Re: how did Roemer know the size of Earth's orbit?
From: Shawn (sdotherecurry_at_bresnanihatespam.net)
Date: 11/28/04
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Date: Sat, 27 Nov 2004 21:34:44 -0700
Alan Horowitz wrote:
> Roemer made the first calculation of the speed of light by noticing
> the different transit times of light from the eclipse of Jupiter's
> moon, as seen from one part of the earth-year, to another part (when
> the earth was farthur away from Jupiter.
>
> So, how did he (well, they) know the absolute size of earth's orbit
> around the sun?
Good question. Cassini used the parallax of Mars as measured
simultaneously from two different points, knew the baseline, and could
thus calculate the Earth-Mars distance. The ratio of Earth's orbit to
that of Mars (but not absolute distances) was known (Kepler IIRC) so the
calculation of Earth to sun distance could then be made.
I suppose the parallax of the sun could be calculated during an eclipse
as well. (thoughts?)
Shawn
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