354 



PHYSIOGRAPHY. 



[chap. 



course of the year, when the earth is at opposite points in 

 its orbit, the days and nights are everywhere equal. These 

 periods are known as the eqiiitioxes?- One occurs in March, 

 and is therefore known as the spring or vernal equinox, and 

 the other in September, and is consequently called the 

 aiitufunai equinox. Again, there aie two other periods, also, 

 when the earth is at opposite points of its path, when the 

 inequality between the days and nights is greatest. These 

 periods are known as the solstices? 



In discussing the earth's revolution round the sun, it 

 should be mentioned that the orbit is not strictly a circle, 



PERIHELION 



APHELJOM 



Fig. 115. — Diagram to illustrate perihelion and aphelion. 



but is a curve of that kind which was described in the last 



chapter as an ellipse. In an ellipse (Fig. 115) the longer 



diameter, AB, is called the major axis, and the smaller 



diameter, CD, the minor axis. On the major axis, there are 



two points which have this property : — any two lines drawn 



from them to the same point of the curve, are always, 



when joined together, of equal length to the same straight 



line. These two points are known as the foci of the 



ellipse. In the great elliptical path of the earth, the sun 



occupies one of these foci, as at S. It is plain, there- 



' Equinox, from Lat. ctquiis, equal ; nox, night. 



^ Solstice, from Lat. sol, sun ; sisto, I stand ; because the sun appears 

 ta stand still in the heavens at these points in its path. 



