Anthony Ayiomamitis wrote:

http://www.perseus.gr/Astro-Lunar-Parallax.htm

The image scale of the resampled image is around 2.51"/pixel. ;-)

I get an estimate of 438,988 km for the distance of the moon from the
earth when, in fact, it was 395,520 km at the time of photography. In
other words, there is an error of approximately 10%.

I got an estimate of 443,368 km. This is assuming an image scale of
about 3.25"/pixel, which I got from the diameter of the Moon,

557 pixels using the ruler tool in Photoshop CS2
1812" according to http://aa.usno.navy.mil/data/docs/diskmap.html

The distance between the two images of Regulus is 337 pixels = 1096".

Moon distance = (Selsey Athens distance / 2) / tan(1096"/2)

I think the error comes from assuming that the Selsey-Athens base of the
triangle is at right angles to the Earth-Moon vector. In general it
won't be. If tilting that line up to make it perpendicular shortens it
to about 2100 km, we get a very accurate estimate.

- Ernie http://home.comcast.net/~erniew