[67] Earth’s diameter, and even Earth’s distance from the sun, is too small a unit. Light, travelling 186,000 miles a second, takes 4¼ years to reach us from the nearest star.
[68] It is impossible to measure the diameter of any star, even with the help of the most powerful telescopes, but in the case of a double star at a known distance the movement of the components as they travel round their common centre of gravity enables us to determine the gravitational force they exercise on each other, and thus their combined mass; and their spectra give some idea of their density. For instance, the mass of the double star Alpha Centauri is nearly twice that of our sun; and as the components appear to be about equal to each other, and both show a spectrum resembling that of the sun, we may conclude that Alpha Centauri consists of two stars, each of which has about the same diameter as our sun. Arcturus has a diameter far greater, some say ten times, some not less than twenty-five times as great as the sun!
[69] The mean length of Earth’s shadow (which varies a little with her distance from the sun,) is 857,000 miles, or 216 times her semi-diameter.
[70] It was also translated from Arabic into Latin at Toledo, in 1175.
[71] Paradise Lost, II. 418
[72] Paradise Lost, VIII. 34-38.
[73] Paradise Lost, VIII. 77-84.
[74] All quotations from Salimbene’s Chronicle are taken from Coulton’s From Saint Francis to Dante.
[75] Croniche Fiorentine, Bk. VII. par. 80.
[76] Inf. xx. 118.