CHAPTER THIRTEEN 



MERCURY AND VENUS 



MERCURY and Venus are the only planets that are 

 nearer to the sun than the earth is. They never appear 

 very far from the sun. Sometimes one or the other 

 of them passes directly between us and the great orb of 

 day. It then appears as a dark speck on the brilliant 

 surface of the sun. 



Mercury. Since Mercury is always so near the sun 

 and is not bright enough to be seen in the daytime, it is 

 visible only low in the west a short time after sunset 

 or in the east just before sunrise. When the sun is not 

 far below the horizon, its light is still so strong as to pre- 

 vent our seeing the stars. A little later the brightest 

 stars come into view. If Mercury is still well above 

 the horizon and the sky is very clear, this planet may 

 then be plainly seen where the sun was an hour or more 

 before. The astronomer Copernicus, who lived near 

 the Baltic Sea, never saw Mercury because the mist near 

 his horizon always shut out so faint a light. In America 

 we have clearer skies, and if you look for Mercury at 

 about the time when it is farthest from the sun you will 

 probably find it. If any bright stars happen to be low in 

 the west where you are looking for Mercury, you can tell 

 them from the planet by noticing that they twinkle more. 



While Mercury never appears to be far from the sun, 

 its actual distance ranges from 28,500,000 to 43,500,000 

 miles. As these figures show, its orbit is not much like 

 a circle. Of all the planets, Mercury has the swiftest 

 motion in its orbit 23 to 35 miles a second ; the slower 

 motion is when it is farthest from the sun. It is the 



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