pole b from the centre of the earth o, then the actual surface of the earth extends at the equator beyond this circle as far as a, where, according to Newton, a a is about 1230 of o b or o a, and according to modern estimates, based on actual measurement of the earth as well as upon theory (chapter x., § 221), it is about 1293 of o a. Both Newton's fraction and the modern one are so small that the resulting flattening cannot be made sensible in a figure; in fig. 72
the length a a is made, for the sake of distinctness, nearly 30 times as great as it should be.
Newton discovered also in a similar way the flattening of Jupiter, which, owing to its more rapid rotation, is considerably more flattened than the earth; this was also detected telescopically by Domenico Cassini four years after the publication of the Principia.
188. The discovery of the form of the earth led to an explanation of the precession of the equinoxes, a phenomenon which had been discovered 1,800 years before
