Until close to Newton’s time, the stars had been accepted as a fixed background to the motions of the Earth and the rest of the solar system. The idea developed that they might be bodies like our sun, but even through a telescope they still looked like luminous points, revealing nothing of their size. Newton found a way to tackle this problem (System 596). He noted that a prominent (first magnitude) star looked about as bright as Saturn. He knew how far away Saturn is; and also knew that we see Saturn by the sunlight that it scatters back towards us. Given that the intensity of light from a source falls off as the inverse square of the distance, he could calculate how far away a star like our sun would have to be to look as bright by direct radiation as Saturn does by reflected light. His result, expressed in modern terms, was about ten lightyears, which is absolutely of the right order of magnitude.”

A.P. French, “”Isaac Newton, Explorer of the Real World,” pp. 50-77, in Stayer, Marcia Sweet, ed., Newton’s Dream. Montreal, CA: MQUP, 1988.

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