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Re: Sun's distance (2)



At 20:12 2/22/00 -0700, you wrote:
How was the earth's distance from the sun determined using the transit of
Mercury?

Gassendi at Paris was first to observe a transit of Mercury across the Sun
in 1631.
The 14th Edition of Enc Britannica reminds us that the objects used to refine
the AU were Mars in opposition viewed from Cayenne and Paris (1672),
and again in 1877 viewed twice from one station;
Venus in transit (1761 & 1769) from multiple positions and again (1874 & 1882)
from multiple positions with the observational difficulty already mentioned
in this thread; - and Eros in opposition.

The objective of these observations was to find the "Solar parallax"
- that is the angle subtended at the Sun by the Earth's disk.
The Earth's radius being known, a determination of the Sun-Earth AU
was then accessible.

Another approach to working the Earth orbital size was provided by Bradley's
discovery of the 'aberration of light' (1728) - that the pointing angle of
the
telescope fixed on a distant star was different in one six month, from the
next. This depended on Roemer's discovery in 1676 of the finite speed of
light (found by timing Jupiter's satellite eclipses.)

You will notice that I did not specify in detail the working out of this
solar parallax from planetary observations in opposition - and I expect
the astronomers on the list would serve better in this role.


Brian W
brian whatcott <inet@intellisys.net>
Altus OK