THE SUN'S MOTION. 



497 



2 1 st of December, at midnight, the sun is at s, from which he arrives again 

 in the space of half a year at s' ; so we are able to represent this annual 

 motion of the sun by a circle, the diameter of which is the line, s' s. This 

 circle is called the Ecliptic. 



The plane of the ecliptic, S'JT, cuts the plane of the equinoctial, A Q, at an 

 angle of 23 J, and the axis of the ecliptic, s"V, makes the same angle with 

 the axis of the heavens, P P. The two parallel circles, s's' and SJ, include 

 a zone extending to both sides of the equinoctial, and beyond which the sun 

 never passes. These circles are called the Tropics, from rpeira), I turn, because 

 the sun turns back at these points, and again approaches the equinoctial. 

 The parallel circles, S''/, and S'"/", described by the poles of the ecliptic, 

 s'V, about the celestial poles, P P, are called the arctic and antarctic circles. 



Whenever the sun crosses the equinoctial, there is the equinox ; but the 

 points of intersection are not invariably the same every year. There is a 



Fig. 546. The Seasons. 



gradual westerly movement, so it is a little behind its former crossing place 

 every year. (See diagram, fig. 547.) 



[This is the " Precession of the Equinoxes," because the time of the 

 equinoxes is hastened, but it is really a retrograde movement. Hipparchus 

 discovered this motion, which amounts to about fifty seconds in a year. 

 So the whole revolution will be completed in about 28,000 years.] 



It is obvious, then, that the sun is the most 

 important star in the universe ; and when we come 

 ^ to speak about the earth we shall consider the seasons, 

 ^^ etc., more fully. Now we must endeavour to explain 



1 what the sun is like, and this can only be done with 

 specially darkened glasses, for a look at the sun through 

 an ordinary telescope may result in great, if not per- 

 manent, injury to the eye. 



The sun is not solid so far as we can tell. It is a 

 mass of "white-hot" vapour, and is enabled to shine by reason of its own 

 light, which the planets and stars cannot do ; they shine only by the sun's 



32 



1 



Fig. 547. Precession of 

 Equinoxes. 



