... Equinox - a moment of the Sun's centre passing through the celestial equator in its visible moving along the celestial sphere. ... The equinox occurs on the 20-21 of March (vernal) and on the 23d of September (autumnal). ...
... They were counted from the point of vernal equinox. But the time is flying away. ... Earth's axis makes the point of vernal equinox move along the ecliptic to the West ... This is called the preliminary of vernal equinox. ...
... 2) a tropical year is a time interval between the Sun's two centre successive passing through the medium point of the vernal equinox (365,2422 days); . ...