Re: [guide-user] comet 45P -- whats going on ?

Bill Gray Feb 12, 2017

Just checked this out, and got results similar to what Roger mentioned.
Before updating, using comet data from 5 February, I had comet parameters
of 13.5 and 8.0. The update knocked those to 9.0 and 4.0, and the
comet had a sudden "outburst" as far as Guide was concerned.

I don't know how MPC computes their magnitude parameters. In particular,
are they fitted to the last few measurements, or the last week, or...
There would be reasons for each of these. You want to give people an idea
as to how bright the object is "right now", but you may not want a brief
surge to mislead people. And they may be trying to figure out how bright
the object was a month ago. They might apply some sort of smoothing that
emphasizes recent observations, but gives some weight to older data.

In any case, the old saw about "comets are like cats: they both have
tails and do what they want to do" applies here: magnitude estimates for
comets tend to be ballpark estimates at best.

-- Bill