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A week is a time unit equal to seven days. It is the standard time period used for cycles of work days and rest days in most parts of the world, mostly alongside (but not strictly part of) the Gregorian calendar. The days of the week were named in different languages after classical planets, various deities (example: Thursday – Thor's day, a variation after Jupiter's day from Roman times)〔(【引用サイトリンク】title=Online Etymology Dictionary )〕 and heavenly bodies (example: Sunday – Sun's day) and other sources.〔(【引用サイトリンク】title=Online Etymology Dictionary )〕 In English, the names are Monday, Tuesday, Wednesday, Thursday, Friday, Saturday and Sunday. ISO 8601 includes the ISO week date system, a numbering system for weeks within a given year – each week begins on a Monday and is associated with the year that contains that week's Thursday (so that if a year starts in a long weekend Friday–Sunday, week number one of the year will start after that). The term "week" is sometimes expanded to refer to other time units comprising a few days, such as the nundinal cycle of the ancient Roman calendar. == Definition and duration == A week is defined as an interval of exactly seven days, so that technically, except at daylight saving time transitions or leap seconds, : 1 week = 7 days = 168 hours = 10,080 minutes = 604,800 seconds. With respect to the Gregorian calendar: * 1 Gregorian calendar year = 52 weeks + 1 day (2 days in a leap year) * 1 week = ≈ 22.9984% of an average Gregorian month In a Gregorian mean year, there are 365.2425 days, and thus exactly or 52.1775 weeks (unlike the Julian year of 365.25 days or ≈ 52.1786 weeks, which cannot be represented by a finite decimal expansion). There are exactly 20,871 weeks in 400 Gregorian years, so 30 April 1611 was a Saturday just like 30 April 2011. A system of Dominical letters has been used to determine the day of week in the Gregorian or the Julian calendar. Relative to the path of the Moon, a week is 23.659% of an average lunation, or 94.637% of an average quarter lunation. In antiquity, days were measured either from sunset to sunset, or from sunrise to sunrise, so that the length of the week (and the day) would be subject to slight variations depending on the time of year and the observer's geographical latitude. Adding one to the remainder after dividing by seven a date's Julian day number (JD ''modulo'' 7 + 1) yields that date's ISO 8601 day of the week,〔Richards, E. G. (2013). "Calendars". In S. E. Urban & P. K. Seidelmann, eds. ''Explanatory Supplement to the Astronomical Almanac'', 3rd ed. (pp. 585–624). Mill Valley, Calif.: University Science Books. 2013, pp. 592, 618. This is equivalent to saying that JD0, i.e. 1 January 4713 BC of the proleptic Julian calendar, was a Monday.〕 For example, the Julian day number of is . Calculating ( mod 7 + 1) yields , corresponding to . 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「Week」の詳細全文を読む スポンサード リンク
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