Names and Order of the Days of the Week
The names of the days of the week in the Julian calendar are Sunday, the Sun's day, Monday, or Moon's day, Tuesday or Tiw's day, Wednesday or Woden's day, Thursday or Thor's day, Friday or Frie's day and Saturday or Saturn's day. At first sight this seems a strange mixture of Sun, Moon and Saturn, clearly of astronomical significance, and some other less familiar names. It makes more sense when we note that Tiw is the Norse god which corresponds to the Roman god Mars, Woden is the Norse god of war corresponding to the Roman Mercury, and Frie is the Norse god of love, similar to the Roman god Venus. And Thor is Jupiter. Now we have Sun's day, Moon's day, Mercury's day, Mars's day, Jupiter's day, Venus's day and Saturn's day. Clearly the days of the week are named for the five planets which are easily visible with the naked eye (not including the Earth, which would not have been considered as a planet by ancient people) and the Sun and Moon.

But why the particular order. The first day of the week is named for the Sun, the brightest object in the sky; the second day for the Moon the second brightest object; the third day is named for the dimmest of all the planets, Mercury. Jupiter is the next brightest object and yet it is the fifth day. The days are apparently not ordered by the brigheness of the astronomical objects they celebrate.

To some ancient observers the slower the object the potentially the more powerful the object. Fast moving objects might have been interpreted as flighty and less serious, slow were ponderous and powerful. Let us consider the speeds at which the Sun, Moon and five visible planets move across the sky against the background of stars. Naturally we will consider their motion as seen from the Earth. The Sun takes a year to pass completely around the celestial sphere, 365 days. The Moon moves along almost the same path, the ecliptic, in just over 27 days. Mercury is always quite close to the Sun and darts 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 back and forth in its dance about the Sun. Because it does move ahead of the Sun at times and then drops behind the Sun, it is clearly moving faster than the Sun, but slower than Mercury. 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. Thus in order of slowest, a most powerful, to the fastest, we have Saturn, Jupiter, Mars, Sun, Venus, Mercury, and the Moon. Well, that is still not the order of the days of the week. But we are on the right track.

The ancient Babylonians ordered the Sun, Moon and planets in the order of slowest to fastest as we have listed them above. They divided each day into twelve hours and each night into twelve hours for a total of twenty four hours a complete day. (Footnote; more correctly, apparently the ancient Egyptian contemporaries of the ancient Bablylonians divided the day and night into 24 hours). Each hour was assigned a celestial god. The first hour for example would be Saturn, the second Jupiter, the third Mars etc. assigned in the order of slowest object to fastest. Because the first hour was assigned to Saturn the day would be known as Saturn's day. Saturn was the guardian of the first hour of the day and was honored to have the day named after him. The assignment of gods continued and when they reached the eighth hour, having exhausted the list of seven gods, they started the list from the top and assigned the eighth hour to Saturn, ninth to Jupiter, etc. The twenty fourth hour of the first day would be assigned to Mars and thus the first hour of the next day would be assigned to the Sun and that would be the Sun's day. Continuing in this way the first hour of the next day would fall to the Moon, Moon's day. The following days would then be Mars's day, Mercury's day, Jupiter's day, Venus's day, and finally back to Saturn's day. Thus, with a little help from the Norse names and some very old ordering due to Babylonian astrologers, we have Saturday, Sunday, Monday, Tuesday, Wednesday, Thursday, Friday, and Saturday again.
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