CHAPTER FOURTEEN 



MARS AND THE MINOR PLANETS 



MARS is sometimes brighter than any star or any 

 other planet except Venus and Jupiter. It comes next 

 after the earth, as we name the planets in the order of 

 their distance from the sun, and is at times comparatively 

 near to us. It is for this reason that it seems bright. 

 In size it is surpassed by all the major planets except 

 Mercury. Its diameter is 4200 miles, and its mean 

 distance from the sun 141,000,000 miles. It revolves 

 around the sun in 687 .days. 



Distance of Mars from the earth. When the sun and 

 Mars are in the same direction from the earth, the dis- 

 tance of Mars from the earth is equal to the sum of the 

 distances of the two planets from the sun (Fig. 139). 

 Mars is then 141 + 93 = 234 million miles away. When 

 Mars and the sun are in opposite directions from us, so 

 that one rises as the other sets, the distance of the planet 

 from the earth is 141 93 = 48 million miles. If the 

 orbits were exactly circular, Mars would never be nearer 

 to us than this, but Mars's orbit deviates from a true 

 circle more than do the orbits of most of the planets. 

 Once in about 17 years it comes within 36 million miles 

 of the earth. As the side turned toward us is then 

 fully illuminated, it may be viewed more satisfactorily 

 with a telescope than any other heavenly body except 

 the moon. Venus comes nearer to the earth, but at 

 such times always turns her face away from us. 



It will be seen that Mars's least distance from the 

 earth is about one seventh of its greatest distance. As 

 49 candles at a distance of 7 yards afford no more light 



241 



