Page:A short history of astronomy(1898).djvu/98

54 The annual motion ♈︎ ♈︎' was, as has been stated, estimated by Hipparchus as being at least 36" (equivalent to one degree in a century), and probably more. Its true value is considerably more, namely about 50".

An important consequence of the motion of the equator thus discovered is that the sun in its annual journey round the ecliptic, after starting from the equinoctial point, returns to the new position of the equinoctial point a little before returning to its original position with respect to the stars, and the successive equinoxes occur slightly earlier than they

Otherwise would. From this fact is derived the name precession of the equinoxes, or more shortly, precession, which is applied to the motion that we have been considering. Hence it becomes necessary to recognise, as Hipparchus did, two different kinds of year, the tropical year or period required by the sun to return to the same position with respect to the equinoctial points, and the sidereal year or period of return to the same position with respect to the stars. If ♈︎ ♈︎' denote the motion of the equinoctial point during a tropical year, then the sun after starting from the