The line of the equinoxes is the imaginary diameter of the celestial sphere which joins them.
It was considered by Biot to have been originally twenty-four, but to have been enlarged to twenty-eight about i ioo B.C., by the addition of determinants for the solstices and equinoxes of that period.
In Europe the frequency near the equinoxes rises above that at midwinter, and we have a distinct double period, with a principal minimum at midsummer and a secondary minimum at midwinter.
In 1749 he furnished a method of applying his principles to the motion of any body of a given figure; and in 1754 he solved the problem of the precession of the equinoxes, determined its quantity and explained the phenomenon of the nutation of the earth's axis.
Owing to the precession of the equinoxes it is longer than a tropical or sidereal year by 25 minutes and 2.3 seconds.