Iridium 12 flare, "phase", solar elev., other 9/23/97 obs

Ed Cannon (ecannon@mail.utexas.edu)
Tue, 23 Sep 1997 23:56:45 -0400

I'm happy to report having seen a monster Iridium flare Monday night!  

NORAD   date       time   Dir  aI eI  aM  eM  aS   eS Phs  Dur Mag Obs  COSPAR
24837  97-09-23  01:58:50  S  131 48  084 57  102 -21 145  02  -6  EdC  97030B

It's hard to say for sure how bright it was.  Jupiter was in the same
general part of the sky, and it was *many* times brighter than Jupiter.  
Its apparent size (partially a function of the visual magnitude?) was 
also larger than Jupiter's.

I think I missed the beginning of it.  Within a couple of seconds after I 
first saw it, it dropped down to about mag. +2 or so but continued to be 
observable at one-power for possibly as long as 15 seconds.

To be accurate, I need to report some uncertainties due to my having only 
an analog watch for this observation.  (For most of my one-power observing
around UT, it's all I need.)  (Maybe Mike M. was observing and saw it also
and got the exact time....)

 time: 01:58:35 to 01:59:05 (1:58:50 +/- 15 seconds; hard to read)
 aI: 123 to 137
 eI: 52 to 44
 Phs: 145-146 (or Quicksat's 35-34?)

I'm happy to say that I learned how to get the Sun's azimuth and elevation 
using the freeware astronomy program Home Planet, which I have had for a 
while but am still figuring out.  So I won't send any more reports of "so 
many minutes before or after the 10-degree twilight time"!

Regarding the phase angle in the Iridium reports, Ron defined it:

> 11) Phs: Sun phase angle. This is the Skymap convention: 
> Satellite-Observer-Sun. Quicksat users should subtract their value from
> 180.

Regarding this, Jim Varney wrote:

> 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 second Sue's point that Quicksat gives the sun - satellite - observer
angle, and I'll repeat her quote from Quicksat's documentation:

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

Continuing on this, Jeff Hunt quoted SatPro:

> "AsPh - The Astronomical Phase Angle in degrees. Phase Angle is defined as
> the angle subtended at the satellite between the Sun and ground station
> (observer). Small angles indicate that a greater portion of the sat is
> illuminated."  This would be sun-sat-observer angle.

And Bram Dorreman wrote:

> 6. My prediction program uses the angle Sun-Satellite-observer.

Again, all of these (except the "Sun phase angle" that Ron has specified) 
are the same as Quicksat's phase angle (Phs).  

>----> The most important thing is that everyone report the value of the 
*same* angle!  So it appears that for Quicksat, SatPro, and the program 
Bram uses, we should report the value of (180-program output angle).

Regarding RA and Dec, Bram also wrote:

> Fortunately my prediction program (written by Patrick Wils, also
> from Belgium) outputs the Right Ascension and declination and
> Azimuth and elevation of the culmination point.

The same is true of Quicksat; it generates Alt/Az and RA/Dec of every 
prediction point -- and prediction points can be generated down to very
small increments (e.g., 50 points per object per pass, or more!).

Other obs:  Monday evening I also saw at one-power objects 80-93B (12072, 
Cosmos 1222 Rk) and 77-91B (10363, Cosmos 955 Rk).  Only three objects 
were predicted to be mag. 3.5 or brighter here this evening -- an 
unusually small number in my (limited) experience.  (By the way, I did 
not see Iridiums 9, 10, 13, or 14, which also passed over here with 
culminations from the zenith to the east.  I didn't try to see two 
others that culminated to the west -- 11 and 16.)

Ed Cannon
ecannon@mail.utexas.edu
Austin, Texas, USA
Observations and report from UT location: 30.29N, 97.74W, 165m