YEAR, a unit of time intended to equal the period of revolution of the earth around the sun. prior to 4000 B. C. the Egyptians established the length of the year as 365 days by counting the days that elapsed between two successive heliacal risings of the star Sirius; that is, two successive appearances of Sirius just before sunrise, each occurring after period of invisibility caused by its nearness to the sun. Astronomers distinguish several kinds of year, of which THREE—the tropical, sidereal, and anomalistic—are most common.

年，相等于地球围绕太阳公转周期的一个时间单位。公元前4000年以前，埃及人通过计算天狼星连续两次的偕日升确立了一年长度为365天。就是说，在日出之前，天狼星连续两次的出现，因它离太阳很近，每次都在看不见的一段时间之后出现。天文学家们区分了几种年份，其中有三个—回归年，恒星年和近点年—最为常见。

The tropical year, also called solar year or equinoctial year, is the year to which the Gregorian calendar is adjusted. It is the interval between two successive returns of the sun to the vernal equinox and is approximately equal to 365.24220 mean solar days, or 365 days 5 hours 48 minutes 46 seconds. Since 1956 the tropical year has been the interval in terms of which the second, the fundamental unit of time in the several systems of units employed in physics and engineering, is defined. In that year the International Committee on Weights and Measures redefined the second as 1/31,556,925.9747 of the tropical year for Dec. 31, 1899 (called January 0, 1900, in astronomical literature) at 12 hours ephemeris time. It was necessary to use a specific date because the length of the year is not precisely constant: the tropical year is slowly decreasing in length, while the sidereal and anomalistic years are slowly increasing. The rate of change, however, is less than 1 second per century.

回归年，也称太阳年或分至年，是公历调整后的年。它是在太阳连续两次出现到春分之间的间隔，大致相等365.2420平均太阳日，或365天5小时48分46秒。自1956年以来，回归年一直是定义秒的时间间隔，是用于物理学和工程学的几种单位系统中的时间基本单位。在那一年，国际度量衡委员会以12小时星历时重新将秒定义为1899年12月31日回归年的1/31,556,925.9747（在天文学文献中称为1900的1月0日）。使用一个特定的日期是必要的，因为一年的长度并不是精确不变的：回归年的长度正在慢慢减少，而恒星年和分至年正在慢慢地增加。然而，每个世纪的变化率不到1秒。

The sidereal year is the average period of revolution of the earth with respect to the fixed stars; its duration is 365.25636 mean solar days. The light-year, a unit used to EXPRESS stellar distances, is the distance light travels in 1 sidereal year; it is equal to 5.879 X 10 Miles. The anomalistic year is the average interval between successive closest approaches of the earth to the sun; its length is 365.25964 mean solar days.

恒星年是地球相对于恒星公转的平均周期；它的持续时间是365.25636个平均太阳日。光年，一个用于表示恒星距离的单位，是光在1恒星年中移动的距离；它等于5.879 X 10¹²英里。近点年是地球连续最接近太阳的平均间隔。它的长度是365.25964平均太阳日。

The calendar year for the Gregorian calendar consists of 365 days (366 in leap years) and is divided into 12 months. It begins on January 1 and ends on December 31. Since the length of the tropical year is nearly 365 1/4 days, it is necessary once every 4 years (with certain exceptions; see LEAP YEAR) to add a day to the calendar year. This day, which falls on February 29, is known as leap day.

公历的日历年由365天组成（在闰年为366天），并被分为12个月。它开始于1月1日，结束于12月31日。由于回归年的长度差不多是365天1/4天，每4年对日历增加1天是必要的（除了某些例外；见闰年）。发生在2月29日的这一天被称为闰日。

See also CALNDAR—The Measurement of Time; CHRONOLOGY; DAY; SIDERAL TIME; TIME—Measurement and Determination of Time.

也可参阅日历—时间的测量；年代学；日；恒星时间；时间---时间的测量与确定。

                                           2024年3月7月译

（译者注：该词条位列《大美百科全书》1985年版，第29卷，第257页）