出典:Wikipedia
出典:『Wikipedia』 (2011/06/14 18:28 UTC 版)
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)