A Brief Guide to the Zodiac
The Zodiac is a set of 12 constellations tracing the yearly path of the Sun, Moon and planets across our sky.
During the Earth's orbit, the Sun appears to pass in front of the constellations of the Zodiac. Our home star seems to move through a band of sky that stretches about 8° north and south of the ecliptic - an imaginary line tracing the annual solar path.
The Moon and some other planets look as if they pass through the 12 constellations too. For most of us, the movement of these bodies through the Zodiac are best known as the basis for predictions made by horoscopes.
Horoscopes and Illusions
While they are not evidence-based, horoscopes are very popular today. Ironically, they have probably had the effect of boosting the profile of the science of astronomy by spreading knowledge about constellations and inspiring star-gazing in general.
When watching the skies, it's interesting to keep in mind that the whole idea of the Sun moving through the sky is an illusion. It was the Greek astronomers some 2,000 years ago who were the first to propose that Earth's rotation and orbit is what causes us to perceive a change in the Sun's position over the course of a year.
The 12 signs of the Zodiac, and the other 76 constellations recognized by the International Astronomical Union, are a product of our human imagination. For thousands of years, people around the world have gazed up at the night sky and created two-dimensional pictures from the star patterns to connect them with their worlds of myth and legend.
All 12 constellations in the modern Zodiac are named after animals and mythical figures familiar to ancient Greek astronomers:
- Capricorn (The Goat)
- Aquarius (The Water Bearer)
- Pisces (The Fish)
- Aries (The Ram)
- Taurus (The Bull)
- Gemini (The Twins)
- Cancer (The Crab)
- Leo (The Lion)
- Virgo (The Virgin)
- Libra (The Scales)
- Scorpio (The Scorpion)
- Sagittarius (The Archer)
The Sun actually passes through another constellation known as Ophiuchus, the Serpent Bearer, which sits between Sagittarius and Scorpius. This sign is not included in the Zodiac because astronomers preferred to divide the ecliptic into twelve equal sections of 30° each, to total the full 360 degrees of a circle.
Our perception of where the Sun sits in the Zodiac is not static. It shifts over time, partly because of precession - a planetary movement that includes the Earth wobbling as it rotates, the way a spinning top does at times. Precession creates a gradual change in the direction of our planet's rotational axis, and this affects solstice and equinox points.
What we think of as the modern Zodiac was outlined some 2,000 years ago. Since then, our planet's movement has caused these solstice and axis points to move about 30 degrees relative to the constellations.
So there has been a change in where we see the Sun at the solstice. When the Zodiac group was established in the second century BCE, the June solstice found the Sun between Gemini and Cancer. Since our calendars are linked to events like solstices and equinoxes, the same date today finds the Sun positioned between Gemini and Taurus, and this difference will only increase with time.
Greeks, Babylonians, and Sumerians
The origin of the Zodiac traces back to ancient times, perhaps as long ago as 1800 BC, when Babylonian astronomers divided the ecliptic into twelve sections. Other ancient cultures in China, Sumeria, and Egypt also developed their own star maps around this time.
Some research indicates that it was Babylonian astronomers who identified constellations like "The Crab", "The Twins", and "The Lion", which may have been the sources of the later Greek versions of Cancer, Gemini, and Leo. Greek astronomers cataloged the Zodiac constellations by the 2nd century BCE, and this knowledge later spread to India, where the Hindu Zodiac was created.
Today, most constellations in the Northern Hemisphere's skies bear Greek and Roman names. The word Zodiac itself is drawn from ancient Greek meaning cycle or circle of little animals.
The Zodiac constellations have served astronomers as useful markers to help track the movement of the Sun, Moon and planets. Constellations have also been helpful in orienting their work and defining what part of the cosmos they study, as well as to help identify objects that appear in space. In addition, constellations are often used to help create names for newly-discovered stars and to identify meteor showers.