The Skeptical Teacher

Musings of a science teacher & skeptic in an age of woo.

The Shifting of the Zodiac & Why Astrology Fails

Posted by mattusmaximus on January 15, 2011

For the last couple of days I’ve been getting questions from some of my colleagues about the “shifting of the zodiac” and today one playfully asked me if they were still a Cancer.  In fact, I’ve seen news headlines stating “Your horoscope could quite possibly be wrong” – this is humorous because I’ve always known horoscopes are wrong & useless :)

Some astrologers and other pseudoscientific goofballs are apparently making a lot of hay out of this (including some doomsayers who have bought into the 2012 hysteria), but I’m here to tell you that this is the effect of nothing more than simple physics.  What is going on is just the effect of the rotational axis of the Earth twisting around in a cone – this is a phenomenon called axial precession. Picture a spinning toy top on the ground – does it stay upright and keep spinning forever?  No, it eventually starts to wobble.  In much the same way, the Earth’s rotational axis wobbles, and it takes about 26,000 years for this cycle to complete.

Image Credit: Wikipedia

For example, many of us have heard of the North Star, also known as Polaris, as the star in the sky right above our north (geographic) pole.  That is, if you were standing at the geographic north pole of the Earth, Polaris would be directly overhead all the time with the rest of the sky appearing to wheel about it.  Well, believe it or not, Polaris hasn’t always been the North Star; in fact, about 13,000 years ago (halfway through our precessional cycle) our North Star was Vega!

Thus, if our North Star can be shifted over time due to precessional movement, then so too can other features of our night sky, such as the zodiac.  The zodiac is a collection of constellations which inhabit a band of sky called the ecliptic – the ecliptic is that region wherein we see the Sun, Moon, and all the planets move from our perspective on Earth, and it basically outlines the plane of our solar system.  The following image explains clearly the arrangement of the zodiac symbols along the ecliptic…

A band around the sky about 18° wide, centered on the ecliptic, in which the Sun, Moon, and planets move. The band is divided into 12 signs of the zodiac, each 30° long, that were named by the ancient Greeks after the constellations that used to occupy these positions; “zodiac” means “circle of animals,” and only Libra is inanimate. Over the past 2,000 years, precession has moved the constellations eastward by over 30° so that they no longer coincide with the old signs. Image Source

So what’s really going on is that, due to the long slow precessional cycle of the Earth, our old star maps which laid down the zodiac we’ve all come to recognize are now getting out of date.  That’s it, nothing more, nothing less.  So relax, it’s not the harbinger of cosmic disaster, it’s just simple physics.  And, I might add, where superstition & astrology have failed, science & astronomy have triumphed – what astrologer predicted the shifting of the zodiac? That is, without consulting the actual scientists first… ;)

If you’d like to see an excellent blog post on this same subject, I highly recommend this entry by Phil Plait over at the Bad Astronomy Blog.

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3 Responses to “The Shifting of the Zodiac & Why Astrology Fails”

  1. Grant Whitesell said

    But I can’t get through the day without making decisions based on an arbitrary allocation of personality traits supposedly intrinsic to every person on earth with a birthday close to mine.

  2. thank you google bacause of u I can learn more about Stars

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