Why are there seven days in a week?
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Why are there seven days in a week? – Henry E., 8, Somerville, Massachusetts
Waiting until the weekend can often feel unbearable, six whole days between Saturdays. Having seven days in a week has been around for a very long time, so people don’t often stop to ask why.
Most of our time is due to the movements of the planets, the Moon and the stars. Our day is equivalent to one complete rotation of the Earth around its axis. Our year is a revolution of the Earth around the Sun, which lasts 365 and ¼ days, which is why we add an extra day in February every four years, for a leap year.
But week and month are a little trickier. The phases of the Moon do not exactly coincide with the solar calendar. The Moon cycle lasts 27 days and seven hours, and there are 13 phases of the Moon in each solar year.
Some of the earliest civilizations observed the cosmos and recorded the movements of the planets, the Sun and the Moon. The Babylonians, who lived in modern-day Iraq, were astute observers and interpreters of the sky, and it is largely thanks to them that our weeks last seven days.
The reason they adopted the number seven is because they observed seven celestial bodies: the Sun, the Moon, Mercury, Venus, Mars, Jupiter and Saturn. This figure therefore had a special meaning for them.
Other civilizations chose other numbers – like the Egyptians, whose week lasted 10 days; or the Romans, whose week lasted eight.
The Babylonians divided their lunar months into seven-day weeks, with the last day of the week having special religious significance. The 28-day month, or a full cycle of the Moon, is a bit too long a period to manage effectively, which is why the Babylonians divided their months into four equal parts of seven.
The number seven is not particularly suited to coincide with the solar year, or even the months, so it has created some inconsistencies.
However, the Babylonians represented such a dominant culture in the Near East, particularly in the 6th and 7th centuries BC, that this notion, along with many of their other notions of time – such as the 60 minutes – persisted.
The seven-day week spread throughout the Middle East. It was adopted by the Jews, who had been captives of the Babylonians at the height of that civilization’s power. Other cultures in surrounding areas adopted the seven-day week, including the Persian Empire and the Greeks.
Centuries later, when Alexander the Great began spreading Greek culture throughout the Near East to India, the concept of the seven-day week also spread. Researchers believe that India may have later introduced the seven-day week to China.
Eventually, once the Romans began to conquer territory influenced by Alexander the Great, they too eventually adopted the seven-day week. It was Emperor Constantine who decreed the seven-day week as the official Roman week and made Sunday a public holiday in 321 AD.
The weekend was only adopted in modern times, in the 20th century. Although there have been some recent attempts to change the seven-day week, it has been around for so long that it appears it is here to stay.
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This article has been updated to correct details of Earth’s revolution around the Sun.
This article is republished from The Conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you make sense of our complex world. It was written by: Kristin Heineman, Colorado State University
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Kristin Heineman does not work for, consult, own shares in, or receive funding from any company or organization that would benefit from this article, and has disclosed no relevant affiliations beyond her academic appointment.
