Earth shadow radius

From: John Locker (
Date: Sun Oct 10 2004 - 08:01:59 EDT

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    Hi all,
    Jeff recently asked about the radius of earth shadow , as cast on the geo 
    arc , under perfect conditions.
    For my location , here in the UK , I should be able to get an accurate 
    "guesstimate" ... if there is such a thing , by observing the track of earth 
    shadow this evening....the peak of the eclipse/outage season for this 
    However , as I seem to be clouded in again , its highly unlikely that I will 
    get the opportunity.
    Looking at the video timings from the other night , I can see that the 
    leading edge of the shadow was moving at a rate of  one degree every 3.5 
    minutes or so , which is about right....and as the satellites at 28.5 
    degrees were coming out of eclipse , the birds at 16 east were just 
    So , this gives a shadow diameter of  12.5 degrees.
    Converting this to  km , I get a figure of  9250km.
    I would expect this figure to be in excess of  12,800 km tonight , when 
    everything is lined up .
    Does this seem right ?
    I'd appreciate any comments from more proficient math students  ;O)
    The video has also revealed that the geos in orbit over Europe are now 
    increasing in magnitude , just prior to eclipse , to a level of mag 8 or 
    better , bringing them into bino range.
    This became apparent when I started to examine the dropped frames from 
    friday's recordings.
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