The magnitude will be based entirely (and many time erroneously) on the latest MPEC that is issued for the updated elements on the comet.  Rarely is the specified magnitude anywhere close to the given magnitude, typically far brighter (given) that the comet actually is.  Note also that magnitudes from MPC on comets are expressed as m1 (total) magnitudes which is horribly misleading in many cases.  Except in rare comets where the head, or nucleus, of the comet is the ONLY measurable parameter (no visible outer coma nor tail), the measurable comet will include the coma as well as the tail that passes through the measurable semi-diameter of the coma; that entire combination of dust and gases is what comprise the TOTAL (m1) magnitude and it is quite difficult to ascertain, even via CCD, accurately. More often - a good case in point at this time is comet 2P Encke - there is a distinct starlike nucleus that should be measured rather than attempt to measure a very vague and extended faint cloud of gas INCLUDING the nucleus.  Such a measurement would be = m1.

 

However, a nuclear measurement (not given on MPECs) would be the measurement of the starlike or disklike structure of the comet structure itself, or m2, nuclear magnitude.

 

In short, never rely on the given magnitudes that you see predicted by JPL or MPC; using current comet lists like:

are far more reliable.

 

But - always record and report what YOU measure, not what you "think" the comet should be or what others report....use lists only as a guide.

 

Dr. Clay
_____
Arkansas Sky Observatories
MPC H45 - Petit Jean Mountain South
MPC H41 - Petit Jean Mountain
MPC H43 - Conway West
http://www.arksky.org/