Saturn has ten satellites, all revolving outside the rings. The names of nine of these in the order of increasing distance are: Mimas, distance 117,000 miles; Enceladus, distance 157,000 miles; Tethys, distance 186,000 miles; Dione, distance 238,000 miles; Rhea, distance 332,000 miles; Titan, distance 771,000 miles; Hyperion, distance 934,000 miles; Japetus, distance 2,225,000 miles; and Phœbe, distance 8,000,000 miles. The last, like the eighth satellite of Jupiter, revolves in a retrograde direction. Only Titan and Japetus are conspicuous objects. The period of Mimas is only about 22½ hours; that of Titan is 15 days, 22 hours, 41 min., and that of Japetus about 79 days, 8 hours. Barnard's measurements indicate for Titan a diameter of 2720 miles. Japetus is probably about two-thirds as great in diameter as Titan. [Illustration: =Photographs of Mars= Made at the Yerkes Observatory by E. E. Barnard, with the forty-inch refractor, September 28, 1909. ] Beyond Saturn, in the order named, are Uranus and Neptune. The mean distance of the former from the sun is 1,782,000,000 miles, and that of the latter 2,791,500,000 miles. The orbit of Uranus is more eccentric than that of Neptune. The diameter of Uranus is about 32,000 miles and that of Neptune about 35,000 miles. The year of Uranus is equal to 84 of our years, and that of Neptune to 164.78. These planets are so remote, and so poorly illuminated by the sun, that the telescope reveals very little detail on their surfaces. Their density is somewhat less than that of Jupiter. Uranus has four satellites, Ariel, Umbriel, Titania, and Oberon, situated at the respective distances of 120,000, 167,000, 273,000, and 365,000 miles. Neptune has one, nameless, satellite, at a distance of 225,000 miles. The most remarkable thing about these two planets is that their axes of rotation, as compared with those of all the other planets, are tipped over into a different plane, so that they rotate in a retrograde or backward direction, and their satellites, in like manner, revolve from east to west. The axis of Uranus is not far from upright to the plane of the ecliptic, so that the motion of its satellites carries them alternately far northward and far southward of that plane, but the axis of Neptune is tipped so far over that the retrograde, or east to west, motion is very pronounced. Neptune is celebrated for having been discovered by means of mathematical calculations, based on its disturbing attraction on Uranus. These calculations showed where it ought to be at a certain time, and when telescopes were pointed at the indicated spot the planet was found. Similar disturbances of the motions of Neptune lead some astronomers to think that there is another, yet undiscovered, planet still more distant.