Bill,
There seems to be some uncertainty about the meaning of "G" in a
comet's elements. My understanding is that it is the negative of the
exponent of the comet-Sun distance in the brightness, and that it is
usually given as 4 (as 2 is the negative of the exponent of the
comet-Earth distance). This corresponds to the expression
Dm1 = m1 - m0 = 5 log Delta + 10 log r.
I find that setting G = 4 in Guide causes it to reproduce the magnitude
variations given by the CFAPS, so Guide must be using it that way.
Under Extras - Edit Comet data - Add a comet it asks for entry of
"Slope parameter (G)". The explanation of G is "This value defines how
the object's magnitude varies according to its distance from the sun (in
the case of a comet) or the angle at which it is illuminated by the sun
(in the case of an asteroid). ... try accepting the default value given
(5.0 for comets, .15 for asteroids"." I believe that the default value
for comets should be given as 4.0. For asteroids the unit is
magnitude/degree (of phase angle).
By the way, does "slope parameter" have a meaning for comets?
Perhaps a different symbol for this parameter should be used for comets
(G is the accepted symbol for "slope parameter" for asteroids).
Truman

Truman P. Kohman, Departments Chemistry and Physics
Carnegie-Mellon University, Pittsburgh, PA 15213 USA
Phone: 412/268-8865, 412/561-8343 Fax: 412/681-0648
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