18 MERCURY. [Lesson iv. 



The diamtter of this planet is nearly one-third 

 of the diameter of the eanh, or about 3000 miles. 

 Hence the surface of Mercury is nearly 1-gth, 

 and his magnitude or bulk l-27th of that of the 

 earth. 



His period of revolution round the sun is 87 

 days, 23 hours, and l-4th : hence you may find 

 that he moves in his orbit about the sun at the 

 amazing rate of more than 95,OOO miles in an 

 hour. His length of day, or time of rotation on his 

 axis, inclination of axis to his orbit, &cc. are yet 

 unknown. 



Mercury changes his phases in a manner simi- 

 lar to the moon, according as he is differently 

 stationed, with regard to the earth and sun : though 

 we may observe that he never appears quite full, 

 because his bright side is only turned directly to- 

 wards us, when he is so near the sun as to be lost 

 from our sight in his beams. These different 

 phases of his make it obvious that he does not 

 shine by any light of his own 5 for if he did, he 

 would always appear round. 



As the orbit of this planet is between the earth's 

 orbit and the sun, if it were in the same plane 

 as the orbit of the earth, Mercury would fre- 

 quently be seen to move across the face or diac 

 of the sun. But as the planes of their orbits are 

 not coincident, this appearance happens less com- 

 monly ; it is denominated by astronomers, a 

 transit of Mercury over the sun's disc, the planet 

 then appearing like a black spot on the face of the 

 sun. The last transit of Mercury happened in the 



year 



