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What's Your Sign?

Bad Astronomy: The zodiac contains 12 constellations.

Good astronomy: By any reasonable definition, the zodiac contains between 13 and 24 constellations.

What's going on:
The creepy lounge lizard walks up to you, shirt unbuttoned down to his navel, gold chains dangling, and asks, 'What's your sign?'

Calmly, you take a sip of your drink, and say, 'Ophiuchus, the serpent bearer. I guess that's appropriate right now.'

The zodiac is defined by the plane of the Earth's orbit around the Sun as it is projected on the sky. This projection is a line that circles the sky and is called the ecliptic. No matter how you slice it, there are more than 12 constellations that cross that line. Perhaps I should be clearer here: there are more than 12 constellations, although in astrological terms there are only 12 signs to the zodiac. As Bad Reader Paul Schylter pointed out to me, the signs originated from the constellations, but are not the same thing. The signs are composed of 12 equal subdivisions of the sky, each 30 degrees in extent. The constellations are not laid out in such an orderly fashion.

Like the Moon Illusion, there is a web site that outlines this problem pretty well. The Real Constellations of the Zodiac is once again brought to you by the people at The Planetarian. The author is Dr. Lee T. Shapiro, Director of the Morehead Planetarium at the University of North Carolina.

A couple of things not noted at that site: if Ophiuchus occupies more of the ecliptic than Scorpius, why is it not in the astrological zodiac instead of Scorpius? Most likely, it's because Ophiuchus is not a particularly bright constellation, while Scorpius is a very bright, vivid group of stars that really does look like a scorpion. Astrologers probably chose it simply because it is easier to see, making it easier to impress the locals. [Note: (added July 21 1998) I have been informed by my friend and colleague James Kaler, a noted teacher of astronomy and author of the excellent textbook "Astronomy!", that I am guilty of a little Bad Astronomy here myself, and as regular readers know, I confess when I err. Ancient astronomers did not place Ophiuchus on the ecliptic; it is the modern construction of official constellation boundaries that gives it so much territory across the ecliptic. This is true, and I was wrong to imply that Ophiuchus eats up more of the ecliptic than Scorpius. Since they are rather arbitrary boundaries, it doesn't make much difference. Still, Scorpius appears to me to be mostly below the ecliptic, while Ophiuchus is mostly above. It still makes me wonder if Scorpius was picked over Ophiuchus because it is easier to see.]

Also, note that the dates the Sun is in a given constellation do not line up with the astrological dates of the constellation. For example, my birthday is in September, but I am a Libra, not a Virgo. This is because the astrological zodiacal signs were invented about 2000 years ago, and since then, the precession of the Earth's axis (the change in the direction that the Earth's axis points) has shifted the constellations over. How come astrologers don't take that into account?

Incidentally, the name of the constellation of the scorpion is Scorpius, not Scorpio. Also, the goat is Capricornus, not Capricorn. So there.

Personal note: the constellation of Capricornus, the Goat, is shaped like an arrowhead, or a chevron. Note also that the French word for 'goat' is 'chevre'. I have been told by Bad Reader Ian Wallis that the Latin word for 'goat' is indeed 'Capricornus', where 'capra' means 'goat' and 'cornus' means 'horn'. The French derived their word through the usual mangling of words as they are handed down over the centuries (thanks to Bad Reader David Sturm for pointing that out to me!). The constellation doesn't look much like a goat, but the 'V' shape is much like the horns of a goat.

©2008 Phil Plait. All Rights Reserved.

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