60 THE WORLD. 



readily, must be caused by a motion of our earth, for it will be 

 noticed, that the inclination of the ecliptic to the equator remains 

 unchanged. 



Let ABC, represent the ecliptic, and D B E, the celestial 

 equator, intersecting each other in two opposite points, one of 

 which is shown at B. Let P P' be the poles of the earth, 9(P 

 distant from the equator F V G, in every direction, and let the 

 star S, in the direction P' P, be the pole of the heavens, every 

 where 90 distant from the celestial equator, D B E, let the point 

 T, be the pole of the ecliptic ABC. We must be careful and 

 not consider the lines F G, H I, marked on the earth as equator 

 and ecliptic, to be fixed, because this would cause the nodes, or 

 equinoctial points, to revolve, apparently, once in a day, through 

 the heavens, but we may suppose them hoops or bands, sta- 

 tionary, while the earth turns around in them. For a moment 

 suppose the diurnal revolution of the earth to be stopped, and let 

 the position of the intersections of the planes of the celestial ecliptic 

 and equator, meet on the earth at V, and let the poles, of the 

 ecliptic H V I thus marked on the earth, be O and R, a spectator 

 at the centre of the earth, would locate the equinoctial point among 

 the stars at B. If, now, the earth should be turned a little, not on 

 its diurnal or equatorial axis P P', but on its ecliptical axis O R, 

 in the direction of the letters C B A, the equinox would appear to 

 shift in the heavens to the star X, and the pole of the heavens S, 

 would appear to have moved partly around the pole of the ecliptic 

 S, and be at Z, This is the fact, whilst the earth is moving around 

 the sun, and all the time turning daily on its equatorial axis, it is 

 making a slow backward revolution around its ecliptical axis, and 

 as the stars are fixed, the equinoctial point continually retrogrades 

 along the ecliptic, thus causing the pole of the heavens continually 

 to shift its place, revolving in a circle whose radius is T S, which 

 is the angular inclination of the axis P P' to the axis O R, or of 

 the plane of the ecliptic, to the plane of the equator. The early 

 astronomers, located the places of the equinoxes in the heavens, 

 and gave the name Aries to the constellation where the vernal, 

 or spring equinox, was located, and the name Libra to the con- 

 stellation where the autumnal equinox was located. Since that 



