OD THE WORLD. 



considerably lower than in the first case. The dotted line POP' 

 will here represent the meridian, which, it must be remembered, 

 is not a fixed direction in space, but simply a plane, extending 

 from the earth to the heavens, and passing through the spectator, 

 wherever he may be, and the poles of the earth. When the sun, 

 after moving through one fourth of his orbit, arrives at the point 

 where the equator and ecliptic cross each other, and which is 

 called the equinoctial point, the days and nights are equal all over 

 the world, and the sun is vertical at noon, at the equator. His 

 apparent diurnal circle will now be the equator E E'. The sun, 

 still moving on in its orbit, finally arrives at S' its greatest southern 

 limit, describing the diurnal circle S' C' at the time of the winter 

 solstice ; after which it again moves northward, rising higher, 

 and higher, each day, until after a tropical year, it arrives at the 

 point S, where we commenced. Now if the points S and S', 

 were fixed points in the heavens, the length of a tropical, or equi- 

 noctial year, would be the same as the length of a siderial year, 

 for the equinoctial points are fixed with regard to the tropical 

 points. It is, for many reasons, more convenient to reckon this 

 year from equinox to equinox, and hence this is generally termed 

 the equinoctial year. 



Let A B C D, represent the sun's path, inclined 23 28' to the 

 equator E D F B, and suppose B, the position of the vernal equi- 



