90 .SEVENTEENTH CENTURY. FT. in. 



examine the surface of that satellite. He saw the mountains 

 of the moon, and the deep hollows buried in darkness, and 

 the wide plains which he mistook for oceans. Then he 

 noticed that curious light called the secondary light, which 

 may be seen on the dark side of the moon when only one 

 quarter of it is bright and shining. Galileo discovered that 

 this curious light is a reflection from the earth ; for you must 

 know that we reflect the sun's light back to the moon just in 

 the same way as the moon does back to us, and at the time 

 when we see a new moon, the man in the moon (if there 

 were such a person) would see a large full earth and could 

 wander about at night by earth-light as we do by moonlight. 

 Look up at the new moon just about dusk in the evening, 

 and if it is a clear night you will most likely be able to see 

 a faint outline of the dark side of the moon, which is 

 caused by our earth-light shining upon it. 



Jupiter's Moons. When Galileo had studied the moon 

 and gazed with intense delight on the myriads of tiny stars 

 in the Milky Way, he next turned his telescope to the planet 

 Jupiter. To his great surprise he saw three small shining 

 bodies like stars close to Jupiter which were quite invisible to 

 the naked eye. Two of them were on the east side of the 

 planet and the other on the west. He waited eagerly for the 

 second night, to see if Jupiter would move away from these 

 stars, but he found them still together, only the two stars 

 which had been on the east side had now moved round to 

 the west, and they were nearer to each other than they had 

 been before. He was quite puzzled as to how this could 

 have happened, and watched and watched for many nights 

 whenever the clouds would allow him ; and at last, on the 

 fourth night after he had first seen them, he came to the 

 conclusion that all three stars were moving round and round 



