The amount of daylight we experience varies throughout the year from place to place. Some places have longer days and nights than others, and the length of each day changes with the seasons. Find out why Earth’s tilt is the reason we have seasons.
[Nearly circular path, centered around Sun, traces Earth’s counterclockwise movement around the Sun.]
The times of sunrise and sunset, and the amount of daylight, vary throughout the year.
Why do we have more light in summer and less in winter?
Earth’s orbit is an ellipse, not a circle.
Could this be the source of our seasons?
[Circle appears, centered on Sun, touches left side of Earth’s path but does not reach right side. Dots appear where the path is farthest from circle, labeled “Aphelion;” and where circle overlaps path, labeled “Perihelion.”]
Aphelion Earth is farthest from the Sun
Perihelion Earth is closest to the Sun
[Aphelion and Perihelion labels change to “July 4” and “January 3.”]
Earth is farthest from the Sun during the Northern Hemisphere’s summer.
So, Earth’s varying distance does not affect the seasons or the amount of daylight we receive.
[Circle disappears]
To see what’s going on, we’ll magnify Earth 3000 times and reduce its spin.
[Earth grows to 3000 times. Slowly rotates so that one side constantly faces the Sun as it orbits. Earth’s North Pole is fully lit and fully dark at different times of the year.]
Here’s a view of that tilt from a different angle.
[Split screen has time-synced side-view of Earth’s orbit. An arrow marks the direction of tilt, in a constant direction throughout the year.]
In December, the North Pole is tilted away from the Sun.
[Dates are labeled on along the first orbit: September 22, December 21, March 20, June 21]
In June, the North Pole is tilted toward the Sun.
[Side view of orbit fades to side view of Earth, North Pole (top) and South Pole (bottom) marked. Left side of Earth is sun-lit, and right side is night-dark.]
The day-night line is called the terminator.
[Arrow on orbiting Earth indicates point of view shown for terminator-view.]
Observing the terminator shows how sunlight varies throughout the year.
[Orbiting Earth pauses at December 21. Terminator line is tilted left so North Pole is dark and South Pole is lit.]
We’ll pause annual motion to compare two Decembers days and nights at different latitudes.
The greatest extremes of daylight and darkness occur at the poles.
[Orbit view fades. Terminator moves as sun sets on that side of the Earth. Small circles Appear near North and South Poles, then grow out to show all- sky, fish-eye view, at 80˚N and 80˚S. 80˚N remains dark with stars rotating across its sky. At 80˚S the Sun skims southern horizon but does not set.]
The time of year, the Sun is never above the horizon at far northern latitudes.
Nor does it dip below the horizon at the southernmost latitudes.
What about mid-latitudes?
[On Terminator-view, three more small circles appear between 80˚N and 80˚S circles, along the same line of longitude. Two grow out to show all-sky views at 40˚N and 40˚S, then center circle grows out to show view at Equator. Sun rises and sets in each view as their marked location on Terminator view are lit and darkened. Path of Sun across day-time skies varies with latitude.
Views return to Terminator with left side lit, and magnified Earth in orbit. Annual time resumes.]
During the equinoxes, everyone experiences equal daylight and darkness.
[Annual time pauses with Earth at March 20. Orbit-view fades. All-skies of the 5 latitudes return. Sun rises and sets simultaneously for all five, due east and due west, respectively, for 2 days. It’s path during the day varies by latitude, closer to the north and south horizons at higher south and north latitudes, respectively.]
Views return to Terminator with left side lit, and magnified Earth in orbit. Annual time resumes.]
In June, the polar extremes of December are reversed, as the North Pole now tilts sunward.
[Annual time pauses with Earth at June 21. Orbit-view fades. All-skies of the 5 latitudes return. 80˚S remains dark with stars rotating across its sky. At 80˚N the Sun skims southern horizon but does not set. Path of Sun across day-time skies varies for other latitudes.
Views return to Terminator with left side lit, and magnified Earth in orbit. Annual time resumes.]
The tilt of Earth’s rotational axis determines the amount of sunlight we receive each day…
…and is responsible for our seasons.
Without the tilt, every day would be an equinox…
…and seasons would cease to exist.
[All fade to credits.]