Page:Astronomy for Everybody.djvu/67

It follows from all this that, since the celestial equator is the circle midway between the two poles, there must be a corresponding shift in its position among the stars. The effect of this shift during the past two thousand years is shown in Figure 9. Since the equinoxes are the points of crossing of the ecliptic and the equator, they also change in consequence of this motion. It is thus that the precession of the equinoxes arises.

The two kinds of year we have described are called equinoctial and sidereal. The equinoctial year, also called the solar year, is the interval between two returns of the sun to the equinox. Its length is—

Since the seasons depend upon the sun's being north or south of the equator, the solar or equinoctial year is that used in the reckoning of time. The ancient astronomers found that its length was about three hundred and sixty-five and one quarter days. As far back as the time of Ptolemy the length of the year was known even more exactly than this, and found to be a few minutes less than three hundred and sixty-five and one quarter days. The Gregorian Calendar, which nearly all civi-