74 The Realm of Nature CHAP* 



well-known twelve " signs of the zodiac." The plane of the 

 ecliptic in space serves as a standard level, to which other 

 directions may be referred for comparison. It seems most 

 natural that the Earth's axis should be perpendicular to the 

 plane of the ecliptic, but, as has been said, this is not the case. 

 The axis is inclined about 23 J from the perpendicular. We 

 have thus to picture the Earth sailing round the Sun, not " on 

 even keel" but with a list or inclination of 23^, and with 

 the north end of the axis always pointing toward the same 

 bright star on the celestial dome. This inclination is not 

 absolutely constant, but like the eccentricity of the orbit is 

 subject to slight increase and diminution in long periods. 



113. Eclipses. Instead of saying that the Earth re- 

 volves round the Sun we should, in order to be accurate, 

 say that " the Earth-Moon System " does so ; for the Moon 

 shares the annual revolution of the Earth as a point on the 

 tire of a wheel shares the onward movement of the centre. 

 If the Moon's orbit lay in the plane of the ecliptic, the Moon 

 would pass between the Earth and Sun once every month, 

 and a fortnight later the Earth would cut off the sunlight 

 from the Moon. In other words, at every New Moon there 

 would be an eclipse of the Sun, at every Full Moon there 

 would be an eclipse of the Moon. But the Moon's orbit is 

 inclined at an angle of about 5 to the ecliptic, and it is only 

 when the Moon happens to be at one of the nodes, or points 

 on the orbit where its plane intersects the ecliptic, that an 

 eclipse can take place. From this fact the ecliptic gained its 

 name. Eclipses of the Moon are common occurrences, for 

 they happen several times in a year and are visible from a 

 large area of the Earth's surface, as the Earth's shadow is 

 wide compared with the angular diameter of the Moon. 

 Eclipses of the Sun are more frequent, but are more seldom 

 seen at a given place, being visible only for a comparatively 

 short time and over a limited tract of the Earth's surface, 

 since the Moon's shadow thrown by the Sun is a com- 

 paratively narrow cone. When the Moon is at its nearest 

 point to the Earth, in the course of its elliptical orbit, its 

 angular diameter is great enough to entirely conceal the 

 Sun, and the eclipse is said to be total. But when the Sun 



