was way off and Sky Safari was spot on.

Dave

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