RE: Standard magnitudes

Ted Molczan (
Wed, 29 Nov 1995 11:43:22 -0500

Edward S Light wrote:

>As a fledgling satellite observer, I noticed that there seems to be
>two different definitions of "standard" magnitudes, namely 1000 km
>distance and zero phase angle, and 1000 km distsnce and 90 degrees
>phase angle. It seems to me  that the zero phase angle definition is
>the more logical. Could someone please explain to me why that is not
>the best choice?

In this case, logic is in the eye of the beholder. I used a ninety degree 
phase angle because Desmond King-Hele used it in his book Observing 
Earth Satellites:

"Obviously, a satellite will be much brighter as a 'full moon' than as a
crescent, and to provide a standard we choose the situation when the
satellite is half illuminated, like the moon at 'first quarter'; that is, when
the 'phase angle', the angle 'Sun-satellite-observer', is a right angle."

Another reference to this topic appears in Appendix C of the Satellite 
Observers Manual, published by the British Astronomical Association:

"The absolute magnitude of a satellite is its half-phase magnitude at a
range of 1000 km, when seen at an altitude of 90 degrees." 

(The specification of a 90 deg altitude was to eliminate the effects of 
atmospheric extinction.)

On average, an observer is more likely to observe satellites at half
illumination than at full illumination or zero illumination, so it seems
reasonable to take that into account when selecting a standard. Of
course we prefer to observe at full illumination, but nature does not
always cooperate, and often we have to accept very poor phase
angles in order to make an important or rare observation.

Also, keep in mind that when King-Hele and the BAA wrote the first
editions of their books, in the 1960's, personal computers did not exist, 
so few amateurs would have included magnitude in their routine ephemeris 
calculations. They made do with rough estimates, and might have
actually preferred a middle-of-the-road phase angle, because they might
be able to more safely ignore it, and perhaps only concern themselves 
with the inverse-squared effects. Range to a satellite could be estimated 
using the information provided by some of the groundtrack prediction 
charts in common use in the 1960's.

Today, there is a proliferation of PC's and satellite ephemeris software, 
so perhaps a different standard might be in order, or for that matter, ANY 
standard might be better than none. The standard magnitude in my n2l
files is derived from the object's dimensions, and is included for the 
benefit of those who do not have any other means to estimate magnitude.
I am open to changing the standard from half to full illumination if there
is sufficient interest. But, keep in mind that there may be software that
depends on my present standard, which would be broken if I changed it.
Also, 90 degrees seems to be well established among British observers, 
who arguably have been the world leaders among amateur satellite 
observers, so perhaps we should respect their de facto standard.

Clear skies and full illumination!