54 IN STARRY REALMS. 



turned towards us, and in about another fortnight we shall 

 have the original hemisphere restored. Each side of the 

 sun is thus equally accessible to our observations ; there is 

 no region withheld from our scrutiny as is the case in the 

 moon. 



Similar language may be used with regard to the 

 attitude of the planets towards the earth. Mars is so much 

 farther from us than the moon that we cannot scrutinize 

 his ruddy globe so minutely as we are able to do that of 

 our own satellite ; but Mars turns round on his axis once 

 every twenty-four hours and a half, so that we are able to 

 form maps of his entire surface, and whatever incomplete- 

 ness those maps may possess arises from such causes as the 

 imperfection of our instruments or the distance of Mars ; 

 for we cannot assert that the movements of the planet are 

 such as to prevent our examination from being extended 

 over the whole of his globe. The giant orb of Jupiter 

 revolves with rapidity on its axis. Suppose an astronomer 

 looks at the great planet at eight o'clock in the evening, 

 observes the belts and other configurations of its surface, 

 and turns to other work until about one o'clock in the 

 morning ; if he now directs his telescope again to Jupiter 

 the panorama is entirely changed. The belts and marks 

 which he saw in the earlier observations have vanished, they 

 have in fact retired to the opposite side of the globe, while 

 all the objects that he sees in his second observation were 

 on the distant side of the globe at the time of the first 

 observation and were consequently invisible. Should his 

 ardour be equal to that of some famous astronomers, as, 

 for instance, to that of Sir William Herschel, whose 

 hours of observing were from dusk to dawn, our observer 



