First Earth-Sun Measurer ```Name: Phyllis Status: other Grade: other Location: NV Country: N/A Date: N/A ``` Question: Historically, who made the first determination of the distance from the earth to the sun? Replies: 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 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 Click here to return to the Astronomy Archives

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