Earth’s Rotation: Day And Night

Why do we have day and night?

Earth is approximately spherical and only the half facing the Sun is lit at any moment. Earth rotates from west to east around an imaginary axis through the North and South Poles. One complete rotation takes about 24 hours. Most places pass through daylight and darkness during that time. Near the poles, Earth's tilted axis allows continuous daylight or darkness at some times of year.

This model explains sunrise and sunset without treating the Sun as if it travels around Earth each day. From the ground, the Sun appears to move from east to west across the sky because the Earth beneath us is turning in the opposite direction. Shadows change direction and length as the apparent position of the Sun changes.

Rotation, orbit and the seasons

Rotation is Earth's daily spin on its axis. Orbit is Earth's movement around the Sun and takes about 365 days. The seasons are linked to Earth's tilted axis as Earth travels around its orbit, not to Earth being much closer to the Sun in summer. Keeping these two movements separate helps distinguish day and night from the seasons.

A day-and-night model can show rotation clearly, but a seasons model also needs the axis to remain tilted in the same direction as Earth moves around the Sun. That changes the angle and duration of sunlight received by each hemisphere through the year.

How time zones fit the model

Different longitudes face the Sun at different stages of Earth's rotation. That is why local time differs around the world and why one country can be starting its day while another is already in darkness. Time zones organise these differences into agreed clock times; they do not cause day and night.

A useful explanation names the movement, links it to the evidence and avoids adding an unnecessary orbit. For example: Earth rotates once in about 24 hours, so our location turns towards the Sun for daylight and away from it for night.

• Rotation: one turn of Earth on its axis, about 24 hours.
• Orbit: one journey of Earth around the Sun, about 365 days.
• Apparent solar movement: the Sun seems to cross the sky because Earth rotates.
• Time zones: agreed clock regions that reflect different local stages of Earth's rotation.