CHAPTER V 



THE EARTH A PLANET 



ioo. The Moon. So far we have looked on the 

 heavenly bodies as convenient marks blazoned on the hollow 

 dome of space around the spinning Earth. In 97 it was 

 implied, however, that the Moon at least was free to change 

 its position on the star-dome. The Moon appears to move 

 amongst the stars, from west to east, so fast that if we 

 observe it rising due east at the same moment as a star, it 

 will be seven times its own diameter behind the star on cross- 

 ing the meridian, and the star will have set about half an 

 hour before the Moon reaches the western horizon. The 

 Moon often passes between us and a star, and occasionally it 

 passes in front of the Sun, causing an eclipse. These facts 

 prove that the Moon revolves round the Earth from west to 

 east, and that it is the nearest of all the heavenly bodies. 

 The diameter of the Earth affords a sufficiently long base- 

 line ( 33) to measure the distance of the Moon accurately, 

 the vertical angle at the Moon of the triangle of which 

 the radius (or semi -diameter) of the Earth is the base 

 being 5 7'. This angle is called the horizontal parallax of the 

 Moon, and shows that the diameter of the Earth as seen from 

 the Moon would be i 54'. The parallax varies somewhat 

 during a month, showing that the distance of the Moon is 

 not always the same ; but from its average value the average 

 distance of the Moon is found to be 238,793 miles, or in 

 round numbers 240,000. The apparent, or angular, 

 diameter of the Full Moon as seen from the Earth is about 



