Precession of the Equinoxes
Earth spins like a top, and like a top it slowly wobbles. Its axis traces a full circle against the stars in about 25,800 years, a span known as the Great Year. Scrub the millennia and watch the north pole drift from star to star (left) as the axis sweeps out its cone (right).
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How long is the Great Year? About 25,772 years at the measured rate, 50.288 arcseconds a year (IAU 2006). That is the time Earth's axis takes to trace one full circle against the stars, the precession of the equinoxes: the 1,296,000 arcseconds of a circle divided by 50.288. The pole star is Polaris now and will be Vega in roughly 11,700 years.
Precession of the equinoxes: the key facts
The precession of the equinoxes is the slow wobble of Earth's spin axis, which traces a full circle against the stars over one Great Year. As the axis turns, the North Star changes and the spring equinox drifts steadily backward through the zodiac.
| Length of the cycle (the Great Year) | About 25,772 years at the measured rate (IAU 2006) |
|---|---|
| Rate of drift | About 50.288 arcseconds a year, one degree in about 71.6 years |
| March equinox (IAU boundaries) | In Pisces since 68 BCE; crosses into Aquarius in 2597 |
| North Star now | Polaris (less than 1 degree from the pole) |
| Previous North Star | Thuban, around 2800 BCE |
| Next bright North Star | Vega, around 13,700 CE |
| Also known as | the Great Year, the Platonic Year |
The wobble of a spinning world
A fast-spinning top does not simply stand upright. Its axis leans over and swings slowly around in a wide circle, a motion called precession. Earth does exactly the same thing, only far more slowly.
The reason is that Earth is not a perfect sphere: it bulges a little at the equator, and the gravity of the Sun and the Moon pulls on that bulge, trying to tug the tilted axis upright. Because Earth is spinning, the axis does not topple. Instead it sweeps around in a great cone, just as a leaning top's axis circles rather than falling over.
One full sweep takes about 25,800 years and carries the axis around a circle 23.4° in radius, centered on the pole of the ecliptic (the point in the sky directly above Earth's orbit). This grand, slow circling is the precession of the equinoxes, also known as the Great Year or Platonic Year.
A changing North Star
Because the celestial pole slowly moves, no single star stays the North Star forever. Thuban, in the constellation Draco, marked the pole around 2800 BCE, shortly before the first Egyptian pyramids were built. Polaris happens to sit nearest the pole in our own era, less than a degree away, which is why it guides us north so reliably today.
In roughly 11,700 years the brilliant summer star Vega will be the closest bright pole star, though even then it will stand about 5° from the exact pole, a looser marker than Polaris is now. Several fainter stars take their own brief turns in between, and after a full turn, about 25,800 years, the pole circles back near Polaris again, around the year 27,900. The left-hand view traces that whole journey among the stars.
The drifting equinox and the constellations
The same wobble slowly drags the equinox point, the spot where the Sun crosses the equator each spring, backward through the constellations of the zodiac, at a rate of about one degree in 71.6 years. Those constellations are unequal, so each passage has its own length: by the official IAU boundaries the equinox spent about 1,797 years in Aries and takes about 2,664 years to cross Pisces.
This backward drift is why the spring equinox, which left Aries for Pisces in 68 BCE, now sits in Pisces and crosses into Aquarius in 2597. (Astrological ages have no single agreed definition; the constellation crossings are the part astronomy can date.)
You will find the Great Year and the other long Earth cycles on the cycles by length page. And it is this very same 23.4° tilt that, taken one year at a time, gives us the seasons.
Earth’s axis also nods: nutation
Precession is the big, smooth wobble. Riding on top of it is a much smaller and faster nodding of the axis, called nutation (from the Latin nutare, “to nod”). Its main beat has a period of just 18.6 years and an amplitude of only about 9.2 arcseconds, roughly one 390th of a degree.
That is far too small to notice with the unaided eye, yet well within reach of careful instruments. Slide through one cycle below: the pole bobs gently along its precession path (left) and traces a tiny ellipse (right).
Why the axis nods, and why it follows the lunar-node cycle
The Moon’s orbit is tilted about 5.1° to the ecliptic, and its line of nodes swings all the way around once every 18.6 years. As that tilted orbit slowly turns, the Moon’s gravitational tug on Earth’s equatorial bulge strengthens and weakens on the same 18.6-year beat, so the axis nods in step with it.
In other words, nutation and the regression of the lunar nodes are the very same cycle: the slow turning of the Moon’s orbital plane is the cause, and the axis’s nod is the effect.
The Sun and the planets add their own smaller contributions, but this 18.6-year lunar nod is by far the largest. The English astronomer James Bradley announced the discovery of nutation in 1748, after more than twenty years of meticulous measurements of the stars.
Frequently asked questions
What is the precession of the equinoxes?
Earth's spin axis slowly wobbles like a spinning top, tracing a full cone against the stars in about 25,800 years. As it turns, the point where the Sun crosses the equator in spring drifts steadily backward through the zodiac.
Has the North Star always been Polaris?
No. Because of precession the celestial pole moves among the stars. Thuban was the pole star around 2800 BCE, Polaris is closest now, and Vega will take a turn around 13,700 CE.
How long is the precession cycle?
One full circuit takes about 25,772 years at the measured rate, 50.288 arcseconds a year (IAU 2006), and is often called the Great Year. At that rate the equinox shifts one degree in about 71.6 years. The rate itself creeps up very slowly, so the figure is approximate.
What is the difference between 25,920 and 25,772 years?
Both describe one full turn of precession, the Great Year. The measured rate, 50.288 arcseconds a year in the IAU 2006 model, is about 71.6 years per degree, so 360 degrees take about 25,772 years. The traditional figure rounds the drift to 72 years a degree, and the 148-year gap between the two is that rounding. The rate itself creeps up very slowly, so even the measured figure is approximate. CycleCalcs computes precession with the measured IAU 2006 rate, 50.288 arcseconds a year, about 25,772 years per turn; 25,920 is the traditional round figure (360 x 72).
What is the Platonic Year?
The Platonic Year, also called the Great Year, is another name for one complete cycle of the precession of the equinoxes, about 25,800 years. The term goes back to ancient Greek philosophy and the idea of a grand cycle after which the heavens return to nearly the same arrangement.
What astrological age are we in now?
Astronomy can date the constellation the March equinox lies in, not an age. By the official constellation boundaries of the International Astronomical Union (IAU), the equinox has been in Pisces since 68 BCE and crosses into Aquarius in 2597, so it is more than three-quarters of the way across Pisces. Astrological ages have no single agreed definition or boundaries; the constellation crossing is the part astronomy can date.
Which star will be the next North Star?
After Polaris, the bright star Vega will be the closest naked-eye star to the celestial pole, around the year 13,700 CE, though it will stand about 5 degrees from the exact pole rather than hugging it the way Polaris does now. Thuban in Draco was the pole star around 2800 BCE, and after a full turn, about 25,800 years, the pole circles back near Polaris, around the year 27,900.
Sources & further reading
- NASA Goddard, Precession: how Earth's equatorial bulge and the pull of the Sun and Moon swing the axis over roughly 26,000 years.
- NASA Earth Observatory, Earth's Orbital Precession: how the changing axis direction shifts the timing of the seasons.
- US Naval Observatory, Astronomical Almanac glossary: the formal definition of precession of the equator and the equinox.
- University of Rochester, Precession of the Earth's Rotation Axis: a university astronomy note on the changing pole star and the drifting equinox.
- Jean Meeus, Astronomical Algorithms (Willmann-Bell): the standard reference for computing precession and equinox positions.
See how these figures are computed on the methodology and sources page.
Keep exploring
Sun's Declination & Seasons
The axial tilt that precession turns is the same tilt that makes the seasons.
InteractiveThe Lunar Nodes
Another slow turning of an orbit: the Moon's nodes, every 18.6 years.
InteractiveApsidal Precession
Orbits rotate too: the Moon's perigee and the planets' perihelia creep forward.
SkyThe 88 Constellations
The patterns precession slowly carries around the sky, with a star chart for each one.
ReferenceThe numbers and where we stand: axial precession
The Great Year in numbers: about 25,772 years per wobble, where the equinox points now, and the pole stars ahead.