THE EARTH'S NEAREST NEIGHBORS 37 



seen because of its cloud-like atmosphere. This atmosphere 

 reflects a great deal of the sun's light, and consequently 

 Venus is the brightest planet. 



28. Mars. Mars has been much studied and some as- 

 tronomers think that light and dark markings upon its surface 

 indicate the presence of land and water like the earth's; 

 that the land is desert at some seasons and covered with 

 vegetation at others; and that around the poles there are 

 snow and ice which change in amount with the seasons. 

 The seasons differ very much as ours do. Mars receives less 

 than half as much light and heat as the earth, because of its 

 greater distance from the sun, so that if there is life on Mars, 

 it cannot be the same kind of life as on the earth. Mars has 

 two moons or satellites, so small that they cannot be seen 

 except through the largest telescopes. 



29. Jupiter. Jupiter is the giant planet, and if it were 

 as near to us as the moon is, it would seem forty times as 

 wide as the moon does. Jupiter has four moons as large as 

 our moon, and five very small ones. Galileo, an Italian 

 astronomer, discovered the four larger ones in 1610, the 

 first time he looked at Jupiter with a telescope. They 

 were the first heavenly bodies discovered. The five 

 smaller satellites have been discovered, one at a time, 

 since 1892. Four of them were discovered by means of 

 photography. Heavenly bodies whose 'light is too faint to 

 make an impression upon the eye, even through a telescope, 

 can be photographed with a telescope-camera. By exposing 

 the plate a long time sometimes for many hours it is 

 possible to get an impression from the faint light of these 

 distant bodies. 



30. Saturn. Saturn, as seen through the telescope, is 

 a wonderful sight, because of the bright, thin, flat rings which 

 revolve about the planet without touching it. When Saturn 

 is situated so as to show the broad side of the rings (as it is 

 once in fifteen years), it is much brighter than when we 



