The French astronomer, M. Lucien Rudaux, finds the following variation in the light of the satellites of Saturn:-- Japetus from 9th magnitude to 12th Rhea " 9 " 10·6 Dione " 9·5 " 10·5 Tethys " 9·8 " 10·5 Titan " 8 " 8·6 The variation of light is, he thinks, due to the fact that the period of rotation of each satellite is equal to that of their revolution round the planet; as in the case of our moon.[173] The names of the satellites of Saturn are derived from the ancient heathen mythology. They are given in order of distance from the planet, the nearest being Mimas and the farthest Themis. 1. Mimas was a Trojan born at the same time as Paris. 2. Enceladus was son of Tartarus and Ge. 3. Tethys was wife of Oceanus, god of ocean currents. She became mother of all the chief rivers in the universe, as also the Oceanides or sea nymphs. 4. Dione was one of the wives of Zeus. 5. Rhea was a daughter of Uranus. She married Saturn, and became the mother of Vesta, Ceres, Juno, and Pluto. 6. Titan was the eldest son of Uranus. 7. Hyperion was the god of day, and the father of sun and moon. 8. Japetus was the fifth son of Uranus, and father of Atlas and Prometheus.[174] 9. Phœbe was daughter of Uranus and Ge. 10. Themis was daughter of Uranus and Ge, and, therefore, sister of Phœbe. In a review of Prof. Comstock's _Text Book of Astronomy_ in _The Observatory_, November, 1901, the remark occurs, "We are astonished to see that Mr. Comstock alludes with apparent seriousness to the _nine_ satellites of Saturn. As regards the ninth satellite, we thought that all astronomers held with Mrs. Betsy Prig on the subject of this astronomical Mrs. Harris." This reads curiously now (1909) when the existence of the ninth satellite (Phœbe) has been fully confirmed, and a tenth satellite discovered. CHAPTER X Uranus and Neptune From observations of Uranus made in 1896, M. Leo Brenner concluded that the planet rotates on its axis in about 8½ hours (probably 8{h} 27{m}). This is a short period, but considering the short periods of Jupiter and Saturn there seems to be nothing improbable about it. Prof. Barnard finds that the two inner satellites of Uranus are difficult objects even with the great 36-inch telescope of the Lick Observatory! They have, however, been photographed at Cambridge (U.S.A.) with a 13-inch lens, although they are "among the most difficult objects known."[175]