Synodic Anomaly or Intrinsic Phenomenon?

Frank Reed (f-reed@metabien.com)
Wed, 24 Jun 1998 15:43:08 -0700

On June 13 UTC I observed blinking and flashing of #24871 Iridium 20aka18 
and reported the data to the SeeSat mailing list.  Using 10x50 binoculars, I 
saw over 100 periodic cycles of blinking at about magnitude 5.  At one point 
in time there was a break in the rhythm that I called a "phase shift."  
Later, Walter Nissen wrote:

>By phase shift, do you mean what Bart and I have referred to as a "synodic 
>anomaly"? I.e., an interval which is different in length, surrounded on 
>both sides by many intervals, all of essentially the same duration. 

Yes, that's exactly what I meant - to avoid confusion I'll denote this event
by the term "Break" in the following.

This Break occurred somewhere within a 30 second interval beginning at 
6/13/98 04:34:06 UTC.  A bright flashing episode of magnitude -3 or brighter 
occurred about a minute later at 04:35:05 UTC.

In further discussion, Bart wrote: "This 'anomaly' ... occurs
when the bisectrix of the angle sun-satellite-observer is almost aligned
with the rotation axis of a reflecting cylinder (such as 91- 29 B)."

And Walter stated: 

"In describing satellite phenomena, and particularly flashing phenomena, a 
distinction is made between intrinsic phenomena, which occur because of 
some characteristic, e.g., rotation, or angular momentum about an internal 
axis, of the satellite itself, on the one hand, and, on the other hand, 
synodic phenomena, which occur because of changing circumstances during 
the progress of the OBS, particularly the changing phase angle at which 
the object is observed, due to its angular momentum about an axis through 
the center of Earth. 

"E.g., the flashing of Ir 18 = Ir 20 = 24871 = 97- 34C is apparently an 
intrinsic phenomenon and this is pretty strongly suggested to an 
experienced observer by its observed appearance. In contrast, the nightly 
Iridial glints produced by the other Iridia in working attitude in working 
orbits are synodic phenomena, which is similarly suggested to the 
observer."

Subsequently Rob Matson has collected observation data of bright flashing 
episodes of this satellite and from his analysis has estimated the position 
of the spin axis to be approximately RA = 4.45 hours, Dec = +29.25.

I ran out some numbers from SatSpy and did some spreadsheet work to find 
the angles between the spin axis from Rob's estimate and the vector from 
the observer to the satellite.  During the time inerval in which I saw 
the Break, the angle increased from 84.7 to 96.8 degrees (assuming I did 
the calculations right).

Therefore the Break may have occurred when I was saw the satellite from a 
direction perpendicular to it's spin axis.

I'll leave it to others to determine whether this was an "intrinsic" or a 
"synodic" phenomenon.



-- Frank Reed
Scottsdale, Arizona, USA  111.898W  33.484 N  1227 feet