From: Lee Daniel Crocker (lee@piclab.com)
Date: Fri Apr 12 2002 - 15:50:46 MDT
> (spike66 <spike66@ATTBI.com>):
> 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.
I think you need to check your geometry there, Spike. 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. To a first
approximation, the Sun's distance can be treated as infinity, and so
the size of a shadow on the Earth's surface is equal to the size of
the object for object altitudes less than millions of miles. We
don't generally see shadows of objects that high because they would
be overwhelmed by atmospheric light scattering and light pollution.
But under good conditions, one does occasionally notice the passing
shadow of an airliner, which is about the size of an ailiner,
regardless of its altitude. I don't think it's unreasonable to think
one might be able to catch a NEA shadow, but I don't know what other
factors (like speed) might get in the way of that.
-- Lee Daniel Crocker <lee@piclab.com> <http://www.piclab.com/lee/> "All inventions or works of authorship original to me, herein and past, are placed irrevocably in the public domain, and may be used or modified for any purpose, without permission, attribution, or notification."--LDC
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