Earth Motion Lab

Overview of Earth's Movements

Earth undergoes two motions: rotation about its axis (about one day) and revolution around the Sun (about one year). Rotation produces day–night cycles and time differences. Revolution, together with axial tilt, changes the subsolar latitude, day length and noon solar altitude, producing the seasons.

Drag to rotate and scroll to zoom. Drag the date control or click Play year to watch Earth move along its orbit.

Earth's axis always points in the same direction (near Polaris), so the Northern Hemisphere alternately tilts toward and away from the Sun.

Use ← → to change chapters, R to reset the view, and F for full screen.

Earth's rotation

Earth rotates west to east about its axis: viewed from above the North Pole, it turns counterclockwise; viewed from above the South Pole, it turns clockwise.

Angular speed: approximately 15°/hour.

Linear speed: greatest at the equator, about 1670 km/h, decreasing toward the poles; at 60° it is half the equatorial speed and at the poles it is zero. Gold arrow lengths indicate linear speed.

Day–night cycle and terminator

Earth is opaque and emits no light. Its sunlit half is the day hemisphere, and the unlit half is the night hemisphere. Their boundary is the terminator. Rotation carries locations from night to day across the dawn terminator and from day to night across the dusk terminator.

Terminator ⟂ Sun raysSolar altitude is 0° at the terminator. It bisects the equator, giving equal day and night there throughout the year.

Watch Beijing, London and New York enter daylight as Earth rotates. A 15° longitude difference corresponds to 1 hour of local time, with eastern locations ahead of western ones.

Switch to the North Pole view, a common classroom diagram: rotation is counterclockwise, crossing the dawn boundary into daylight.

Earth's revolution

Earth orbits the Sun west to east in one sidereal year (365 days, 6 h, 9 min, 10 s). The orbit is a nearly circular ellipse, with the Sun at one focus. At perihelion in early January, orbital speed is higher; early July brings aphelion, when orbital speed is lower.

Obliquity 23°26′The angle between the equatorial plane (blue ring) and ecliptic plane (gold ring). Earth's axis keeps pointing near Polaris throughout the orbit.

Axial tilt makes the subsolar point move between the Tropics of Cancer and Capricorn; see the next chapter.

Earth is closer to the Sun and moves faster during the Northern Hemisphere's winter half-year, making it shorter (about 179 days) than the summer half-year (about 186 days).

Subsolar point movement

Axial tilt makes the subsolar point move between the Tropics of Cancer and Capricorn over one tropical year (365 days, 5 h, 48 min, 46 s).

Day length

The terminator divides a latitude circle into a day arc (gold) and a night arc (blue). A longer day arc means a longer day: day length = day-arc angle ÷ 15°.

The hemisphere tilted toward the Sun has longer days than nights and day length increases toward its pole. Within its polar circle, polar day.

The equator has equal day and night all year; at the equinoxes day and night are equal worldwide.

Northern Hemisphere day length is greatest at the June solstice and least at the December solstice.

As the subsolar point moves north, northern days lengthen; as it moves south, they shorten.

Noon solar altitude

Noon solar altitude is the day's maximum solar altitude. It decreases northward and southward from the subsolar point, depending on latitude difference:

North of the Tropic of Cancer: maximum at the June solstice, minimum at the December solstice.

South of the Tropic of Capricorn: maximum at the December solstice, minimum at the June solstice.

Between the tropics: the Sun is overhead twice a year (once on each tropic).

Building spacing: to give the ground floor sunlight at noon on the winter solstice, spacing ≥ building height × cot H_winter. The lower-right panel calculates this for the chosen latitude.

Solar water heaters: collector tilt = 90° − H, so sunlight strikes the panel perpendicularly.

changing of seasons

Seasonal changes in day length and noon solar altitude change the solar energy received, producing four seasons. Astronomically, summer has the longest days and highest Sun, while winter is the opposite; spring and autumn are transitions.

In-class testing