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. The 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.

axis
The Unisphere tilts on its axis.

Right-Hand Rule
The "right-hand rule" helps amateur astronomers understand axial tilt. When the fingers of the right hand are curled in the direction of the planet's rotation, the thumb points in the direction of the planet's North Pole.

astronomer
Noun

person who studies space and the universe beyond Earth's atmosphere.

axial tilt
Noun

angle between an object's axis of rotation and its orbital axis, perpendicular to the orbital plane. Earth's axial tilt is about 23.5 degrees.

Noun

an invisible line around which an object spins.

celestial
Adjective

having to do with the sky or heavens.

center of mass
Noun

point where an object appears "balanced," where an outside force acting on the object acts as if the object were located at just that point.

Noun

our planet, the third from the Sun. The Earth is the only place in the known universe that supports life.

Noun

imaginary line around the Earth, another planet, or star running east-west, 0 degrees latitude.

Noun

distance north or south of the Equator, measured in degrees.

mass
Noun

measure of the amount of matter in a physical object.

North Star
Noun

the star Polaris, located roughly above the North Pole. Also called the Lodestar or Pole Star.

orbital axis
Noun

angle perpendicular to an object's orbital plane

orbital plane
Noun

flat space in which a body orbits.

particle
Noun

small piece of material.

perpendicular
Noun

at a right angle to something.

Noun

large, spherical celestial body that regularly rotates around a star.

Polaris
Noun

star that is currently located roughly over the North Pole. Also called the North Star or Lodestar.

precession
Noun

slow change in the direction of the axis of the Earth or another rotating body.

rotate
Verb

to turn around a center point or axis.

Noun

object's complete turn around its own axis.

Noun

period of the year distinguished by special climatic conditions.

Noun

fixed point that, along with the North Pole, forms the axis on which the Earth spins.

star
Noun

large ball of gas and plasma that radiates energy through nuclear fusion, such as the sun.

Noun

rise and fall of the ocean's waters, caused by the gravitational pull of the moon and sun.