The tropical year is the interval of time between one vernal equinox and the next and is a standard unit of time used by astronomers, physicists, and navigators. It is equivalent to approximately 365.24 mean solar day s or 3.1557 x 10 7 second s.
As the Earth revolves around the Sun, the Sun appears to move northward in latitude for six months from about December 21 to June 21, and southward in latitude for six months from about June 21 to December 21. On March 20 or 21, the Sun is precisely over the Earth's geographic equator, moving northward. This moment is called the vernal equinox.