CHAP, vi The Solar System and Universe 



the Moon, those of Venus being clearly visible by the aid 

 of an opera-glass. Signor Schiaparelli has recently proved 

 that the period of rotation of Mercury is equal to its period 

 of revolution round the Sun ; and this is probably true of 

 Venus also. Solar tidal friction has evidently acted on 

 these planets as the tidai- friction of the Earth has acted 

 ) 



THE PLANETS. 



on the Moon ; and it is interesting that the two planets 

 nearest to the Sun, and receiving enormously more heat 

 and light than the Earth, have perpetual day in one 

 hemisphere, and perpetual night with a cold approaching 

 the absolute zero in the other. Mercury and Venus 

 occasionally pass between us and the Sun, the planet 

 appearing to pass across the solar disc lilce a small black 

 spot. A -transit of Venus affords, the best opportunity of 

 measuring the solar parallax, and hence the Sun's distance, 

 by noticing how far the path of the planet across the disc 

 is altered when viewed from distant parts of the Earth. 



128. Mars, the first planet beyond the Earth, most 

 resembles it. The rotation period is nearly the same, and 

 the surface is diversified by marks which evidently indicate 

 continents and seas, while at each pole a gleaming white 

 patch increases and decreases as the planet wheels round 



