Lunar transit by ???

25 Jul 1997 13:16:47 -0800

Bill Kahn wrote:

"I believe I saw a satellite crossing the face of the nearly full moon.
Is this common? possible? ridiculous?


Observing nearly full moon on evening of July 17, 1997.
Lat: N39.692  Lon: W75.567 (30 miles west of Philadelphia)
Using 12.5" f4.84 Newtonian with 32mm Plossel eyepiece for 48x and 1*fov
Time 10:32:50 EDT  (WWV checked watch)
Observed: A black resolvable round circle, estimated at 10" diameter crossed
from left to right in the scope taking about 1 second to traverse the
approximately 20' of illuminated disk, approximately 5' below lunar equator.

If this was a satellite, do folks do this lunar silhouette satellite
observing often?

Which satellite was it?

- - - - -

Hi Bill,

What you probably saw was a weather balloon.  10 arcseconds is huge, even for
a low-earth orbit object making a zenith pass.  At the time of your
observation, the moon was near culmination in the south, less than 45 degrees
above the horizon.  Thus the range to a satellite transiting the moon would
have been at least 300 km (for an object on the verge of reentry) and probably
more than 500 km.  At 300 km range, 10 arcseconds represents a linear
dimension of 14.5 meters -- there's nothing in orbit of this size at such a
low altitude.  At 500 km range, the size has to increase to 24 meters; the
Shuttle and Mir are the only candidates of this size, and their shapes would
have been pretty obvious in your telescope.

As a check, I ran SkyMap against ALLDAT.TLE to see if anything was close to
crossing the moon at that time, moving roughly west to east (or right to left
as you look at the moon).  Only Norad #00059 was close to the right
trajectory, but it still missed the moon by 5 degrees, and the timing was

 I continue to be amazed by the number of weather balloons that are spotted in
telescopes observing the sun and moon -- just how many of these things are
released each day??  --Rob