Your turn
Each lab is one part of the video. Drag, slide and see what changes.
Each lab isolates one part of the geometry. The drawings are not to scale.
Slide the sliders and watch what happens.
Slide through the year. Earth is closest in early January, when the north has winter.
The distance varies by only about 3% (91.4 to 94.5 million miles), and the hemispheres always have opposite seasons.
Slide through the year. Is it summer when the Earth is close to the Sun?
The orbit is stretched here so you can see the change. Really it is nearly a circle.
Drag Earth around the Sun. Which half leans toward the Sun in June? In December?
The axis keeps pointing at Polaris all year; only Earth’s position in its orbit changes.
Move the Earth around the Sun. Watch the top half lean in, then lean away.
Raise and lower the Sun. How much light lands on each square of ground?
Energy per square metre scales with sin(altitude): Denver’s noon Sun gives about 96% in June and 45% in December.
Move the Sun up high, then down low. When is the light spot small and bright?
Pick a latitude and a date. How many hours is the Sun up?
Try 66.6°N in June and December, then the equator: the swing grows with latitude.
Pick a place and a month. Is the day long or short?
Set the tilt, then drag Earth around the Sun. Watch your noon Sun and a friend’s in the south.
At 0° the noon Sun never changes; at about 98° (Uranus) each pole gets a 21-year dark winter. The south is always opposite.
Tap the ring or slide the date. Watch the tree, the sky and the day length change.
The ground tint lags the solstice by weeks, as the warmest weather does: land and oceans heat slowly.
Slide through the year. When does the tree have leaves? When is there snow?
In winter it's cold and the days are short. In summer it's warm and the days are long. Why? It's not because the Earth gets closer to the Sun. The Earth is closest to the Sun in January, when it's winter in the north!
The Earth spins like a top. One spin takes one day: in most places that brings a daytime and a night. But the Earth does not stand up straight. It leans over a little, and it keeps leaning the same way all the way around the Sun.
So first the top half of the Earth leans toward the Sun. That's summer up top. Later, the bottom half leans in and gets its summer. When it's summer in the USA, it's winter in Australia!
Shine a flashlight straight down, and you get a small, bright spot. Tip it, and the light spreads out, big and dim. In summer the Sun is high, and its light is strong. In winter it's low, and its light is spread thin.
In summer the Sun comes up early and goes down late. In winter it gets dark early. Near the North Pole, the Sun does not come up at all in winter!
Earth has seasons because its axis is tilted by about 23.5 degrees (23.4° to be precise).74 As Earth travels around the Sun, that tilted axis keeps pointing the same way in space, toward the North Star.3 So for part of the year the Northern Hemisphere leans toward the Sun, and six months later the Southern Hemisphere does.1
The half that leans in gets two advantages: higher, more direct sunlight and longer days. That's summer. The half that leans away gets low, spread-out sunlight and short days. That's winter.4
This is the most common wrong answer, and it sounds sensible. Earth's orbit really is a slightly stretched circle, so our distance from the Sun does change.1 But look at the dates:
| When | Distance from the Sun | Northern Hemisphere season | |
|---|---|---|---|
| Perihelion (closest) | Early January (January 3 in 2026) | About 91.4 million miles | Winter |
| Aphelion (farthest) | Early July | About 94.5 million miles | Summer |
Sources: NASA Space Place and NASA APOD.111 The north has winter when we're closest to the Sun. The whole change is only about 3%. That changes the sunlight a little, but it can't explain strong seasons that are opposite in the north and the south.4 And there's a second clue: if distance caused the seasons, the whole planet would have summer at the same time. Instead, the two hemispheres always have opposite seasons.4
Earth spins around an imaginary line through its poles called its axis, one turn a day. That spin gives us day and night.1 Scientists think that long ago, something very big (a planet-sized body nicknamed Theia) crashed into the young Earth and knocked it over. The rubble from that crash probably became our Moon.12 The tilt isn't perfectly fixed, either. Over very long stretches of time it slowly shifts between about 22 and 24.5 degrees.6
The tilt changes two things, and they work together.
Try this with a flashlight and a sheet of paper. Shine the light straight down and you get a small, bright circle. Now tip the paper: the circle stretches into a bigger oval. It's the same amount of light, but it's spread over more paper, so each square centimetre gets less.5
Sunlight works the same way. When the Sun is high in the sky, its light lands almost straight down and heats the ground strongly. When the Sun is low, the same light is spread thinly and heats less.54 Outside the tropics (in Denver, for example), the noon Sun's height changes by about 46 degrees between the two solstices.4
sunlight on each square ≈ sin(Sun's height)
For example, in Denver (about 40°N) the noon Sun is roughly 74° high in late June but only about 27° high in late December. That means a patch of ground there gets about 96% of full-strength sunlight at June noon, but only about 45% in December: less than half.45 (Our calculation, from the tilt and the spread-out-light rule.)
Because the axis leans, the line between day and night misses the poles for most of the year (it passes through them only at the equinoxes). In June, places in the north spend most of each spin in daylight. In parts of the United States the Sun is up for as long as 15 hours in June, but only 9 or 10 hours in December.46 More hours of stronger sunlight means more heating: summer. The farther you go from the equator, the bigger the swing:
| Place | Daylight in mid-June | Daylight in mid-December |
|---|---|---|
| Near the equator | About 12 hours | About 12 hours |
| Athens, Greece | 14.5 hours | 9.5 hours |
| Helsinki, Finland | 18.5 hours | Less than 6 hours |
| Arctic Circle (about 66.5°N) | 24 hours (midnight Sun) | No sunrise at the solstice |
| North Pole | About 6 months of daylight, then 6 months of darkness | |
Sources: National Geographic Education, OpenStax, NWS.1046 Near the equator the seasons are mostly about rain, wet and dry, rather than temperature.4
When the North Pole leans toward the Sun in June, the South Pole must lean away. So June is winter in Argentina and Australia, and December is summer there.103 NOAA's version: if you go to South Africa for the December holidays, bring your swimsuit and leave your skis at home.2
| Event | Usual date | 2026 (Eastern Time) | What happens |
|---|---|---|---|
| March equinox | March 20–21 | Mar 20, 10:46 am | Sun over the equator; about 12 h of day and night everywhere |
| June solstice | June 20–21 | Jun 21, 4:24 am | North leans in most; Sun overhead at the Tropic of Cancer (23.5°N); longest day in the north |
| September equinox | September 22–23 | Sep 22, 8:05 pm | Sun over the equator again |
| December solstice | December 21–22 | Dec 21, 3:50 pm | South leans in most; Sun overhead at the Tropic of Capricorn; shortest day in the north |
Dates from the U.S. Naval Observatory via the National Weather Service.510 Solstice comes from Latin for "the sun stands", and equinox means "equal night".5 Day and night aren't exactly equal at an equinox: the air bends sunlight, so we see the Sun a few minutes before it really crosses the horizon in the morning and a few minutes after it crosses it in the evening.54
Because land, air and oceans take time to warm up, just as a pond is warmest in the late afternoon rather than at noon. In the Northern Hemisphere the hottest months are July and August, and the coldest weather comes a month or more after the December solstice.4
Astronomers start the seasons at the solstices and equinoxes, so each season lasts 89 to 93 days. Weather scientists use whole months instead: spring is March–May, summer June–August, autumn September–November and winter December–February, in the north.9
With no tilt at all, the noon Sun would reach the same height every day of the year, and there would be no seasons. A smaller tilt would give milder seasons.4 Other planets show what a different tilt does. Mars is tilted 25°, close to Earth, and has seasons that last longer than ours because its year is longer.13 Uranus is tipped about 98°, nearly on its side, so it has the most extreme seasons in the solar system: each pole gets a dark winter that lasts 21 years.8
Those long summer days and short winter days are where clocks come in, too: see why daylight saving time exists. The Moon, probably made from the rubble of the crash that tipped Earth over, has its own cycles: see why the Moon has phases and what causes a solar eclipse.
Earth's axis is tilted by about 23.5 degrees. As Earth travels around the Sun, first the northern half and then the southern half leans toward the Sun. The half that leans in gets higher, more direct sunlight and longer days, so it has summer. The half that leans away has winter.
In summer the Sun climbs higher in the sky, so its light hits the ground more directly and each patch of ground gets more energy. The Sun is also up for more hours each day. In winter the low Sun spreads its light thinly, and the days are short.
Not in northern summer. Earth is closest to the Sun in early January (about 91.4 million miles) and farthest in early July (about 94.5 million miles). That's northern winter and summer. The distance changes by only about 3%, which is too little to cause seasons.
When the North Pole leans toward the Sun in June, the South Pole leans away, so Australia, Argentina and South Africa have winter while North America and Europe have summer. Six months later it's reversed.
No. Any planet with a tilted axis has seasons. Mars is tilted 25 degrees and has seasons that last longer than ours. Uranus is tipped about 98 degrees, so each pole gets 21 years of darkness in its winter.
Shine a flashlight straight down onto paper, then tip it: the spot gets bigger and dimmer. Sunlight does the same. Then tilt an orange on a pencil and walk it around a lamp, keeping the pencil pointing at the same wall. The top half gets the direct light for half the trip, and the bottom half gets it for the other half.
At a solstice (around June 21 and December 21) one hemisphere leans toward the Sun as much as it can, giving the longest and shortest days of the year. At an equinox (around March 20 and September 22) neither hemisphere leans toward the Sun, and day and night are each about 12 hours everywhere.