The difference between the zodiac and the constellations

Constellation: a region of the sky on the map

Constellations are not real physical groupings of stars but patterns our ancestors drew around stars that happen to appear side by side as seen from Earth. Modern astronomy tidied these patterns up: in the first half of the 20th century the International Astronomical Union divided the sky into 88 constellation regions with precisely drawn borders. What is meant by "constellation" today is these mapped regions, rather like national borders. The borders have a practical function: the address of a comet appearing in the sky or of a newly discovered object is reported by the constellation region it lies in, much as one says which district an event took place in.

The zodiac: twelve equal segments of the Sun's path

Through the year the Sun traces a circle against the backdrop of the stars; this path is called the ecliptic. The zodiac is the division of the band around the ecliptic into 12 equal segments of 30 degrees each. The tropical zodiac, common in Western astrology, fixes these segments to the seasons rather than to the stars: the segment of Aries begins at the spring equinox. The signs, in other words, are really a seasonal calendar; they are tied to the Sun's seasonal position, not to the star patterns in the sky. One consequence of this definition is that in the tropical system the dates of the signs barely change from year to year, because the system is anchored to the seasons; the stars fall outside this calendar.

There are 13 constellations on the ecliptic, not 12

Because constellation borders are not drawn at equal intervals, the Sun's path passes through Ophiuchus (the Serpent Bearer) as well as the classical twelve signs. The transit times are also very uneven: the Sun spends about a month and a half in front of Virgo, but only about a week in front of the constellation Scorpius. An arrangement of 12 equal segments therefore cannot map one-to-one onto the real regions of the sky. This unevenness is not an error; it is the clearest sign that the two systems were built for different purposes. One maps the sky, the other divides the year into equal parts.

Precessional drift: why don't the dates match?

Because of the Earth's axial wobble of about 25,772 years, the equinox points move slowly backwards against the stars; we describe this phenomenon in our guide to precession. In antiquity the zodiac segments roughly coincided with the constellations they were named after; over the two thousand years since, the drift has reached about one sign. Today, during the period your birth dates make "Aries", the Sun is mostly in front of the constellation Pisces.

Different skies, different patterns

The zodiac we use today is the product of a single tradition, born in Mesopotamia and shaped in the Greek world; yet every culture has drawn its own patterns around the same stars. Chinese astronomy divides the sky into entirely different regions and uses lunar mansions, based on the Moon's path, rather than a zodiac. Some of the Aboriginal peoples of Australia, meanwhile, see the pattern not in the stars but in the dark dust clouds of the Milky Way: the giant "emu" figure in the sky is made not of bright points but of starless dark regions. This variety tells us two things at once: star patterns are a product of human creativity, and no pattern is the "natural" division of the sky. The International Astronomical Union's list of 88 constellations is likewise not a discovery but a convention chosen out of the world's astronomers' need for a common language; it took the patterns of the Western tradition as its basis but redrew the borders for scientific purposes.

So has my sign "changed"?

The stories that surface from time to time saying "the signs have changed, a 13th sign has been added" are the product of a confusion. Astronomically nothing has changed; constellation borders and precession have been known for a long time. Tropical astrology, for its part, defines its segments by the seasons and so is consistent within its own system; most astrologers use the segments rather than the constellations in any case. The real gain here is this: knowing that the zodiac segments and the star patterns in the sky are separate things allows you to read sky stories with a cooler head. We take up the scientific standing of astrology separately in our guide to whether astrology is a science.

Frequently asked questions

This guide draws on the International Astronomical Union's constellation definitions and on the content of standard astronomy sources.