Re: Sun's distance

[brian whatcott <inet@INTELLISYS.NET>]

At 15:16 2/19/00 -0700, you wrote:
> Where could I find some information on how the distance to the Sun was
measured?

The story is one of extending a measured baseline.
Eratosthenes had the length of the noon sun shadow measured
at two places along a meridian whose distance had been measured.
Supposing the Earth to be spherical, he deduced its radius.

 Hipparchus found a position where the moon was overhead
and compared it with the position where the Moon was seen on the horizon.

 Beginning with his knowledge of the distance between these two places,
he drew a right triangle depicting the horizon point, the Earth center
and the Moon center. The size of the Earth radius and the angular distance
between the observatories gives a measure of Moon distance.
(about 59 Earth radii, he found)

Aristarchus had also (earlier) estimated the Earth Sun distance with a
comparable method. This was a comparison of the local noon, and the time
of the Moon when just half-full.
He pushed the best estimate from a few Earth-Moon distances to as many as
nineteen, but though bold, his finding was not bold enough: the distance
is now taken to be about 390 Earth-Moon baselines.

Brian W



brian whatcott <inet@intellisys.net>
Altus OK