Days of the Week: an Astronomical Explanation

Have you ever thought of why do we have seven days in a week? Why do we have them in a particular order? After reading many resources and research papers, I have found something interesting – it has a long-lost connection from astronomy.

In this article, you will know about –

  1. Why the week has seven days?
  2. Why is the particular order of days?  
  3. Why so many cultures have the same names for days?

In visual astronomy, which we humans have practiced until the invention of the telescope, we can see five planets, the sun, and the moon. Objects in the sky were mainly divided into two categories – constellations/asterisms and planets (including sun and moon). Unlike stars, planets describe odd paths in the sky. They travel relatively faster than stars and go backward in the sky from time to time – the retrograde motion of planets (planetai ~ Greek for wanderers). Sun, Moon, Mercury, Venus, Mars, Jupiter, and Saturn are visible to naked eyes.

In the case of stars, no relative motion can be detected. So it is fairly logical to believe that the stars are fixed in the background and nothing was interesting in their appearances or motion. What interesting was, the motion of the planets, sun, and moon. So ancient astronomers (whoever first developed the week system) thought of them as gods/something that influences the life of people on earth – astronomy and astrology were the same fields for a long time.

This is the reason why the week has 7 days. Earth was not included in the week system as it was not considered as a planet till then.

In Hindu tradition, names are pretty straight forward – they are named directly on the sun, moon, and 5 planets. In the Julian calendar, however, Sun, Moon, and Saturn are named as such. For Tuesday, Wednesday, Thursday, and Friday it is Tiw’s day, Oden’s day, Thor’s day, and Frie’s day. They all are just Norse gods. Roman names for planets and gods were the same. They believed planets to be gods. While Indians used names of planets – following the astronomical way. The Julian calendar names are mythological.

Bracelet representing the days of the week. Source: Wikipedia

But why this particular order? The most logical way should be to order them in decreasing brightness like Sun, Moon, then it should be Jupiter (brightest planet) but we have Mars. So, brightness is not the way. Others should be Size (apparent size), Sun, Moon, Jupiter – again we have Mars.

Now, you must have heard of Babylonia, an ancient state and cultural area based in central-southern Mesopotamia (present-day Iraq and Syria). A small Amorite-ruled state emerged in 1894 BC, which contained the minor administrative town of Babylon. It greatly expanded during the reign of Hammurabi in the first half of the 18th century BC and became a major capital city. During the same period in India, there was Indus Valley Civilization which was later followed by the Vedic Period.

Babylonian empire (1800-500 BC). Source: Wikipedia

There are many remarkable similarities between ancient India and Babylonia. One possibility is that the diffusion of religious ideas occurred during the period of contact between South India, the Indus Valley, and Sumer. One important thing here to note is both cultures co-existed during the same time, so there were cultural influences both ways.

Astronomy developed by Babylonians and Indians can be found in Vedāṅga Jyotiṣa, compiled in the Vedic Period. Astronomy and astrology were commonly called Jyotish in India.

Indus Valley (3300-1300 BC). Source: Wikipedia

Babylonians and Indians believed that the slower the planet is moving more powerful it is (Astrologically). They have accurately calculated the orbital period of all planets, moon, and sun. We get the orbital period of the Sun as 365 days and the Moon 27 days. Mercury is always very close to the Sun and wanders from one side of the Sun to the other completing a full cycle around the Sun in 116 days. Venus can wander a bit farther from the Sun and takes about 584 days to cycle the Sun. Mars takes about 780 days to move around the celestial sphere, longer than the 365 days that the Sun takes. Jupiter takes longer than Mars and Saturn longer than Jupiter. Here you can see that Venus and Mercury were considered orbiting around the Sun. Orbital periods of outer planets, Sun, and moon were calculated against background stars.

These calculations were later mentioned in detail by 5th-century Indian Astronomer Aryabhata in his work – Aryabhatiya.

Now, planets in the order of increasing speed are – Saturn (the slowest), Jupiter, Mars, Sun, Venus, Mercury, and the Moon. Still not correct order. Babylonians divided days into 24 hours (like Indians) and they assigned each hour to a God in order of increasing speed. Day for Babylonians started at sunrise and Indians followed the same until Aryabhata, who changed that to midnight based on more precise astronomical calculations and rules.

If we pick a random day and assign the first hour (at sunrise) to the slowest, i.e. Saturn. Then the following hours would be for – Jupiter, Mars, etc. assigned in the order of slowest object to fastest. Since the chosen day has the first hour assigned to Saturn, it is known as Saturn’s day – Saturday. After the seventh hour, they repeated from the slowest planet again. Thus, the first hour of the next day will go for Sun (do the calculations yourself) – that will be Sun’s day, Sunday. The next day will be Monday and so on.

Thus, we get the days of the week as – Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday. Later this new system from Babylonia reached Europe in the coming years and from India, it reached east – China and Japan.


Published by Anand Krishna

Amateur astronomer and astrophotographer. Interested in astrophoto processing, astrostatistics, comet hunting, visual and radio astronomy.

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