12 THE SKY. 



the south pole during the other half. There is therefore at the 

 poles but one day and one night in the year the day being 

 summer and the night winter. In less northern climates, 

 as England, this exists to a much smaller extent ; for half 

 the year the sun is longer above than below the horizon, 



FIG-. 9. 



constituting our summer, the other half of the year the sun 

 is longer below the horizon than above it, and this is winter. 

 Hence, in each case, midway between, the sun is twelve 

 hours above and twelve below the horizon, which occurs in 

 spring and autumn, being called the "equinoxes" (equal 

 nights) ; the vernal or spring equinox takes place on the 

 21st of March, the autumnal on the 21st of September. 

 The times just between these, when the sun is the longest 

 and shortest time below the horizon, are called the summer 

 and winter solstices, and occur on the 21st of December 

 and the 21st of June. 



The earth has a smaller sphere or " satellite," circulating 

 round it, this is the Moon; it is placed at a distance of 

 about 238,000 miles from the earth, is about one-fiftieth of 

 its bulk or volume, and revolves round it every 27 days, 

 keeping always the same side towards it, so that the other 

 side has never been seen. This is effected by rotation on 

 its axis, which takes place once for every circuit round the 

 earth ; if it did not rotate, or kept the same side always to 

 the same point of the heavens, then, when it had half-way 

 revolved, the other side would be turned towards the earth. 

 There are good reasons for believing that the moon has no 

 atmosphere or air around it, for the concave edge (when 

 only a small portion of the half illuminated by the sun is 



