Phase Angle

Sue J. Worden (worden@uts.cc.utexas.edu)
Tue, 23 Sep 1997 20:24:21 -0500 (CDT)

> Date: Tue, 23 Sep 1997 13:08:59 -0700 (PDT)
> From: Jim Varney <sat_watcher@rocketmail.com>
> Subject: re: the Iridium format
> To: SeeSat-L@cds.plasma.mpe-garching.mpg.de
> [ ... ]
> 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.

	I was under the (mistaken?) impression that that angle
	*was* the phase angle.  From the QuickSat documentation:

	   "The Phs value is the sun-object-observer angle.
	   A value of 0 indicates full phase, a value of 180
	   indicates new phase."

	Perhaps this is a case of terminology confusion, but I
	learned in high school geometry that the designation
	"angle ABC" was the angle formed by travelling from
	point A to point B to point C in that order, and that,
	by convention, of the two possible angular values, the
	value smaller than 180 degrees was understood to be the
	angle being described.

	Is not the QuickSat "sun-object-observer angle", i.e.,
	angle sun->satellite->observer, the same as the Jim's
	"angle subtended from sun to satellite to observer"?
	
	I am very confused at this point.  I would appreciate
	someone shedding some light on this, and at full phase
	illumination, please. :-)

	--Sue (worden@uts.cc.utexas.edu)