"David Woolley" <david@djwhome.demon.co.uk> wrote in message
news:T1067070959@djhwome.demon.co.uk...

In article <744cc401.0310231817.2dd47387@posting.google.com>,
stevejdufour@yahoo.com (Steve Dufour) wrote:

From space.com

Well, forget it. The Great Wall is about 15 feet wide, which even from
as little as 200 miles up (Shuttle cruising altitude) subtends an
angle of only about one-twentieth of a minute of arc. The human eye
can see detail down to one minute of arc, which is obviously far too

Whilst I seem to remember that the Chinese astronaut said they
couldn't see it, being below the resolution of the eye doesn't mean
that you can't see something, especially if it is a linear feature.

Because it is linear, the contrast (in linear optical power terms)
will be 1/20th, or better[1], of that of a large object, not the 1/400th
one would get for a circular object of that diameter.  The brain ought to
be able to integrate the wall along any runs in more or less a constant
direction and see it even if the constrast for a short segment would
leave it in the background noise.  Knowing what you are looking for
will, of course, help, and there is a strong tendency to find such
features when they don't exist.

People do this integration along a line when looking at waterfall displays
from amateur SETI (or moonbounce) systems.

[1] I think there may be a factor close to pi/2 involved.

You might also want to add the effect of a shadow.