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Seth Shostak Headshot

Time for Astrology to Sign Out

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The caller on my voice mail was clearly concerned: "So what's going to happen to my personality, now that my sign is Ophiuchus? And, oh, how do you pronounce 'Ophiuchus' anyway?"

I figured it was a joke. The shift in the signs of the zodiac, whose constellations are the one-and-only basis for the tens of thousands of astrology forecasts routinely printed in newspapers and magazines, is a story old enough to be chiseled in stone. Despite the fact that it's suddenly in the news, this bit of celestial mechanics was worked out by the Greek astronomer Aristarchus of Samos 23 centuries ago. I mean, that's a pretty stale story. (Note to academics: Aristarchus' track record of astronomical research would probably have guaranteed him tenure somewhere, if tenure had been invented. His stack of reprints included measuring the distances of the Moon and Sun. He also argued that Earth was not the center of the solar system, an idea that, at the time, wouldn't have made it past the referees.)

What Aristarchus noted is that the positions of the stars are slowly changing, and en masse. This is not because the stars themselves are moving around (they are, but at their enormous distance the motion is far too subtle to have been seen by Aristarchian eyeballs). It's because the Earth wobbles like a top. Remember tops? Spin one up, and you'll notice that its axis doesn't remain precisely vertical; it moves around in a circle, like a wagging finger.

Thanks to the fact that the Earth isn't a perfect sphere, and invoking a bunch of Newtonian physics, you can deduce that our planet wobbles too, taking roughly 26,000 years to trace out a small circle on the sky, a phenomenon known as precession. So stars seem to lazily bob up and down, changing their position relative to the horizon (or the Sun) about 1 degree every century. That's not breathtakingly fast, but it was enough for Aristarchus to notice. It also means, for instance, that when the great pyramids of Egypt were built, the north star wasn't very close to being north -- a fact that seems to puzzle a lot of people who wonder about the very precise alignment of these pointy structures.

But the bottom line is that the position of the Sun relative to the stars slowly changes for any given date, and over the course of 26,000 years, it can easily slide between constellations. So you may think you're a Pisces, but you're actually an Aquarius. Maybe if life hasn't been going your way, it's because you've been reading the wrong horoscope entry.

Maybe, but probably not. To begin with, a variant on astrology that determines your sign based on the time of the year, not the position of the Sun, isn't affected by this precession. So invoking the so-called "tropical zodiac" lets astrologers off the hook.

But there's this too: the whole idea that events in the sky at the time of your birth can influence your personality is fundamentally suspect. It lacks any physical basis. Granted, if an asteroid is hurtling to Earth as you're being born, and then vaporizes the hospital a few minutes later, that will affect your personality. But just because Mars is in Aquarius when you pop into the world... well, how could that possibly matter?

Maybe it matters because Mars and the Sun are now pulling on your fetal frame at the same time? You know, the extra tug Mars lends to the Sun if it's in the same direction?

Well, it can't just be the tug of Mars, because Mars is always in the sky. It's just that now it's in a different part of the sky (namely, your zodiacal sign). OK, work it out, and you'll find that the Red Planet's gravitational attraction differs from month to month by less than the pull of the cars in the hospital parking lot. Even Jupiter -- which is really the only planet that counts in this gravitational sweepstakes -- has a variable tug that, from one month to the next, differs by less than that of the office building down the block. Maybe horoscopes should be cast based on city maps.

Frankly, I'll believe in horoscopes the day I can describe my personality to an astrologer, and they tell me what date I was born.

Oh, and by the way, it's OH-FEE-YOU-CUSS.

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