78. Q. What is said of the day and night of Jupiter? A. The sun is always nearly over the equator of Jupiter, and every place has nearly its five hours day and five hours night. 79. Q. How do the seasons of Earth, Mars and Saturn compare? A. They are much alike, except in length. 80. Q. How long are Saturn’s seasons? A. Each is seven and a half years long. The alternate darkness and light at the poles is fifteen years long. 81. Q. In what form are the orbits of the planets? A. Not in the form of exact circles, but a little flattened into an ellipse, with the sun always in one of the foci. 82. Q. What is that point called where a planet is nearest the sun, and what where it is farthest from it? A. The point nearest the sun is called the perihelion, and the farthest point the aphelion. 83. Q. What is the plane of the ecliptic? A. It is the plane of the earth’s orbit extended to the stars. 84. Q. What is said of the densities, sizes, and relations of the collections of matter smaller than the planets, scattered through space in the solar system? A. They are of various densities, from a cloudlet of rarest gas to solid rock; of various sizes, from a grain’s weight to little worlds; of various relations to each other, from independent individuality to related streams millions of miles long. 85. Q. By what names are they known when they become visible? A. Shooting-stars, meteors, and comets. 86. Q. How far above the surface of the earth do shooting-stars appear and disappear? A. They appear about seventy-three miles above the earth, and disappear about twenty miles nearer the surface. 87. Q. What is their velocity? A. Their average velocity is thirty-five miles a second, and it sometimes rises to one hundred miles a second. 88. Q. What does Prof. Peirce state as the result of his investigation in regard to meteors? A. That the heat which the earth receives directly from meteors is the same in amount which it receives from the sun by radiation, and that the sun receives five-sixths of its heat from the meteors that fall upon it. 89. Q. When the bodies are large enough to bear the heat, and the unconsumed center comes to the earth, what are they called? A. Aerolites or air-stones. 90. Q. What is said of the distribution of these bodies through space? A. They are not evenly distributed through space. In some places they are gathered into systems which circle round the sun in orbits as certain as those of the planets. 91. Q. How many such systems of meteoric bodies has it been demonstrated that the earth encounters in a single year? A. More than one hundred. 92. Q. What are comets? A. They are clouds of gas or meteoric matter, or both, darting into the solar system from every side, at every plane of the ecliptic, becoming luminous with reflected light, passing the sun, and returning again to outer darkness. 93. Q. What appendage do comets usually have? A. A tail, which follows the comet to perihelion, and precedes it afterwards.