Tropical year

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A tropical year (also known as a solar year), for general purposes, 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. Since antiquity, astronomers have progressively refined the definition of the tropical year, and currently define it as the time required for the mean 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). (Meeus & Savoie, 1992, p. 40)




The word "tropical" comes from the Greek tropikos meaning "turn". (tropic, 1992) Thus, the tropics of Cancer and Capricorn mark the extreme north and south latitudes where the Sun can appear directly overhead, and where it appears to "turn" in its annual seasonal motion. Because of this connection between the tropics and the seasonal cycle of the apparent position of the Sun, the word "tropical" also lent its name to the "tropical year". The earliest Chinese, Hindus, Greeks, and others made approximate measures of the tropical year; early astronomers did so by noting the time required between the appearance of the Sun in one of the tropics to the next appearance in the same tropic. (Meeus & Savoie, 1992, p. 40)

Early value, precession discovery

In the 2nd century BC Hipparchus introduced a new definition which was still used by some authors in the 20th century, the time required for the Sun to travel from an equinox to the same equinox again. He measured the length of the year to be 365 solar days, 5 hours, 55 minutes, 12 seconds. A modern computer model gives 365 solar days, 5 hours 49 minutes 19 seconds. He adopted the new definition because the instrument he used, the meridian armillae, was better able to detect the more rapid motion in declination at the time of the equinoxes, compared to the solstices. (Meeus & Savoie, 1992, p. 40)

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