re: Iridium flare and flasher observed, TiPS

Bart De Pontieu ((no email))
Fri, 19 Jun 1998 22:47:26 -0700 (PDT)

On Fri, 19 Jun 1998, Frank Reed wrote:

> Since there is increasing interest in spinning satellites and observers
> often have occasion to describe a periodic shape, and departures therefrom,
> of their brightness waveforms, it would be nice if there were a term whose
> meaning would be clear to all.  If the common usage for the term "synodic
> anomaly" is as Walter defines it above, that's fine with me.  However, it
> doesn't necessarily imply (to me anyway) a _time_ shift in the cyclic
> behavior, nor does it quite fit the definitions in my old (1950) college
> Webster:
> 
> synodic ... 2. _Astron._ Pertaining to the conjuctions, esp. to the period
> between two successive conjunctions of the same bodies, as of the moon or a
> planet with the sun.
> 
> anomaly  1. Deviation from the common rule; irregularity. ... 3.  _Astron._
>  The angular distance of a planet from its perihelion, as seen from the sun.
> 
> I thought of something like "anomalous period", but that might be construed
> to be something like the mean motion.  Maybe "short cycle" or "long cycle"
> or "cyclic time shift" ???

The term 'synodic anomaly' was coined by either Patrick Wils or myself, I
can't remember right now, when describing Walter Nissen's observations of
91- 29 B. Its origin is pretty straightforward: Walter saw an extreme
version of the 'synodic effect', i.e. an anomaly. This 'anomaly' (used as
'deviation from the common rule', *not* definition 3 in the above) 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). If
this happens a small change in the orientation of the satellite with
respect to the observer leads to a large perceived change in flashing
period. 

The synodic effect is a common term in astronomy, we're not making up
terms here. It describes the change in e.g. phase or flashing period due
to the relative motion of a celestial object (due to its orbital motion).
The flashing period of a tumbling satellite is hardly ever identical to
its rotation period (as measured in an inertial frame), and this is due to
the orbital motion of the satellite. Whilst researching the synodic
effect for the english language brochure of the Belgian Working Group
Satellites a few years ago, Mike McCants and I were discussing the synodic
effect and this lead to:
 
http://www2.satellite.eu.org/flash/90/chapter2_6.html

This reference explains the term synodic anomaly.

You're probably right, Frank, that this may not precisely describe a time
shift, but it's the historical term and I imagine most SeeSat-L readers
that were around 2 years ago got used to it :-)

Now as far as spinning satellites enjoying an increasing interest, I hope
you're right. People interested in watching and measuring spinning
satellites might want to check out the activities of the Belgian Working
Group Satellites which has been concentrating on flashing satellites since
at least 1986 and has collected a database of almost 50000 measurements of
about 1500 different satellites called the PPAS (Photometric Periods of
Artificial Satellites). More info at:

http://users.skynet.be/satimage/bwgs/bwgs.htm
http://www2.satellite.eu.org/ppas/ppas7.html

Cheers,
     Bart

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Bart De Pontieu -- bdp@lmsal.com -- Belgian-in-exile :-)
!!!!!! Join us at Eurosom 3, October 24/25, Edinburgh, Scotland !!!!!!
Solar Physicist at Lockheed Martin Solar & Astrophysics Lab, Palo Alto
bart@satellite.eu.org -- www2.satellite.eu.org/satintro.html
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