01Apr2017

What Is The Zodiac?

The zodiac, the twelve signs listed in a horoscope, is closely tied to how the earth moves through the everlasting heavens.

The signs are derived from twelve constellations that mark out a path.  It’s the path on which the sun appears to travel over the course of a year. The dates in a horoscope correspond, in principle, to when the sun passes through each constellation. But a closer examination of the motion of the earth, the sun, and the stars shows the zodiac to be more complex than you might imagine!

As the earth orbits the sun, the sun appears to pass in front of different constellations. Much like the moon appears in a slightly different place in the sky each night, the location of the sun relative to distant background stars drifts in an easterly direction from day to day. It’s not that sun is actually moving—the motion is entirely an illusion caused by the earth going around our star.

The path of the sun through the zodiac.

Over the course of a year, the Sun appears to be in front of, or “in”, different constellations. One month, the Sun appears in Gemini; the next month, in Cancer. The dates listed next to your sign in the newspaper’s horoscope identify when the sun appears in that constellation. So March 21 through April 19 are set aside for Aries because that’s when the sun is sitting in that constellation! Your sign tells you what constellation the sun was in the day you were born.

If only that were actually true!

To understand why the horoscope dates are all off, we need to know a little bit more about how the earth moves. And something about how we measure time.

Time is a fiendishly difficult thing to define, especially if we insist on using the sun and stars as a reference. Our calendar is, for better or worse, tied to the seasons. June 21—the summer solstice above the equator and the winter solstice below—marks the day the sun appears at its most northerly point in the sky. At the June solstice, the North Pole is tilted fully towards the sun.

What makes this complicated is that the North Pole is not always pointing in the same direction. Our planet spins like a top. And like a top, the earth also wobbles! A wobbling Earth makes the North Pole trace out a circle on the celestial sphere. Now, the wobble is quite slow—it takes 26,000 years to wobble around once—but as the years go by, the effect accumulates.

The wobbling Earth.

Over the course of one orbit around the sun, the direction of the earth’s axis drifts ever so slightly. This means that where along our orbit the solstice occurs also changes by a very small amount. The solstice actually occurs about 20 minutes earlier each year!

Since we tie our calendar to the solstices, the earth does not actually complete an entire orbit in one year. A year is actually a hair less than a full orbit. This means that, each year, where the sun is relative to the stars on any given day—June 21, for example—drifts a very tiny amount.

But wait about 2000 years, and the sun will be sitting in an entirely different constellation!

On the June solstice 2000 years ago, the sun was sitting almost halfway between Gemini and Cancer. On this year’s June solstice, the sun will be sitting between Gemini and Taurus.

The dates in the newspaper are all wrong!

They were right when the modern Western zodiac was defined roughly 2000 years ago. But in the intervening centuries, the slow wobble of the earth’s axis has shifted the signs by roughly an entire month!

The changing location of the vernal equinox.

To complicate matters more, the constellations themselves are arbitrary. The stars that make up a constellation are not, for the most part, physically related. The constellations are just patterns that our ancestors saw as they gazed skyward and tried to make sense of it all. Today’s constellations are specific to ancient Greek culture. Most of them were introduced by the Greek astronomer Ptolemy in the 2nd century (who in turn borrowed them from ancient Babylonian texts). Different cultures have seen patterns in the sky unique to their history. Some constellations are shared by many cultures (Orion is a notable example), but most are not.

The modern constellation boundaries weren’t defined until 1930 by the International Astronomical Union. With the current boundaries, there are actually thirteen constellations that lie along the sun’s path. The extra one not listed in any horoscope is Ophiucus, the serpent bearer, who sits between Sagittarius and Scorpius.

While the zodiac may not be a great predictor of love, fortune, and health, it is a great tool for better understanding the motions of the sun, the earth, and even the cultures that have come and gone on our little planet. The zodiac signs, derived from the constellations which lie along the sun’s path in the sky, track the orbit and wobble of Earth and remind us of astronomy’s humble roots. Source: Earth and Sky

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