Kepler knew, for instance, that Mars’s distance from the sun is 1.53 times Earth’s distance, and Mars’s year is 1.88 times Earth’s year. He saw—somehow—that 1.53 × 1.53 × 1.53 = 1.88 × 1.88. The other planets all told the same story. (Put another way, the length of a planet’s year depends not on its distance from the sun, or on that distance squared, but on something in between—the distance raised to the 3/2 power.) But why? What did it mean?
