Why SSMTWTF?

There are a lot of things in life we take for granted and hardly think about its origin or reason. We rarely think that at some stage of history obvious looking things were a point of research or debate. Sequence of days in a week is one such thing about which I never thought. Why it should be Monday, Tuesday, Wednesday and so on? Why not any other sequence? Few days back I was reading a book about Indian astronomy and found the answer.

There are five visible planets in the sky - Saturn, Jupiter, Mars, Venus and Mercury, and in older days Sun and Moon were also categorised as planets. So there were total 7 visible planets. (Navgrah concept in Indian astrology includes Rahu and Ketu, which are not physical objects but imaginary points, intersection points of Moon's orbit with the plane of Earth's revolution around Sun.)

Now arrange all the 7 planets in decreasing order of period (time taken to make a complete circle with reference to stars as seen from the earth)- Saturn, Jupiter, Mars, Sun, Venus, Mercury and Moon.

Each day has 24 hours. Assume that each hour has one ruler planet. Suppose that today's first hour ruler planet is Saturn, second hour ruler will be Jupiter, Third will be Mars, fourth will be Sun and so on. First hour ruler decides the day. Hence today is Saturday.

Repeating the above mentioned sequence we see that 22nd hour ruler is again Saturn, 23rd is Jupiter, 24th is Mars and 25th will be Sun. Now 25th hour is 1st hour of tomorrow, so it is Sunday tomorrow. Similarly tomorrow's 22nd hour is ruled by Sun, 23rd by Mercury, 24th by Venus and 25th by Moon. So day after tomorrow is Monday.

Wow.... its simply amazing to learn the way sequence of weekday has been setup.