Zodiac signs versus constellations

Several traditions, one table

Astrology is not one system; the table on this site is the tropical Western twelve-sign system.

Astrology has several traditions. Vedic astrology, Jyotish, measures its signs against the stars; the sidereal offset record on this site has its history. The Western teaching (opens in a new tab) most people meet in horoscope columns uses the tropical zodiac, and that is the one this section's twelve-sign table describes.

In that system, as Britannica (opens in a new tab) summarises it, the twelve signs fall into four groups by element (fire, earth, air, water) and into three modes: cardinal, linked to beginnings; fixed, to sustaining; mutable, to change. Each sign also has a traditional ruling planet. The twelve signs lists them.

Sources for this section Faculty of Astrological Studies, Foundation Course (course outline) (opens in a new tab) · ; Encyclopaedia Britannica, Zodiac (opens in a new tab)

Constellations are regions of sky

To astronomers a constellation is an uneven area of sky with official borders.

The International Astronomical Union (opens in a new tab) agreed its list of 88 constellations 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 (opens in a new tab) of all, Virgo the second largest (opens in a new tab). The IAU notes (opens in a new tab) that because of this the Sun spends different lengths of time in each.

Sources for this section International Astronomical Union, The Constellations (opens in a new tab) · ; Ian Ridpath, Star Tales: Crux (opens in a new tab) · ; Ian Ridpath, Star Tales: Virgo (opens in a new tab) · ; International Astronomical Union, Astronomy FAQs (opens in a new tab)

Signs are equal slices of a circle

Each sign is 30° of the Sun's path, counted from the March equinox, whatever stars lie there.

The signs are twelve equal divisions of the Sun's yearly path, like a pizza cut into 12 equal slices (opens in a new tab), in NASA's phrase, each 30° long.

In the tropical zodiac 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.

Sources for this section NASA, Constellations and the Calendar (2016) (opens in a new tab) · ; Ptolemy, Tetrabiblos I.10, tr. F. E. Robbins (Loeb, 1940), LacusCurtius (opens in a new tab)

Element, mode and ruler are terms of the system

These labels describe how the tradition sorts its signs; they are not measurements of the sky.

Ptolemy sorted the signs into solstitial, equinoctial, solid and bicorporeal (opens in a new tab) kinds, gave each to a planet as its house (opens in a new tab), and grouped them in four triangles (opens in a new tab). Modern teaching calls the kinds modes and the triangles elements; this site credits each label to the tradition that uses it.

A birth chart records where the Sun, the Moon and the planets were in the zodiac at the moment of birth (opens in a new tab), whether or not they were above the horizon, so it is not a map of what you could have seen. The sky finder shows what is actually above you, and it reads the same in every lens.

Sources for this section Ptolemy, Tetrabiblos I.11, Of Solstitial, Equinoctial, Solid, and Bicorporeal Signs, tr. F. E. Robbins (Loeb, 1940), LacusCurtius (opens in a new tab) · ; Ptolemy, Tetrabiblos I.17, Of the Houses of the Several Planets, tr. F. E. Robbins (Loeb, 1940), LacusCurtius (opens in a new tab) · ; Ptolemy, Tetrabiblos I.18, Of the Triangles, tr. F. E. Robbins (Loeb, 1940), LacusCurtius (opens in a new tab) · ; Encyclopaedia Britannica, Zodiac (opens in a new tab)

Sidereal zodiacs

Vedic astrology measures its signs against the stars, so its signs sit about 24° from the tropical ones.

The gap between the two frames is the ayanamsha (opens in a new tab), the angle between the starting points of the tropical and sidereal zodiacs, and practitioners do not all use the same value; most accepted values are within a degree or two of each other.

By the value this site uses, Lahiri's, which the same source calls the most widely used (opens in a new tab), the two zodiacs are about 24° apart today; the record on this site has the history.

Sources for this section Ayanamsha Principles, Vedic Planet (Jyotish teaching site) (opens in a new tab)

Why they drift apart

Earth's axis wobbles, so the equinox, and the tropical zodiac with it, slides slowly against the stars.

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.

NASA notes that the sky has shifted (opens in a new tab) since the zodiac was devised because the axis no longer points the same way, which is why the sign dates in horoscope columns no longer match the constellations.

Sources for this section NASA Science, Milankovitch (Orbital) Cycles and Their Role in Earth's Climate (2020) (opens in a new tab) · ; NASA, Constellations and the Calendar (2016) (opens in a new tab)

Thirteen constellations, twelve signs

The Sun crosses thirteen constellations, Ophiuchus included; the twelve signs did not change.

The Sun's path crosses thirteen of the IAU's constellations, and in astronomy Ophiuchus counts (opens in a new tab) as one of the zodiac's. The IAU and NASA both repeat a story that the Babylonians left it out to fit a twelve-month calendar; the British Museum's Babylonian tablet (opens in a new tab), though, pairs months with figures that do not match the Greek constellations one for one, so the history is less tidy than that.

When the thirteenth constellation 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. Ophiuchus is not an automatic thirteenth sign in the twelve-sign system.

Sources for this section International Astronomical Union, Astronomy FAQs (opens in a new tab) · ; British Museum, tablet 77824 (1885,0430.15): months and signs of the zodiac (opens in a new tab) · ; NASA, Constellations and the Calendar (2016) (opens in a new tab)

No single zodiac, and no evidence either way that it works

Tropical and sidereal are conventions; neither is a measurement of a person.

Tropical and sidereal zodiacs start from different points (opens in a new tab), the equinox and the stars, and each is consistent on its own terms. Neither is the sky's official version.

NASA's point is the one this site keeps: astronomy follows evidence and data, while astrology is important to some cultural traditions but its claims are not based on scientific evidence (opens in a new tab). Britannica (opens in a new tab) adds that no correlation has been found between birth dates and personality.

Sources for this section Ayanamsha Principles, Vedic Planet (Jyotish teaching site) (opens in a new tab) · ; NASA Space Place, What Are Constellations? (updated 2022) (opens in a new tab) · ; Encyclopaedia Britannica, Zodiac (opens in a new tab)

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