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( Solar year)
A tropical year (also known as a solar year) is the length of time that the Sun takes to return to the same position in the cycle of seasons, as seen from Earth; for example, the time from vernal equinox to vernal equinox, or from summer solstice to summer solstice.[1] A tropical year can equivalently be defined as the time taken for the Sun's tropical longitude (longitudinal position along the ecliptic relative to its position at the vernal equinox) to increase by 360 degrees (that is, to complete one full seasonal circuit). For the reasons explained below, the length of a tropical year varies slightly, by up to a minute or two, depending on the seasonal starting point. The tropical year measured between (northern) vernal equinoxes (one of the four cardinal points along the ecliptic), is called the vernal equinox tropical year, or just vernal equinox year. The mean tropical year is calculated by averaging the (slightly differing) tropical years over all possible starting points through the four seasons. When used without qualification, the term "tropical year" often refers to the mean tropical year. Because of a phenomenon known as the precession of the equinoxes, the tropical year, which is based on the seasonal cycle, is slightly shorter than the sidereal year, which is the time it takes for the Sun to return to the same apparent position relative to the backdrop of stars. This difference was 20.400 minutes in AD 1900 and 20.409 minutes in AD 2000. Because it is desirable for everyday-use calendars to keep in synchronisation with the seasons, it is the tropical year that, in principle, these calendars track. Although the yearly differences are small, they are cumulative, and after many years amount to a very noticeable discrepancy.
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