Star symbolism · Symbolic interpretation
Zodiac signs versus constellations
Aries, Taurus and the rest name two different things: twelve signs used in astrology, and constellations used by astronomers. They share names and a history, but they do not line up, and astrology itself has more than one zodiac.
Constellations are regions of sky
For astronomers a constellation is an area. The International Astronomical Union (opens in a new tab) agreed its list of 88 in 1922 and adopted Eugène Delporte's boundaries in 1928, so that the regions cover the whole sky with no gaps. They are very uneven: Crux is the smallest of all, while Centaurus is ninth largest.
Signs are equal slices of a circle
Astrology's signs are twelve equal divisions of the Sun's yearly path, each 30 degrees (opens in a new tab) long, whatever stars happen to lie there. In the tropical zodiac of Western astrology they are counted from the March equinox. Ptolemy (opens in a new tab) explained the choice in the second century: a circle has no natural beginning, so astrologers take the sign that starts at the spring equinox, Aries, as the first.
Sidereal zodiacs
Vedic astrology, Jyotish, measures its signs against the stars instead. The gap between the two frames is the ayanamsha (opens in a new tab), and practitioners do not all use the same value. By the value this site uses (Lahiri's), the two zodiacs are about 24° apart today; the record on this site has the history.
Why they drift apart
Earth's axis wobbles slowly, once in about 25,771 years (opens in a new tab). That carries the equinox, and so the whole tropical zodiac, backwards against the stars by about a degree every 72 years, which is how the two zodiacs came apart.
Thirteen constellations, twelve signs
The Sun's path crosses thirteen of the IAU's constellations. The IAU notes (opens in a new tab) that the Babylonians, who devised the zodiac, left one out, Ophiuchus, to keep in step with their twelve-month calendar. When this made headlines in 2016, NASA pointed out (opens in a new tab) that it had not changed anyone's sign; it had only done the arithmetic.
No single zodiac
Tropical and sidereal zodiacs are different conventions, each consistent on its own terms. Neither is the sky's official version, and neither is a scientific measurement of anything about a person. The constellations are real regions of sky; the signs are a symbolic system laid over the same circle.
Reflect
A Cool Star Facts reflection prompt
- Write down three things you have heard about your sign.
- Look at the constellation that shares its name, as a patch of stars, on a star chart or in the sky finder.
- Which of the three things feel true because they describe you, and which because you were told them? There is no right answer.
This prompt is this site's own. It is not an astrological technique, and it predicts nothing.
Sources
Practitioner sources tell you what those practitioners say. They are not evidence that the stars do it.
- Astronomy International Astronomical Union, The Constellations (opens in a new tab) for the 88 constellations and their boundaries
- History Ian Ridpath, Star Tales: Crux (opens in a new tab) for Crux as the smallest
- History Ian Ridpath, Star Tales: Centaurus (opens in a new tab) for Centaurus as ninth largest
- Reference Encyclopaedia Britannica, Zodiac (opens in a new tab) for signs of 30 degrees
- Practitioner Ptolemy, Tetrabiblos I.10, tr. F. E. Robbins (Loeb, 1940), LacusCurtius (opens in a new tab) for the tropical starting point
- Practitioner Ayanamsha Principles, Vedic Planet (Jyotish teaching site) (opens in a new tab) for the ayanamsha
- Astronomy NASA Science, Milankovitch (Orbital) Cycles and Their Role in Earth's Climate (2020) (opens in a new tab) for the precession cycle
- Astronomy International Astronomical Union, Astronomy FAQs (opens in a new tab) for Ophiuchus and the Babylonian twelve
- Astronomy NASA, Constellations and the Calendar (2016) (opens in a new tab) for NASA on the 2016 headlines