--On Tuesday, September 23, 2003 12:09 AM +0200 Jean-François Coliac
<jfcoliac@...> wrote:

> I am trying to get the magnitudes for comets with Astronomical
> algorithms p 216 but the results are different from those obtained in
> Guide.
> I used mag = H + 5*log delta + G * log r
> Does anyone knows the formula used in Guide to calculate the comet
> magnitude ?

The correct formula is:

m1 = H + 5*log delta + 10*log r

This assumes that the brightness is inversely proportional to the square
of the comet-Earth distance (delta) and inversely proportional to the
fourth power of the comet-Sun distance (r). If the brightness is assumed
to be proportional to the inverse nth power of r, the formula would be:

H + 5*log delta + 2.5*n*log r

Guide incorrectly uses G for n. G is the "slope parameter", used mainly
for asteroids, and refers to the phase angle (Sun-asteroid-Earth angle).
It has no relevance to comets.

The default value for n (or G) should be 4. I always use n = 4, and
assume that any deviation from this is due to variations in H (the absolute
magnitude). I calculate H from the most recently reported comet magnitudes
http://cfa-www.harvard.edu/icq/CometMags.html
using n = 4, and averaging. Then using n = 4 for calculating m1 (total
magnitude) is consistent.

Truman

Truman P. Kohman, Departments of Chemistry and Physics
Carnegie-Mellon University, Pittsburgh, PA 15213 USA
Phone: 412/279-6056 Fax(CMU): 412/681-0648 Internet:
tk11@... (best for correspondence, ASCII documents),
tpkohman@... (best for images, coded attachments, etc.)