Earth's rotation and the day–night cycle

Earth spins about an imaginary line called its axis. This axis is tilted, and Earth completes approximately one rotation every 24 hours.

At any moment, the side of Earth facing the Sun receives sunlight and experiences daylight. The side facing away from the Sun experiences night. As Earth rotates, a location is carried from the daylight side to the night side and back again. This produces the repeating cycle of day and night.

The Sun’s apparent daily motion

During the day, the Sun appears to move across the sky, generally from east to west. This is an apparent motion caused by Earth’s rotation: as the ground beneath an observer turns, the direction in which the observer sees the Sun changes. The Sun does not travel around Earth each day to produce this motion.

Earth's orbit and the seasons

Earth travels around the Sun once approximately every 365 days. Its rotation axis is tilted relative to its orbit. As Earth travels around the Sun, the axis keeps approximately the same orientation in space.

Because of this tilt, each hemisphere is tilted towards the Sun during part of the orbit and away from it during another part. The amount and direction of sunlight it receives therefore change through the year.

How the tilt produces seasons

When a hemisphere is tilted towards the Sun, it has longer daylight hours, giving the ground more time to warm. Sunlight also reaches it more directly, so the energy is concentrated over a smaller area. Together, these effects produce warmer summer conditions.

When the same hemisphere is tilted away from the Sun, it has shorter daylight hours and receives sunlight less directly. The energy is spread over a larger area, and there is less time for the ground to warm. These conditions produce a colder winter.

The hemispheres experience opposite seasons: when the Northern Hemisphere is tilted towards the Sun and has summer, the Southern Hemisphere is tilted away and has winter. About half an orbit later, the situation is reversed. The pattern repeats as Earth completes each orbit.

The Moon's phases

The Sun illuminates one half of the Moon at any time. As the Moon orbits Earth, our view of that illuminated half changes. The different apparent shapes of the illuminated portion are called the phases of the Moon.

At new Moon, the illuminated side faces mostly away from Earth, so little or none of it is visible to us. As the Moon moves around Earth, more of its illuminated side comes into view. The visible illuminated portion is then waxing, or growing.

At full Moon, the illuminated side faces mostly towards Earth, so we see almost all of it. As the Moon continues its orbit, the visible illuminated portion becomes smaller. It is then waning, or shrinking, until the Moon returns to the new Moon phase.

The Moon orbits Earth in approximately one month, and the cycle of phases repeats approximately monthly. The phases are caused by our changing view of the Moon’s sunlit side, not by Earth’s shadow falling on the Moon.