# What Causes the Seasons? (It’s Not How Close We Are to the Sun) Science · 6 min read · updated 2026-10-30 > Earth is closest to the Sun in January, in the northern winter. So what really causes the seasons? The tilt, how it spreads sunlight, and how long the days are. ## The short answer The seasons are caused by Earth’s tilt, not by its distance from the Sun. Earth’s axis leans about 23.4 degrees, so for half the year the northern hemisphere tips towards the Sun and gets higher midday sunshine and longer days, and for the other half the southern does. Earth is actually closest to the Sun in early January, during the northern winter. In 1987, a film crew went to a graduation ceremony at Harvard and asked new graduates, and a few of their professors, a simple question: why is it hotter in summer than in winter? Nearly all of them got it wrong. Most said the Earth is closer to the Sun in summer. They answered confidently, in caps and gowns, and the film, *A Private Universe*, became a classic in science teaching because of what it showed: that a wrong idea picked up in childhood can survive an excellent education intact. The closer-in-summer idea is natural. Stand near a fire and you get hotter. Step back and you cool down. Why wouldn’t the Sun work the same way? The answer is that Earth’s distance from the Sun does change a little, but it runs the wrong way, and the real cause is something else entirely. ## The distance runs backwards Earth’s orbit isn’t quite a circle. It’s a very slightly squashed ellipse. At its closest, called *perihelion*, Earth is about 147 million kilometres from the Sun. At its farthest, *aphelion*, about 152 million. Now the dates. Perihelion comes in early January. Aphelion comes in early July. So in the northern hemisphere, we’re *closest* to the Sun in the middle of winter and *farthest* in the middle of summer. If distance drove the seasons, January would be the hottest month in London and New York. Two more clues finish off the distance idea. The difference is small: about 3%, which changes the sunlight reaching Earth by roughly 7%, not enough to turn summer into winter. And the two hemispheres have opposite seasons at the same moment. In December, when Britain is cold, Australia is having summer. Both are, near enough, the same distance from the Sun. ## The real cause: the tilt Earth’s axis, the line it spins around, isn’t upright compared with its path around the Sun. It leans by about 23.4 degrees. And it keeps pointing the same way in space all year, towards a spot near the North Star, as Earth travels around its orbit. So for part of the year, the northern half of the planet leans towards the Sun. Six months later, on the other side of the orbit, the axis still points the same way, which means the north now leans away and the south leans towards. The tilt changes two things, and both make summer warmer. ### 1. The angle of the sunlight When your hemisphere leans towards the Sun, the midday Sun is higher in the sky. Sunlight arrives at a steeper angle, so the same beam is concentrated on a smaller patch of ground. In winter the Sun is low, the beam arrives at a shallow angle, and the same energy is spread over a much bigger area. Shine a torch straight down at a table and you get a small, bright circle. Tilt the torch and the light smears into a long, dim oval. Same torch, same light, less of it on each square centimetre. The toy at the top shows exactly this with the Sun. *(An interactive illustration belongs here. Try it on the page: https://cognosc.com/blog/why-seasons-happen)* In London, which is about 51.5 degrees north, the midday Sun stands about 62 degrees above the horizon in late June and only about 15 degrees in late December. At noon, a patch of ground in June gets around three and a half times as much sunlight as the same patch in December. ### 2. The length of the day When your hemisphere leans towards the Sun, it also spends more of each turn of the Earth in daylight. In London, the longest day has over 16 and a half hours of daylight, and the shortest has under 8. More hours of sunshine and stronger sunshine together make summer. Near the poles, it gets extreme. Inside the Arctic Circle the summer Sun never sets for weeks, and in winter it never rises. Near the equator, the tilt makes little difference to the angle or the day length, which is why the tropics don’t have warm summers and cold winters, but wet and dry seasons instead. ## The four turning points The tilt gives the year four landmark days: | Date (approximately) | What happens | In the north | |---|---|---| | 21 June | The north leans most towards the Sun | Longest day, the summer solstice | | 22 September | Neither hemisphere leans towards the Sun | Day and night roughly equal, the autumn equinox | | 21 December | The north leans most away | Shortest day, the winter solstice | | 20 March | Neither leans towards the Sun | Day and night roughly equal, the spring equinox | South of the equator the names swap: 21 December is their longest day and 21 June their shortest. *(An interactive illustration belongs here. Try it on the page: https://cognosc.com/blog/why-seasons-happen)* ## See it with a lamp and an orange You can model the whole thing on a kitchen table. Push a pencil through an orange to make an axis, and tilt it a little, about a quarter of the way to horizontal. Put a lamp in the middle of the table for the Sun. Now carry the orange around the lamp in a circle, keeping the pencil pointing at the same corner of the room the whole way. On one side of the lamp, the top of the orange leans towards the light and gets the most direct beam; that’s the northern summer. Walk halfway round, still pointing the pencil at the same corner, and the top now leans away; that’s northern winter. In between, neither end leans towards the lamp: the equinoxes. Notice that at no point did you move the orange closer to or further from the lamp. The seasons appeared anyway. That’s the whole point. ## Why the hottest weeks come later If the strongest sunshine is in late June, why are July and August usually hotter? Because the land, and especially the oceans, take time to warm up. In June the north gains more heat each day than it loses, so temperatures keep climbing for weeks after the solstice, the same way a pan of water keeps warming after you’ve turned the heat up. This delay is called *seasonal lag*. The coldest weeks lag the shortest day too, coming in January and February. ## Seasons elsewhere, and in the past The tilt idea predicts that a planet without one would have no seasons, and a planet with a big one would have extreme seasons. Both are true in the solar system. Jupiter is nearly upright and barely has seasons. Mars leans about 25 degrees, close to us, and has seasons too, each about twice as long because its year is nearly twice as long. Uranus is knocked over at about 98 degrees, so each pole spends about 42 years facing the Sun and then 42 years in darkness. Earth’s own tilt isn’t fixed for ever. Over a cycle of about 41,000 years it wobbles between roughly 22 and 24.5 degrees. A bigger tilt means sharper seasons. Together with slow changes in the shape of Earth’s orbit, these cycles, named after the Serbian scientist Milutin Milanković, help explain the timing of ice ages. ## Why the wrong idea is so hard to shift The distance explanation feels right because it matches everyday experience: closer to a heat source is warmer. It’s also often reinforced by diagrams that exaggerate Earth’s orbit into a long oval, which makes the distance look important when it isn’t. The tilt explanation asks for something harder: holding in your head a planet that leans, keeps leaning the same way, and carries that lean around a circle. Once it clicks, though, it explains a lot at once: why the seasons are opposite in the two hemispheres, why days are long in summer, why the tropics are different and why the poles have midnight sun. A good explanation does that. It doesn’t just answer the question; it answers questions you hadn’t asked yet. ## Test yourself The seasons question is in the free [physics test](/test/physics) and the [space test](/test/space), along with other everyday science intuitions that turn out to be wrong. And if you liked this, [why the Moon has phases](/blog/moon-phases-explained) clears up another sky myth that catches out a lot of graduates. ## What to remember - Earth is closest to the Sun in early January, in the northern winter, so distance can’t explain the seasons. - The axis leans about 23.4 degrees and keeps pointing the same way as Earth orbits. - When your hemisphere tilts towards the Sun, sunlight arrives at a steeper angle, concentrated on less ground, and the days are longer. - The two hemispheres have opposite seasons at the same moment. - The hottest weeks come after the longest day, because land and sea take time to warm up. ## Questions people ask ### What causes the seasons? The tilt of Earth’s axis, about 23.4 degrees. As Earth orbits the Sun, each hemisphere spends half the year leaning towards the Sun and half leaning away. Leaning towards means a higher midday Sun, so sunlight is more concentrated, and longer days, so there’s more of it. ### Is Earth closer to the Sun in summer? No. Earth is closest to the Sun, about 147 million kilometres, in early January and farthest, about 152 million, in early July. That’s the northern winter and summer, so distance runs the wrong way. The difference is too small to matter much anyway. ### Why are the seasons opposite in the southern hemisphere? Because when the northern half of Earth tilts towards the Sun, the southern half must tilt away, and vice versa. So when it’s summer in Europe and North America, it’s winter in Australia, South Africa and Argentina. ### Why is the hottest weather after the longest day? Because land and especially the oceans take time to heat up. In late June the north receives the most sunlight, but the ground and sea are still catching up, so temperatures keep rising for several weeks. This is called seasonal lag. ### Do other planets have seasons? Yes, if they’re tilted. Mars leans about 25 degrees and has seasons roughly twice as long as ours. Uranus is tipped over by about 98 degrees, so each pole spends about 42 years in sunlight and then 42 in darkness. ## Sources and further reading - NASA Space Place. What causes the seasons? - Schneps, M. H. and Sadler, P. M. (1987). A Private Universe (documentary). Harvard-Smithsonian Center for Astrophysics. - US Naval Observatory. Earth’s seasons: equinoxes, solstices, perihelion and aphelion. - Met Office. What is seasonal lag? - Berger, A. (1988). Milankovitch theory and climate. Reviews of Geophysics, 26(4), 624–657. --- Canonical page: https://cognosc.com/blog/why-seasons-happen From Cognosc (https://cognosc.com): free tests, articles and an AI tutor that has you reason things through.