Hi Rolf,

Hmmm... there are occasional wild variations in the absolute magnitudes
given in the MPECs for comets. Comets are notorious for deciding to drop
or rise in brightness, and I've sometimes seen the absolute magnitude
rise and drop by a lot.

MPC's earlier result was based on data from discovery up till September
22. Using those observations, I get an absolute mag of 4.6. MPC does
some different things with weighting of observations, and got 4.0.

MPC's second result uses data up to November 24. Doing the same, I
get an absolute mag of 6.1. However, that gives computed magnitudes
that are (relative to C/2009 P1's current brightness) too bright by a
couple of magnitudes. The problem is that I'm computing an absolute
magnitude based on _all_ observations, and the comet is intrinsically
a lot dimmer now.

So I _assume_ MPC has some method in place that is weighting "current"
photometry very highly, so that they can compute an absolute magnitude
that does a good job of computing current brightness. Which means that,
just as the brightness of a comet jumps around a lot, so will MPC's
absolute mag.

-- Bill

Rolf Stadelmaier wrote:

> Hi Bill,
>
> when Guide 9.0 DVD finally had passed customs (without fee) I installed it
> and saw I would be pretty content with your great work - thank you a lot.
>
> One thing puzzled me though:
> Guide9 displays comet Garradd with mag 12.7 in comparison to Guide8
> which displays it with mag 7.7 - what should be the right magnitude.
>
> When comparing the entries in the MPCORB files
>
> 30.09.2011 (Guide8):
> Garradd (C/2009 P1) 23.6755 12 2011 0.0
> 1.550564 1.001011 106.1774 90.7454 325.9972 2000.0 4.0 4.0
> MPEC 2011-S56
>
> 30.11.2011 (Guide9):
> Garradd (C/2009 P1) 23.6755 12 2011 0.0
> 1.550564 1.001011 106.1774 90.7454 325.9972 2000.0 9.0 4.0
> MPEC 2011-W48
>
> one can see the dufference, so it's not a matter of Guide.
>
> Is this all right or should'nt be the actual magnitude of comet Garrad
> 7.x instead of 12.x? Perhaps you can shed some light on this?
>
> CS and many thanks,
> Rolf Stadelmaier
>