Earth's Motions: Rotation, Revolution & the Seasons
Why we have day and night, why seasons happen (the tilt, not distance), and the evidence that Earth rotates and revolves.
Rotation — day and night
Rotation is Earth spinning on its axis, once every 24 hours, from west to east. Because it turns 360° in 24 hours, it sweeps 15° of longitude every hour — which is exactly why time zones are spaced about 15° apart. Rotation gives us day and night and makes the Sun and stars appear to move across the sky from east to west.
Evidence Earth rotates: a Foucault pendulum appears to change its swing direction over the day, and moving objects (air, ocean currents) curve due to the Coriolis effect — deflecting to the right in the Northern Hemisphere.
Revolution — the year
Revolution is Earth's orbit around the Sun, once every 365¼ days. The orbit is a slightly eccentric ellipse (e ≈ 0.017), so Earth is actually closest to the Sun in early January — a fact that surprises Northern Hemisphere students freezing in winter, and a strong hint that distance is not what causes seasons.
What actually causes the seasons
The cause is the 23.5° tilt of Earth's axis, which stays pointed at Polaris all year. As Earth revolves, different hemispheres lean toward the Sun:
- Summer (for a hemisphere): tilted toward the Sun → the Sun rises higher, sunlight strikes more directly, and days are longer. More energy per square meter.
- Winter: tilted away → low Sun, slanting rays spread over more area, short days.
- Equinoxes (around March 21 and September 23): the Sun is directly over the equator, day and night are equal everywhere.
- Solstices (around June 21 and December 21): the Sun is over a Tropic — longest and shortest days.
The key idea is the angle of insolation: direct (high-angle) sunlight delivers more energy to a given patch of ground than slanting (low-angle) sunlight, which is spread thin.
The Sun's apparent path
Over a day the Sun appears to arc across the sky, highest at solar noon. In summer it rises north of east, climbs high, and sets north of west — a long, high path. In winter the path is short and low. Shadows are shortest at noon (Sun highest) and longest near sunrise/sunset (Sun low).