From: spike66 (spike66@attbi.com)
Date: Wed May 15 2002 - 22:06:32 MDT
>
>
>>Mike Lorrey wrote:
>>
>>>I find this rather interesting because a year or so ago I recall working
>>>outside one day with a cloudless sky when a rather large shadow passed
>>>through the valley and over me. Looking up I saw no aircraft or clouds
>>>anywhere near the sun at all, which I though was rather strange. Could
>>>this shadow have been from an NEA eclipse?
>>>
>>No. If a shadow passed over you, then the object would have had
>>to be large enough to eclipse a large part of the sun. Recall that the
>>sun is half a degree or close enough to a hundredth of a radian. If
>>you saw nothing, no contrail, it would need to be well outside of the
>>atmosphere, perhaps a good 200 km up, in which case it would need
>>to be 2 km across. Anything that big would set off radar all over the
>>globe.
>>
>Lee Daniel Crocker wrote:
>I think you need to check your geometry there, Spike.
>
OK I checked it.
> The critial
>measurement is not the angle subtended by the disk of the sun, but
>the overwhelmingly large distance from Earth to Sun; the former would
>determine the sharpness of the shadow, but not the size.
>
I claim that any NEA large enough to create a noticeable shadow would
need to be at least half a kilometer across.
Suppose a NEA was passed overhead at 200 km (much lower than
that it would leave a very prominent contrail. If it were half a kilometer
across, it would subtend about a 2.5 milliradians. The sun's diameter
is close enough to 10 milliradians. Square it out, the object would
eclipse about 6% of the suns rays as it passed in front of the sun's disc.
One *might* notice a temporary 6% reduction in sunlight, but it wouldn't
be a large effect.
Even then, if a half km object passed overhead at 200 km, the military
would become quite concerned.
Im as puzzled as you are about what it was that Mike saw that day.
The best I can come up with is a verticle axis vortex which caused
condensation at the center which blocked sunlight as it passed but
could not be easily seen against a blue sky. spike
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