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The base unit of time in the International System of Units (SI), and by extension most of the Western world, is the second, defined as about 9 thousand million periods of radiation of the caesium atom. Historically units of time were defined by the movements of astronomical objects. * Sun based: the year was the time for the earth to rotate around the sun. Year-based units include the olympiad (four years), the lustrum (five years), the indiction (15 years), the decade, the century, and the millennium. * Moon based: the month was based on the moon's orbital period around the earth. * Earth based: the time it took for the earth to rotate on its own axis, as observed on a sundial. Units originally derived from this base include the week at seven days, and the fortnight at 14 days. Subdivisions of the day include the hour (1/24th of a day) which was further subdivided into seconds and minutes. * Celestial sphere based: as in sidereal time, where the apparent movement of the stars and constellations across the sky is used to calculate the length of a year. These units do not have a consistent relationship with each other and require intercalation. For example, the year cannot be divided into 12 28-day months since 12 times 28 is 336, well short of 365. The lunar month (as defined by the moon's rotation) is not 28 days but 28.3 days. The year, defined in the Gregorian calendar as 365.25 days has to be adjusted with leap days and leap seconds. Consequently, these units are now all defined as multiples of seconds. Units of time based on orders of magnitude of the second include the nanosecond and the millisecond. ==Historical== (詳細はday, the solar year and the lunation. Such calendars include the Sumerian, Egyptian, Chinese, Babylonian, ancient Athenian, Hindu, Islamic, Icelandic, and Mayan, and French Republican calendars. The modern calendar has its origins in the Roman calendar, which evolved into the Julian calendar, and then the Gregorian. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「unit of time」の詳細全文を読む スポンサード リンク
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