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Introduction to Astronomy


Planets and the Days of the Week


  • With the naked eye, seven objects can be observed to move relative to the stars: the Sun, the Moon, and the five planets Mercury, Venus, Mars, Jupiter, and Saturn.

  • Given the power of the Sun to heat the Earth, and the Moon to move the tides, it was natural to assume that the planets were also "godlike" in character, and from earliest times they have been associated with the deities of various cultures.

    (This was also the genesis of astrology, although in fact the the planets have little or no effect on the Earth.)

    Naming the planets after gods goes back at least as far as the cultures of Mesopotamia, but the tradition was continued by the Romans, using similar gods.

    English, a hybrid germanic-romance language, uses the same names for the planets as the Romans.

  • Seeing significance in the number of these visible, moving objects, the ancient Babylonians chose to organize their calendar around a seven-day week.

    The names of the days were taken from these objects/gods, and the Romans followed suit.

    The Germanic tribes of northern Europe also adopted the seven-day week, and substituted their own gods in the names of the days.

    English continued to use the Germanic names for days, thereby disconnecting them, for the most part, from the planetary names:
    • Sunday: The Sun's Day => Sol's Day

      Sol was the Roman god of the Sun.
    • Monday: The Moon's Day => Luna's Day

      Luna was the Roman goddess of the Moon.
    • Tuesday: Tiu's Day => Mars' Day

      Tiu was the Germanic god of war, corresponding to the Roman god Mars.

      This is an appropriate choice given the planet Mars' blood reddish color.
    • Wednesday: Woden's Day => Mercury's Day

      Woden or Odin is sometimes described as the supreme Germanic god, which sounds like the Roman god Jupiter.

      However, early Germanic tribes associated Woden with the Roman god Mercury.

      Mercury was the swift messenger of the gods, an appropriate characteristic for the fastest moving planet.

      Similarly, Woden daily sent out his two ravens to gather information about the world, and was also the god of writing.

      Probably more important, Woden and Mercury were both bearers of the dead to the afterlife.
    • Thursday: Thor's Day => Jove's Day

      Thor was the Germanic god of thunder and lightning, and since Jupiter was known to throw a few thunderbolts himself, this seems an understandable correspondence.

      Jupiter, the supreme god, seems an appropriate choice for this planet, given its brightness and the relative longevity of its appearances (and as we now know, its huge size).
    • Friday: Freya's Day => Venus' Day

      Freya was the Germanic goddess of love and beauty, corresponding closely to the Roman goddess Venus.

      The planet Venus can be the brightest object in the sky besides the Sun and Moon, quite beautiful and hard to miss when it is far from the Sun, so this seems an understandable association.
    • Saturday: Saturn's Day

      Saturn was the Roman god of agriculture and of time; he was the father of Jupiter, so his association with the farthest visible planet seems appropriate.

      The Germanic tribes did not appear to have a similar god themselves, and they may have adopted him as their own.

  • If you know Spanish, the day names closely match the Latin (except for Sunday, Domingo, "the Lord's Day"): Lunes, Martes, Miercoles, Jueves, Viernes, Sabado.

The star chart background was produced on a Macintosh with the Voyager II program, and are ©1988-93 Carina Software, 830 Williams St., San Leandro, CA 94577, (510) 352-7328. Used under license.

©2001 Scott R. Anderson
Last update: 2001 June 5
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