re: the Iridium format

Jim Varney (
Tue, 23 Sep 1997 13:08:59 -0700 (PDT)

---Richard Clark <rclark@LPL.Arizona.EDU> wrote:

> are just pointing out an alternative and not suggesting change? I
> that most people will have an easier time reporting in az-el,
> for recording visual (usually naked eye) observations in the field.

I don't follow how it is "easier" for visual observers to report in
az-el.  I think most of us judge a satellite's position using
reference stars.  That naturally leads to RA and Dec as the "easier"
quantity to report; az-el requires a conversion calculation from
> But what I was going to say...  As useful as the phase angle is, it
may be
> safer to use the sun and flare az-el and calculate the illumination
> from that. In any case, the phase angle doesn't fully define the 3
> dimensional geometry of a flare.

You're right.  I assume the whole point of gathering all of these
flare observations is to eventually ferret out some sort of
relationship that will lead to the prediction of flares.  With that in
mind the phase angle has very little value.

Phase angle is useful for diffusely illuminated objects -- the theory
being that the percentage of the body that is sunlit and in shadow has
a direct relationship to the object's visual magnitude.  For specular
reflections from mirror-like surfaces, however, the only factor that
matters is the relationship between the reflector's surface (actually
a normal to it), the observer and the sun.  The overall illumination
of the satellite body is irrelevant.

What we really need to report is the angle subtended from sun to
satellite to observer.  Unfortunately I don't know of any readily
available program that calculates that.

> Speaking of 80 characters/line... Sorry to get a little off topic by
> bringing up an 'internet 101' sort of issue but the problem that
> (irritates:-) me to point it out just occurred in the very message 

...and in addition to Mozilla there's these really obnoxious "free
e-mail" services that add taglines without your permission :)

 -- Jim
Jim Varney   121.398W  38.458N 8m   Sacramento, CA,
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