12 PHYSICAL GEOGRAPHY. It is difficult to obtain clear conceptions of distances that are represented by millions of miles. We may learn the numbers, but in general they convey no definite ideas. Should a man travel forty times around the earth at the equator, he would only have gone over about 1,000,000 miles. Now, Mercury, the nearest of the planets, is thirty- six times farther from the sun than the entire distance the man would have travelled, while Neptune is nearly three thousand times the distance he would have travelled. 9. The Satellites—A satellite is a body that revolves around another body: the planets are satellites of the sun; the moon is a satellite of the earth. Mars has two moons. So far as is known, neither Mercury nor Venus has a satel- lite. All the planets whose orbits are beyond the orbit of the earth have moons: Jupiter has five, Uranus six, Saturn eight, and Neptune one. Be- sides its moons, Saturn has a number of curious ring-like accumulations of separate solid or liquid particles revolving around it. The earth’s moon is about 240,000 miles from the earth. Its vol- ume is about one-forty-ninth that of the earth’s. 10. The Sun is the great central body of the solar system. Around it move the planets with their satellites, receiving their light and heat from it. The sun is a huge heated mass about 1,300,000 times the size of the earth. Its diam- eter is about 866,500 miles. It appears the largest self-luminous body in the heavens because it is comparatively near the earth. Many stars which appear as mere dots of light are much larger than the sun. The sun is a body heated to luminosity, and gives out or emits light and heat like any other highly-heated body. If no causes exist to maintain its heat, it will eventu- ally cool and fail to emit light. The sun’s heat is partly kept up by a variety of causes, the principal of which is the heat developed by meteoric showers that fall on its surface. If a meteor fall toward the sun from inter- planetary space, it will reach the surface with enormous velocity, and its motion will there be converted into heat. Since, however, the increase of the sun’s mass so necessitated’ is not confirmed by astronomical observa- tions, itis believed that the sun’s heat is not being main- tained in this way, and that the sun must eventually cool —an event, however, so remote in time that the life of the solar system may be regarded as practically infinite. Size of the Sun.—Were the sun hollow and the earth placed at its centre, there would not only be sufficient room to enable the moon to revolve at its present actual distance around the earth, but it would still, in all parts of its orbit, be nearly 200,000 miles below the surface of the sun. All the fixed stars are distant suns, and probably have worlds like our own moving around them. From the enormous distances of the fixed stars, we are obliged, in estimating their distances, to use for our unit of measurement the velocity of light. Any other common unit would be too small. Light moves through space at the rate of about 186,000 miles a second, which is over 11,000,000 miles a minute. Notwithstanding this prodig- ious velocity, it would take over three thousand years for light to reach the earth from some of the stars that are visible to the naked eye. But beyond these stars the tele- scope reveals myriads of others, whose number is limited only by the power of the instrument. We may conclude that the universe is as boundless as space; that is, light can never reach its extreme limits. 11. Cause of the Harth’s Revolution.—The earth continues its motion through space solely on account of its inertia. Its curved path around the sun is a resultant caused by the constant action of two forces: one, a pro- jectile force probably imparted to it when it began its separate existence; the other, the sun’s attraction, which causes the earth to fall toward the sun. Under the infiu- ence of the projectile force alone the earth would, in a given time, move from a to d (Fig. 3); but during this time Fig. 3, Cause of the Curved Shape of the Barth's Orbit, it has been continually changing its direction by an amount equivalent to a direct fall from 6 to ¢ along bd; hence its real orbit, during this time, is along the curved line ac. 12. Position of the Solar System in Space— The sun, with all the bodies which move around it, is in that portion of the heavens called the Milky Way. The sun is an insignificant star among the millions of other stars the telescope has revealed to us. It was formerly believed that the sun was stationary, for it was not then known that the positions of the fixed stars were undergoing slight variations as regards the earth. It is now generally conceded that the sun, with all the planets, is moving through space with tremendous veloc- ity, the direction at present being toward the constella- tion Hercules. The astronomer Maedler, however, believes that the grand centre around which the solar system is moving is Aleyone, the brightest star in the constellation of the Pleiades. The estimated velocity of the sun in its immense orbit is 1,382,000,000 miles per year. As the earth is carried along with the sun in its orbit, it is continually entering new realms of space. 13. The Earth.—The-shape of the earth is that of a round ball or sphere slightly flattened at two opposite sides. Such a body is termed a spheroid. There are two kinds of spheroids—oblate and pro- late; the former has the shape of an orange, the latter that of a lemon.