Earth in the Solar System

Earth is the third planet from the Sun and the largest of the four rocky inner planets. It is the only world known to support life, a circumstance linked to its distance from the Sun, its substantial atmosphere, and the presence of liquid water across much of its surface. Earth orbits the Sun at an average distance of about 150 million kilometres, a span used as a standard unit of measurement in astronomy.

Two motions govern everyday experience of time on Earth. The planet spins on its axis roughly once every twenty-four hours, producing the cycle of day and night, and it completes one orbit of the Sun in about 365 days, defining the year. Because Earth's rotation axis is tilted relative to its orbit, different parts of the planet receive sunlight more or less directly through the year. This tilt, not any change in distance from the Sun, is what produces the seasons, which occur in opposite halves of the year in the northern and southern hemispheres.

Earth has a single large natural satellite, the Moon, which orbits it roughly once a month and whose changing illumination gives the lunar phases. The gravitational pull of the Moon, together with the Sun, drives the ocean tides. Seen from space, Earth appears as a blue and white sphere, its colour reflecting the oceans, atmosphere and clouds that distinguish it from its drier, more barren planetary neighbours.

Frequently asked questions

What causes day and night on Earth?

Earth rotates on its axis about once every twenty-four hours, so each location turns toward the Sun and then away from it, giving day and then night.

Why does Earth have seasons?

Its rotation axis is tilted, so over the year sunlight strikes each hemisphere more or less directly. This changing angle, not distance from the Sun, produces the seasons.

How far is Earth from the Sun?

About 150 million kilometres on average, a distance defined in astronomy as one astronomical unit.