An axis is an invisible line around which an object rotates, or spins. The object can be a tiny particle, smaller than a single atom. Or it could be a star with the mass of a thousand suns.

In either case, an object's axis runs through its center of mass, or barycenter. An object's center of mass is a point where an outside force acting on the object acts as if the object were located at just that point—where the object appears "balanced." Earth's center of mass actually varies. Ocean tides shift the center of mass, although not enough to radically shift the planet's axis.

Each planet in our solar system rotates on its axis. So, each planet has a North and South Pole, the points where an axis meets the planet's surface.

The time it takes for a planet or other celestial object to complete one spin around its axis is called its rotation period. Earth's rotation period is about 24 hours, or one day.

Axial Tilt

Some planets, such as Mercury, Venus, and Jupiter, have axes that are almost completely perpendicular, or straight up-and-down.

Earth's axis is not perpendicular. It has an axial tilt, or obliquity. Axial tilt is the angle between the planet's rotational axis and its orbital axis. A planet's orbital axis is perpendicular to to the ecliptic or orbital plane, the thin disk surrounding the sun and extending to the edge of the solar system.

Earth's axial tilt (also known as the obliquity of the ecliptic) is about 23.5 degrees. Due to this axial tilt, the sun shines on different latitudes at different angles throughout the year. This causes the seasons.

Uranus has the largest axial tilt in the solar system. Its axis is tilted about 98 degrees, so its north pole is nearly on its equator. Astronomers suspect that this extreme tilt was caused by a collision with an Earth-sized planet billions of years ago, soon after Uranus formed.

Axial Precession

Earth's axis appears stable, but it actually wobbles very slowly, like a spinning top. It takes Earth's axis about 26,000 years to complete a circular "wobble." This wobble is called axial precession.

Earth’s axis helps determine the North Star, and axial precession helps change it. Currently, for instance, Earth's axis points toward a star called Polaris. Polaris, which gets its name because it is almost directly above the North Pole, is the current North Star.

Polaris will not always be the North Star, however. Earth's axis is slowly wobbling away from Polaris. In another 13,000 years, it will point toward the new North Star, a star called Vega.