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 CHAPTER V THE DISCOVERY OF URANUS

The third paper sent by Herschel to the Royal Society was in the form of a letter to Dr. Watson from Mr. William Herschel of Bath, dated October 18, 1780. It was a record of observations made in the three years from 1777 to 1779, with the view of determining whether our day is of the same length year after year. A point so difficult could be settled, he thought, only by observing the length of the day in other planets. This had been done, or attempted, for Venus and Jupiter, by watching the time it took for a spot on the face of the planet to return to the same position. But in Venus, on account of her exceeding brilliance, it had been done so imperfectly that her day was put down roughly as of 23 hours' length. For Jupiter the time of rotation on his axis was set down more precisely at 9 hours 56 minutes, a result arrived at by keeping careful watch on spots that may not be fixed points on his disc, but movable on what we may call trade-wind belts of clouds in his equator. These spots "change so often that it is not easy, if at all possible, to ascertain the identity of the same appearance for any considerable length of time." Sometimes a bright, at other times a dark spot, or belt, was observed, but the time of its revolution round the