Precession of the Equinoxes

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 25,920 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 3000 BCE, when the Egyptian pyramids were being 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 Great Year the pole circles back to Polaris again. The left-hand view traces that entire 25,920-year journey among the stars.

The drifting equinox and the ages

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 every 72 years. A full zodiac sign, roughly 30° wide, therefore takes about 2,160 years to cross, the span people call an astrological "age." This backward drift is why the spring equinox, which lay in Aries two thousand years ago, now sits in Pisces and is edging toward Aquarius. (The constellation boundaries are not sharp, so the dates of the ages are only ever approximate.)

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.