242 Trees, Stars, and Birds 



than i candle at i yard, so Mars, when nearest, would 

 give us 49 times as much light as when farthest away, 

 supposing that it were equally illuminated by the sun 

 in both cases. 



Mars will rise in the east at the same time that 

 the sun sets in the west, on May 10, 1920, and 

 July 4, 1922. On any clear night for weeks before 

 or after these dates, Mars may be seen as a bright red 

 star. 



Satellites of Mars. Mars has two satellites, Pho- 

 bos (Fear) and Deimos (Fright). These revolve around 

 Mars as the moon goes about the earth. They were 

 discovered in 1877 by Professor Hall, using the great 

 telescope of the Naval Observatory at Washington. 

 So small are they that their size cannot be measured, 

 but must be merely estimated from the amount of light 

 they give us. The smaller one, Deimos, is perhaps some- 

 thing like 10 miles in diameter. Seeing it at a distance 

 of 35 million miles is like seeing a baseball at a distance 

 of 200 miles, which is farther than from Washington to 

 New York. The larger satellite, Phobos, is about 3650 

 miles from the surface of Mars and completes a revolu- 

 tion about the planet in 7 hours and 39 minutes. As 

 Mars requires 24 hours for a rotation, this satellite 

 goes around it one and a half times before the planet 

 turns halfway around, so that the inhabitants of Mars, 

 if it has any, would see this little moon rise in the 

 west, cross the sky, set in the east, and a second time 

 rise in the west, all in the course of one night. -Al- 

 though a tiny object as compared with the rest of the 

 heavenly bodies we know, yet it is so close to Mars that 



