E218 -- Brief von L. Euler an E Pontoppidan vom 11. Mai 1754

(Letter from Euler to Pontoppidan of May 11, 1754)

(based on Eric J. Aiton's introduction (written in English) to Opera Omnia Series 2, Volume 31)
This is Euler's response to six questions that Pontoppidan asked Euler in response to reading E183; Pontoppidan wanted to write an essay on the newness of the world and sought Euler's help for some clarification. Euler reiterates his belief that the gradual approach of the planets toward the sun provides convincing evidence that the system of the world, as we now know it, had a beginning and will also have an end. He also shows why an aether exists and then goes on to say that Newton's theory of light is inconsistent with Newton's hypothesis that space is a void. He explains this by saying that if luminous matter does emanate from the sun with the speed of light that experiment has revealed, then all of space must be filled with this matter. On the other hand, if, as Euler believes, light is a phenomenon similar to sound, then it must be transmitted through an elastic fluid that fills up space; such a fluid, however, resists the motions of the planets, and the approach of the planets to the sun then follows by the laws of mechanics. In answer to Pontoppidan's first question, Euler says that luminous matter tends to drive the earth away from the sun, but this effect is overwhelmed by the force of gravity. As to Pontoppidan's other questions, Euler says that they are based on the false assumption that light actually emanates from the sun. Euler concludes by saying that even though the length of each year has become shorter, so have the days; as a result, the years have continued to contain the same number of days as before.


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