v The Earth a Planet 75 



is at its nearest, its disc appears larger than that of the 

 Moon at its farthest ; and if an eclipse occurs in such con- 

 ditions it is said to be annular, the black disc of the Moon 

 being surrounded by a narrow bright ring of the Sun, like 

 a penny lying on a half-crown. 



1 1 4. Solar Tides. The differential attraction of the Sun 

 on the opposite sides of the Earth has a tide-raising power 

 like that of the Moon ( 103). But the Sun is so distant that 

 in spite of its vast mass the difference in its attracting power 

 on opposite sides of the Earth, due to the distance of 8000 

 miles, is only two-fifths as great as the difference in the 

 attracting power, of the nearer Moon. At New Moon and 

 at Full Moon the tide-raising power of Sun and Moon is 

 exerted in the same direction, and produces Spring-tides in 

 the ocean ; the tidal wave rises highest and sinks lowest 

 or has its greatest amplitude. At the quarters, on the 

 other hand, the Sun is raising high water where the 

 Moon is producing low water, and consequently the ampli- 

 tude is much less, the tidal wave not rising to the average 

 height nor sinking to the average depth. These are called 

 neap-tides, and represent the difference, as spring-tides 

 represent the sum, of the tide-raising power of Sun and 

 Moon, the relative amplitudes being as 3 to 7. 



115. Precession of the Equinoxes. The tropical year 

 or apparent time of the Sun's circuit of the heavens is 20 

 minutes shorter than the Earth's revolution period ( no); 

 in other words, if the Sun starts from that point of the 

 ecliptic known as the vernal equinox it will reach it again 

 20 minutes before completing the annual circuit of the 

 heavens. Thus the equinox seems to be moving slowly along 

 the ecliptic to meet the Sun, and so every year it precedes 

 or comes before its former position, the phenomenon being 

 known as the precession of the equinoxes. The star-dome, 

 not sharing the movement, appears to rotate about an axis 

 at right angles to the plane of the ecliptic, but so slowly that 

 2 5,000 years are required for a single turn. Consequently the 

 constellations on the zodiac have ceased to correspond with 

 the " signs " of 30 each which formerly included them. This 

 apparent movement of the heavens must be produced by a 



