E184 -- Part of a Letter from Mr. Professor Euler to the Reverend Mr. Wetstein, Chaplain to His Royal Highness the Prince, concerning the contraction of the orbits of the planets.

(based on Eric J. Aiton's introduction (written in English) to Opera Omnia Series 2, Volume 31)
In the paper, Euler says that in order to remove any doubt that the orbits of the planets are continually contracted and their periodic times consequently reduced, he needs some good ancient observations. He explains that the problem he ran into with Ptolemy's observations is that the times are probably one or two days earlier than the chronologers had calculated. Euler then asks if the length of the day has always been the same. He also argues that the force of Jupiter accelerates the earth in its orbit so much that the resulting diminution in the length of an earth year would be too great to be in agreement with observation, unless the diurnal motion had likewise been accelerated in the same proportion.


According to the remark on p. 356, the letter is dated December 20, 1749 and was presented to the “Royal Society” on March 1, 1750 [that is, 17495].

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