The Earth is not a perfect sphere: it bulges at the equator.
The pull of the Sun and the Moon on this bulge tugs on its axis, which does not stay fixed and slowly traces a cone, like a spinning top that tilts as it turns. A full turn takes about 25,800 years. The tilt of the axis hardly changes; only its direction does.
A visible consequence: the point of the spring equinox moves backward along the ecliptic, by one degree every 72 years or so. In Hipparchus’s time, this point lay in Aries; with the official constellation boundaries, it lies today in Pisces, and it will enter Aquarius around the year 2600. The North Star is one of the effects: in about twelve thousand years, it will be Vega that marks north.
For astrology, precession raises a question of reference point. The tropical zodiac ignores it by design, since it is tied to the equinox. The sidereal zodiac corrects for it by tying itself to the stars. It is precession that, century after century, moves the signs away from the constellations that gave them their names. Since Hipparchus, the accumulated shift has reached about thirty degrees, a whole sign: to follow the Sun against the stars, you must therefore always specify the date of the calculation.
Family
The sky and the zodiac
The diagram
Precession in numbers
Read next
Tropical zodiac vs. sidereal zodiac
History, debates, and sources
A little history
Hipparchus, around 130 BCE, compares his measurements of star positions with those of his predecessors and notes a regular offset: the equinoxes are moving.
Ptolemy takes up the measurement in the Almagest and puts it at one degree per century, a value too small but right in principle. Copernicus, in 1543, sees in it a motion of the Earth’s axis, not of the stars. Newton then supplies the cause, the pull of the Sun and the Moon on the equatorial bulge. Modern formulas, such as those of Jean Meeus, are used for the calculations this site makes.
Sources
3
Oldest
Hipparchus, c. 130 BCE
Most recent
Jean Meeus, 1998
What is debated
Precession is a measured astronomical fact, and no one disputes it.
The debates concern what astrology makes of it. Some see it as the reason to prefer a sidereal zodiac; others consider it irrelevant, since the signs depend on the seasons. The notion of the Age of Aquarius, derived from precession, circulates with very different dates depending on the author, because one must choose where the constellation begins and ends.
Schools do not all agree: the site sets out the positions without deciding for you, and states the setting it uses by default.
Where this concept comes from
The authors and texts that establish or discuss this concept, with the passage when it is known.
c. 130 BCEHipparchus(reported by Ptolemy), see the Almagest, Book VII
First to measure the displacement of the equinoctial points against the background of the stars.
2nd centuryClaudius PtolemyAlmagest, Book VII
Takes up Hipparchus’s measurement and estimates it at one degree per century (the modern value is about one degree in 72 years).
The diagram’s figures are calculated by the site’s engine (Astronomy Engine) or taken from the recorded facts, never copied by hand.
Precession in numbers
Speed
50.29″ per year
One degree
in 71.6 years
One sign (30°)
in ≈ 2150 years
A full circle
in ≈ 25,770 years
March equinox 2026
Sun in PiscesConstellation, calculated by the site: the sign itself begins at Aries
Gap between the zodiacs
≈ 24° 13′Lahiri ayanamsa, 2026
Frequently asked questions: the precession of the equinoxes
What is the precession of the equinoxes?
It is the slow change in direction of the Earth’s axis, comparable to that of a spinning top. It moves the equinox points along the ecliptic, by about 50 arcseconds each year. The stars thus seem to slide very slowly relative to the seasons.
How long does a precession cycle last?
About 25,800 years. That is the time the Earth’s axis takes to trace a full cone. It is sometimes called the Great Year or the Platonic year. Over this long cycle, each tropical sign therefore passes once through all the constellations of the zodiac.
Does precession change the astrological signs?
Not in the tropical zodiac, which is tied to the equinox: Aries always begins in spring. It does, however, change the position of the signs relative to the constellations, and therefore the result of a sidereal calculation. This is what explains the gap of about 24 degrees between the two zodiacs.