First Earth-Sun Measurer




Name: Phyllis
Status: other
Grade: other
Location: OR

Question: Historically, who made the first determination of the distance 
from the earth to the sun?
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Dear Phyllis,

Good question!  I think that Edmund Halley first made a serious attempt, 
suggesting that a transit of Venus would give us a way to calculate the Earth 
Sun distance. Until the 10th century, transits were considered the way to do 
it.

David H. Levy
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Hello Phyllis,

Your question can actually be broken into two variations.  Who was the
first person to attempt such a measurement and who was the first to get
it reasonably correctly.  Aristarchus of Samos is one of the earliest
(if not the earliest) person to attempt to calculate the sun-to-earth
distance based on geometric arguments.   However, he was a little off
the mark and got it about 400 times too short.

Next comes an interesting enigma from Eratosthenes (who is also credited
with obtaining the first accurate measure of the earth's circumference).
 There is a brief quotation of a number for the distance attributed to
him, but with no mention of how it was obtained.  There are other
numbers mentioned in the same passages that are somewhat inaccurate, but
in this case his distance from the earth to the sun is quite close, only
off by 2%.
Thus we have a quandary.  Could he have really measured the distance of
the earth to the sun so accurately without measuring the earth to moon
distance with the same(or similar) precision?

The first really accurate and reliable measure of this distance comes
much later from observations of apparent parallax of Mars and Venus.
Cassini and Richer were the first to do this by making simultaneous
measurements of the mars at various dates in different locations.
Subsequent measurements of parallax from observing Venus have also been
used. In all of these cases, the there is an important phenomena.  It
takes a third object (or point of reference) in order to obtain a
measurement of the distance through geometric means.

cheers,

Michael

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Michael S. Pierce
Materials Science Division
Argonne National Laboratory
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