A tropical year is the length of time that the Sun, takes to return to the same position along the ecliptic. The precise length of time depends on which point of the ecliptic one chooses. If it starts from the vernal equinox, then it yields the vernal equinox year; averaging over all starting points on the ecliptic yields the mean tropical year.
On Earth, we notice the progress of the tropical year from the slow motion of the Sun from south to north and back. The tropics of Cancer and Capricorn mark the extreme north and south latitudes the Sun reaches during this cycle.
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 days or 3.1557 x 107 seconds.
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.
A Modified Tropical Year is the time span between the Sun's apparent passage of the Prime Meridian on the days of the Vernal Equinoxes.
In the Northern Hemisphere, the lowest elevation of the Sun occurs at the Winter Solstice. The highest elevation occurs at the Summer Solstice. The time when the elevation crosses the equator in its travel toward the North or Summer Solstice is the Vernal Equinox. The time it crosses the equator in its travel toward the South or Winter Solstice is the Autumnal Equinox.
The time span between successive Vernal Equinoxes changes every year. A Mars Tropical Year can be derived from the relative position of the Sun with Respect to Mars. The Mars Modified Tropical Year can be derived by first finding the Vernal Equinox and then finding the time of Prime Meridian passage closest to that time.