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Why are there 7 days in a WEEK ?

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The length of Day and Year are based on Earth's movement around Sun. Similarly months are are made according to Moon's phases. But what made us put seven days in a week ? It could have been of 5 or 10 days. Why did we make a week of exactly 7 days ? The answer to this question is not as simple as others. There are a number of facts that are responsible for having 7 days in a week. Some of them are present in our religions, which are strongly affected by celestial movements of Earth, Sun, Moon and Planets. Once you read these facts, you will feel that you have learned something new.

Why are there 7 days in a week ?

  • Babylonians, which were present even before Greece or Rome, found 7 celestial objects in the sky, which were close to us and they distinguished they are not just like any other star in sky. They observed these objects without any telescope. These objects were Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. Due to these 7 celestial objects, Babylonians used seven-day week.
  • Calendars of ancient China and Japan also used seven-day week due to the same 7 celestial or heavenly objects.
  • A lunar month usually consists of 30 days (one full-moon to another), which can be further divided into two sections for increasing phase and decreasing phase of 15 days each. 15 days must have been considered lengthy for a week, so they might have divided into two parts and made a week of seven days. Due to this fact Babylonians used to add intercalary days in the last week of month, to match it with new moon.
  • Babylonian week was adopted by Jewish but moon-dependency was removed. Babylonian calendar and week system was adopted by Greeks in 4th Century BC.
  • In Judaism, creation story took seven days to complete and hence 7 is an important number there.
  • Roman calendar adopted the same 7 day week based on prior beliefs, as these 7 heavenly bodies (which are actually part of our solar system) were considered as Gods looking over us.
  • Same seven-day week was adopted by Persian Empire, Hellenistic astrology, Gupta India and Tang China.
  • Roman Emperor Constantine, who was a Christian convert, adopted the seven day week and declared Sunday as the first day. Saturday, which is the Jewish day of rest, was taken as the last day. So since year 321, the same trend is going on.
If you take a close look at week days name, you will understand that they are connected to objects of our solar system.
  • Sunday - Sun
  • Monday - Moon
  • Tuesday - Mars
  • Wednesday - Mercury
  • Thursday - Jupiter
  • Friday - Venus
  • Saturday - Saturn
If week days are named after planets, then why are they not in correct order ?
We may think that they are not in correct order, but thousands of years ago when these planets were discovered by ancient civilizations, they were not able to calculate their distance from Sun. At that time they didn't even know that these celestial objects are moving around Sun except moon. Those people use to think that Earth is the center on universe and every thing in revolving around it. Coming back to the point, the order of days ! Those ancient people must have ordered them according to these planets, but not by distance, but either by their size observable from Earth or their brightness in the night sky. However the astrological concept of planetary hours of Hellenistic astrology is also considered as reason behind the present pattern of days.

There were some calendars and week systems where number of days in a week were not 7.
  • 13-day week was used by the Aztecs and Mayas in the Mesoamerican calendars.
  • 10-day week, called décade was used in France in the 18th Century, but that trend lasted only for 9 and half years.
  • 8-day week was used in ancient rome.
  • 6-day week is used in Akan Calendar.
  • 5-day week were used in  Icelandic calendar, the Javanese calendar, and the traditional cycle of market days in Korea.
A lot of information about week system is available on internet, but the best extract of that information is provided here. If you feel that any important point is missed here them you may write it in comments section.

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