It’s easy to imagine Earth’s orbit as a circle around the Sun. But in truth, its path is more complicated.

In AD 127, the Greek astronomer Hipparchus compared star maps made centuries before to his own maps–and found the stars were in different places.

He realized that Earth must be slowly wobbling on its axis like a top wobbles as it spins, causing it to be oriented differently to the stars.

He called this precession, which happens on a 26,000-year cycle.

In 1500, Copernicus put forth the idea that planets circled the sun.

But 100 years later, Johannes Kepler’s calculations would not match a circular orbit, so he tried an ellipse… and suddenly things lined up, establishing elliptical orbits for planets.

Later astronomers calculated that these elliptical orbits also vary—from nearly circular to much more oblong—as they’re influenced by the gravities of other planets.

These eccentric orbits cycle at intervals of about 100,000 and 400,000 years.

Around 1900, German scientist Ludwig Pilgrim found a third important cycle: the tilt of the planet’s axis changes by two and a half degrees, and back, every 40,000 years.

This change in tilt, called obliquity, combines with Earth’s wobble and eccentric elliptical orbit to dramatically impact the amount of sun the planet receives.

We’ll talk about why this matters in another EarthDate.