Science Fair Project Encyclopedia
A week is a unit of time longer than a day and shorter than a month. In most modern calendars, including the Gregorian Calendar, the week is a period of seven days, making it the longest conventionally used time unit that contains a fixed number of days. Although having no direct astronomical basis, it is widely used as a unit of time.
Weeks can be thought of as forming an independent continuous calendar running in parallel with various other calendars. However, some calendars have been designed so that a given date occurs on the same day of the week each year. This can be done by making the week dependent on the year, with some days each year that do not belong to any week: the proposed World calendar has 52 weeks and 1 or 2 extra days each year, while the French Revolutionary Calendar had 36 weeks of 10 days and 5 or 6 extra days. The year can also be made dependent on the week: the former Icelandic calendar had years of 52 or 53 weeks.
- The article days of the week covers in detail the order and names of the days of the week.
Origin of the seven-day week
The ancient Babylonians are known to have observed a seven-day week; each day dedicated to a different deity. The significance of seven comes from Babylonian astronomy. There are the seven heavenly bodies or luminaries normally visible to the naked eye (the Sun, Moon, and 5 visible planets), and they associated each with a deity. The biblical account of the creation also includes a seven-day week; according to which God laboured for six days and rested on the seventh, and the Ten Commandments which contained God's instruction to observe the week. This Islamic Calendar includes a seven-day week as well.
Others speculate that the fixed 7-day period is a simplification of 1/4 of a lunar month.
Later use of the week
Various groups of citizens of the Roman Empire adopted the week, especially those who had spent time in the eastern parts of the empire, such as Egypt, where the 7-day week was in use. Contemporaneously, Christians, following the biblical instruction, spread the week's use along with their religion.
As the early Christians evolved from being Jewish to being a distinct group, various groups evolved from celebrating both the Jewish Sabbath (Saturday) and the first day or the Lord's Day (Sunday), to celebrating only Sunday. See: Sabbath (Christian); Shabbat (Jewish).
In the early 4th century, the Roman Emperor Constantine regulated the use of the week due to a problem of the myriad uses of various days for religious observance, and established Sunday as the day for religious observance and rest for all groups, not just those Christians and others who were already observing Sunday.
The Jews retained their (at least) 800-year-old tradition of Saturday observance. Later, after the establishment of Islam, Friday became that religion's day of observance -- however the Islamic week still begins on Sunday and ends on Saturday, just like the Jewish-Christian week.
The 7-day week soon became a practice among Christians, Jews, and Muslims. Following European colonization and the subsequent rise of global corporate business, the 7-day week has become universal in keeping time, even in cultures that did not practise it before.
Facts and figures
- 1 week = 7 days
- 1 week = 168 hours = 10,080 minutes = 604,800 seconds (except at daylight saving time transitions or leap seconds)
- 1 year = 52 weeks + 1 day (2 days in a leap year)
- 1 week = 23% of an average month (almost exactly)
In a Gregorian mean year there are exactly 365.2425 days, and thus exactly 52.1775 weeks (unlike the Julian year of 365.25 days, which does not divide evenly into weeks). There are exactly 20871 weeks in 400 Gregorian years, so 25 December 1601 was a Tuesday just like 25 December 2001.
ISO 8601 includes a numbering system for weeks; each week is associated with the year in which Thursday occurs. Thus, for example, week 1 of 2004 (2004W01) ran from Monday 29 December 2003 (2003-12-29), to Sunday, 4 January 2004 (2004-01-04). The highest week number in a year may be 52 or 53. This style of numbering is commonly used (for example, by businesses) in some European countries, but rare elsewhere.
- Falk, Michael (1999). "Astronomical Names for the Days of the Week", Journal of the Royal Astronomical Society of Canada , Vol. 93, p.122. 1999JRASC..93..122F. 
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