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an interval of time approximating the duration of one revolution of the earth around the sun.
Since ancient times the determination of the length of a year by observing recurring astronomic phenomena has been an important scientific problem having great practical significance. Ancient China and ancient Egypt both had quite accurate measurements of the year. The Greek scholar Hip-parchus in the second century B.C. calculated the length of a year as exactly 365.25 days minus 1/300 of a day, which varies from contemporary measurements by only 6½ minutes.
The complexity of the movement of the earth around the sun accounts for the various values of the year that are used in different fields of science and everyday life. The stellar (sidereal) year is the interval of time in which the sun completes its apparent annual path in the heavens with respect to the stars. The tropical year is the interval of time between two successive transits of the sun through the point of the vernal equinox. It corresponds to the period in which the earth experiences the change of seasons—spring, summer, autumn, and winter. The anomalistic year, the interval between two successive transits of the sun through the perihelion, is used in celestial mechanics. The Draconic year is the interval in which the sun returns to the same (ascending or descending) nodal point on the lunar orbit. It is used for predicting solar and lunar eclipses. The Julian year is the basis for the Julian calendar (old style); now it is sometimes used in astronomy for computing large intervals of time (the Julian century is 36,525 days). The Gregorian year serves as the basic unit in the Gregorian calendar (new style). The lunar year equal to 12 (or 13) synodical months, is used in lunar calendars. Table 1 shows the duration of a year in each system.
|Table 1. Systems of years|
|Duration in ephemeris days (for 1950)||100 years’ deviation (in days)|
|Sidereal year...............||365.256360||+0.11 x 10−4|
|Tropical year...............||365.242196||-6.16 x 10−4|
|Anomalistic year...............||365.259641||+3.04 x 10−4|
|Draconic year...............||346.620047||+32 x 10−4|
|Lunar year(in 12 months)...............||354.3670||-2.4 x 10−4|
|Gregorian year(average duration)...............||365.2425||—|
N. P. ERPYLEV