A band around the ecliptic
The zodiac is the band of the sky about 8° to 9° on either side of the ecliptic, the Sun's apparent yearly path. Because the orbits of the Moon and the planets are only slightly inclined to the Earth's, the Moon and the planets stay within it. The name comes from the Greek zōidiakos kyklos, the circle of little animals, after the figures of its constellations.
The zodiac was organised by Babylonian astronomers in the 5th century BC, who divided it into twelve equal parts of 30° to track the motion of the Sun, the Moon and the planets. Greek astronomers adopted this scale, and ecliptic longitude is still reckoned in the same way: 0° to 360° from the vernal equinox.
Signs and constellations are not the same
A sign is a 30° segment of the ecliptic, counted from the vernal equinox: Aries from 0° to 30°, Taurus from 30° to 60°, and so on. A constellation is a region of the sky with boundaries fixed by the International Astronomical Union in 1930. The constellations have very unequal sizes, and the ecliptic actually crosses thirteen of them, including Ophiuchus between Scorpius and Sagittarius.
The table gives the dates when the Sun is in each constellation of the zodiac, using the IAU boundaries. They vary by a day from year to year.
| Constellation | Sun in the constellation | Days |
|---|---|---|
| Capricornus | 20 Jan – 16 Feb | 27 |
| Aquarius | 16 Feb – 11 Mar | 23 |
| Pisces | 11 Mar – 18 Apr | 38 |
| Aries | 18 Apr – 13 May | 25 |
| Taurus | 13 May – 21 Jun | 39 |
| Gemini | 21 Jun – 20 Jul | 29 |
| Cancer | 20 Jul – 10 Aug | 21 |
| Leo | 10 Aug – 16 Sep | 37 |
| Virgo | 16 Sep – 30 Oct | 44 |
| Libra | 30 Oct – 23 Nov | 24 |
| Scorpius | 23 Nov – 29 Nov | 6 |
| Ophiuchus | 29 Nov – 17 Dec | 18 |
| Sagittarius | 17 Dec – 20 Jan | 34 |
The Sun spends 44 days in Virgo but only six days in Scorpius, and is in Ophiuchus for longer than in Scorpius. Ophiuchus has always been crossed by the ecliptic; it was simply never one of the twelve traditional signs.
Precession shifts the signs
Precession moves the vernal equinox westward along the ecliptic by about 1° every 72 years. Two thousand years ago the equinox was in Aries, so the sign and the constellation of Aries roughly coincided. The equinox has since moved about 28° and lies in Pisces; with the IAU boundaries, it will enter Aquarius around the year 2600.
As a result, on the date given for a sign in a horoscope, the Sun is usually in the constellation before it: someone born on 1 April has the Sun in the sign of Aries but in the constellation Pisces.
Tropical and sidereal zodiacs
- The tropical zodiac, used in Western astrology and in the natal chart on this site, starts the signs at the vernal equinox. It is tied to the seasons: the Sun always enters Aries at the March equinox.
- The sidereal zodiac, used in Indian astrology, keeps the signs attached to the stars. The offset between the two, the ayanamsa, grows by about 50″ a year and is close to 24° today (in the common Lahiri convention).
- Astronomy uses neither: positions are given in ecliptic longitude, in degrees from the equinox of a stated date, such as J2000.0 or the equinox of date.
The zodiac on AstroAlgos
The all-sky disk draws the ecliptic and the zodiac constellations. Set the timeline to any date and you can check which constellation is behind the Sun, and see the planets threaded along the band. The natal chart computes the ecliptic longitudes of the Sun, the Moon and the planets with Meeus's algorithms and reports them in tropical signs.
Explore it on AstroAlgos
Frequently asked questions
Is Ophiuchus a zodiac sign?
It is a zodiac constellation but not a traditional sign. The Sun crosses Ophiuchus from about 29 November to 17 December, but the twelve signs are fixed 30° segments that ignore constellation boundaries.
Why don't the zodiac signs match the constellations?
Because of precession. The signs start at the vernal equinox, which has moved about 28° westward along the ecliptic since the zodiac was defined, so each sign now lies mostly over the preceding constellation.
How wide is the zodiac?
Traditionally about 8° to 9° on each side of the ecliptic, wide enough to contain the paths of the Moon and the planets.
