The Zodiac
The zodiac is a special group of 12 constellations that sit in a wide band right around the sky, along the path that the Sun appears to follow over the course of a year. That path is called the ecliptic. The Sun moves through each of the 12 constellations in turn, spending about a month in each one. The same band of sky is also the path the Moon and the planets take, which is why all the most important objects in our solar system are always found in a zodiac constellation.
- Zodiac constellations12Plus a 13th, Ophiuchus, often left out
- EclipticSun's yearly pathA great circle round the sky
- Time in eachapprox. 30 daysBut not exact: Sun spends 45 days in Virgo
- Oldest knownapprox. 3,000 yearsBabylonian astronomers, approx. 1,000 BC
- Origin of nameGreek "zodiakos"Meaning "circle of animals"
- Largest zodiac signVirgoSun spends 45 days passing through
The 12 constellations of the zodiac
In the order the Sun moves through them across the year:
- Aries (the Ram), 19 April to 13 May
- Taurus (the Bull), 14 May to 19 June
- Gemini (the Twins), 20 June to 20 July
- Cancer (the Crab), 21 July to 9 August
- Leo (the Lion), 10 August to 15 September
- Virgo (the Maiden), 16 September to 30 October
- Libra (the Scales), 31 October to 22 November
- Scorpio (the Scorpion), 23 November to 29 November
- Ophiuchus (the Serpent Bearer), 30 November to 17 December
- Sagittarius (the Archer), 18 December to 18 January
- Capricornus (the Sea Goat), 19 January to 15 February
- Aquarius (the Water-Bearer), 16 February to 11 March
- Pisces (the Fish), 12 March to 18 April
That is actually 13 names. The astronomical zodiac genuinely has 13 constellations along the Sun's path. Astrological zodiac signs use only 12 and leave Ophiuchus out, but astronomers count it.
Why are these constellations special?
The zodiac matters because the solar system is mostly flat. The 8 planets all orbit the Sun in roughly the same plane (with only Mercury tilted a few degrees). So when you look up from Earth, the Sun, Moon and planets all appear to move along the same line across the sky, the ecliptic. That line passes through the 12 (really 13) zodiac constellations.
This is why, throughout human history, almost every great civilisation noticed the zodiac first. The Egyptians, Babylonians, Greeks, Chinese, Indians and Mayans all picked the same 12-or-so star groups around the ecliptic as specially important. They are the constellations where all the action happens.
Where the zodiac came from
The zodiac in its modern form was put together by the Babylonians around 3,000 years ago. They divided the ecliptic into 12 equal slices, one for each of the 12 lunar months. Then they named each slice after a constellation that sat in it, mostly animals or mythological characters. The Greeks borrowed and renamed the Babylonian system around 500 BC, and Greek names are the ones we still use today.
The Babylonian zodiac was deeply practical. Knowing which constellation the Sun was "in" let farmers track the seasons exactly: when to plant, when to harvest, when the Nile would flood, when storms would come. The whole calendar of life depended on it.
Why the dates have drifted
If you look up your astrological birth sign, the dates will not match the dates above. That is because astrology uses dates fixed about 2,000 years ago, but the sky has been slowly shifting since then.
Earth's axis slowly wobbles in a circle that takes 26,000 years to complete, a motion called precession (see Ursa Minor for more). Over the past 2,000 years, the constellations have appeared to drift by almost a full month's worth of zodiac sign compared to where they were when the Greeks set up the original system. So someone born on 24 July, whose astrological sign is "Leo", actually has the Sun in Cancer when they were born.
Deeper dive: astronomy vs astrology, and why scientists do not study horoscopes
The zodiac sits at the meeting point of two very different things. Astronomy is the science of stars, planets and space, based on observation, measurement and physics. Astrology is an ancient belief that the positions of stars and planets at the moment of your birth influence your personality and your future. For most of human history the two were the same thing: ancient astronomers were also astrologers, because nobody understood the difference between predicting eclipses and predicting kings' fortunes. They were both "reading the sky".
By the 1600s, scientists like Galileo, Kepler and Newton began to separate the two. Newton showed that the planets move because of gravity, a precise mathematical force, not because of any influence on human life. By the 1700s most serious astronomers had stopped doing astrology.
Modern scientific studies have tested astrology many times. They have looked at large groups of people and checked whether the personality traits predicted by their zodiac signs match what people are actually like. They have compared the predictions of professional astrologers against random guesses. In every well-designed test, astrology has failed: it makes no better predictions than chance. The reason is straightforward: the stars are so far away that their gravity at your birth is utterly tiny compared to, say, the gravity of the doctor delivering you. There is no known mechanism by which they could shape your life.
The zodiac itself is still very important to astronomy, though, just for a different reason: the planets, the Sun and the Moon always move along the same band of sky. So when telescopes look for new comets, asteroids or exoplanets, they often watch the zodiac constellations first.
Want a closer look at a single zodiac sign? Start with Leo. For non-zodiac patterns try Orion, Ursa Major or Cassiopeia.