216 PHYSIOGRAPHY 



the sun is vertical at the equator, days and nights are equal every- 

 where, and when the sun is vertical south of the equator, days are 

 longer than nights in the southern hemisphere, and the sun's rays 

 are more nearly vertical there than in the northern hemisphere. 

 The northernmost parallel where the sun's rays are ever vertical 

 is called the tropic of Cancer. The corresponding southernmost 

 parallel is the tropic of Capricorn. The tropics are nearly 23^ 

 (23 27'+) from the equator, because the axis of the earth is in- 

 clined by that amount to the plane of its orbit The sun is vertical 

 at the tropic of Cancer at the time of the summer solstice, and at 

 the tropic of Capricorn at the time of the winter solstice. The 

 parallels just touched by the circle of illumination at the time of 

 the solstices are the polar circles. They are as far from the poles 

 as the tropics are from the equator. They are therefore in lati- 

 tude about 66]/2 . The one in north latitude is the Arctic circle, 

 and the one in south latitude is the Antarctic circle. 



The Solar System and the Stars 



The solar system includes the sun and all the bodies which 

 revolve about it. There are eight planets, of which the earth is 

 one. To us, all the planets except our own appear as stars, but 

 in their motions they behave differently from the other stars. 

 Named in the order of their distance from the sun, commencing 

 with the nearest, the planets are: Mercury, Venus, Earth, Mars, 

 Jupiter, Saturn, Uranus, and Neptune. Most of the planets have 

 satellites corresponding to our moon. 



Besides the planets and their satellites, the solar system in- 

 cludes numerous (more than 400) asteroids, bodies much smaller 

 than the planets, intermediate in position between Mars and 

 Jupiter, and those comets which revolve about the sun. These 

 bodies have little influence on the earth, and nothing further need 

 be said of them in this place. 



The stars, comets, etc. Beyond the solar system there are 

 thousands of stars, each of which may be compared to our sun. 

 We do not know, however, that they have planets circling about 

 them. There are also some comets which do not belong to our 

 solar svstem. 



