Space · Model
Seasons and the tilt
The Earth is five million kilometres closer to the Sun in January than in July, and January is the cold one. Whatever causes seasons, it is not how far away we are.
Start here
We are closest to the Sun in the first week of January.
The Earth’s orbit is very slightly oval. It reaches its closest point — about 147 million km — around 3 January, and its furthest — about 152 million km — in early July.
So what causes summer?
When your half is tilted towards the Sun. The Earth is actually about 5 million km closer to the Sun in early January — northern midwinter — than in early July, so distance cannot be the answer, and in any case it is summer in Australia while it is winter here. What changes is the tilt: 23.4° of it, fixed in direction, so that for half the year one hemisphere leans sunwards and for the other half it leans away.
Start with the motion that has nothing to do with seasons. The Earth turns once on its own axis every 24 hours. Half of it faces the Sun and has day; the other half faces away and has night; and as it turns, every place passes through both. The Sun is not travelling across the sky — we are turning underneath it, which is why it appears to move. That daily spin is a separate motion from the year-long journey round the Sun, and it is the year-long one that the rest of this lesson is about.
The Earth’s axis is tilted by about 23.4° away from upright, and that tilt keeps pointing in the same direction in space all year round. It does not wobble to follow the Sun. What changes is where the Earth is in its orbit, and therefore which hemisphere the fixed tilt happens to be leaning towards.
When your hemisphere leans towards the Sun, two things happen together. The Sun is above the horizon for longer, so there are more hours of warming. And it climbs higher at noon, so its light strikes the ground more squarely and the same beam is concentrated onto a smaller patch. More hours and stronger sunlight is summer.
Six months later the Earth is on the other side of its orbit, the same unchanged tilt now leans the other way, and both effects reverse. This is also why the two hemispheres always have opposite seasons, and why places on the equator — where the Sun is high all year and the day is close to twelve hours all year — barely have seasons at all.
At the bench · four dates in the orbit, three places on the Earth
Same Sun, same year, three different experiences of it.
Change a control to begin
Pick a date and a place. The tilt never changes and the Sun never changes — everything on this bench follows from which way your part of the Earth happens to be leaning.
Commit first. It is 21 June. What is happening in Sydney?
Date in the orbit
London, 52° N
What the tilt does at each of the three places, on this date
Date
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Daylight
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Sun at noon
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Energy per square metre
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Key fact
Seasons are caused by the Earth’s 23.4° axial tilt, which stays pointing the same way as the Earth orbits. The hemisphere leaning towards the Sun gets longer days and a higher noon Sun, so each square metre receives more energy. Distance from the Sun has nothing to do with it — the two hemispheres have opposite seasons at the same distance.
Think again
“It is summer when the Earth is closer to the Sun.”
The Earth is closest in early January, which is midwinter in the northern hemisphere and midsummer in the southern. One distance, two opposite seasons — so distance cannot be the cause. The variation is about 3% anyway, which is far too small to produce the difference between a January and a July in Britain. The idea survives because it sounds sensible and because nobody ever checks the date.
“The Earth’s tilt changes through the year, leaning towards the Sun in summer and away in winter.”
The axis holds an almost constant direction in space — it currently points at Polaris, which is why that star sits nearly still while the rest of the sky wheels around it. The tilt does not move; the Earth does. In June the planet happens to be on the side of its orbit where that fixed northward lean points sunwards, and in December it is on the opposite side, where the same fixed lean points away.
Mastery ladder
Not started yet.
Rungs 3 and 4 you mark yourself.
Rung 1 · Read the model
The Earth is closest to the Sun in early January. Why is that not the reason for summer?
Rung 2 · The one that catches people
Why does a high Sun warm the ground more than a low one, even for the same length of time?
Rung 3 · Explain
Explain why the UK has summer in June and Australia has summer in December, using the tilt.
Rung 4 · Take it somewhere new
Inside the Arctic Circle the Sun does not set for weeks in summer, and does not rise for weeks in winter. Explain why, and predict what the seasons would be like if the Earth had no tilt at all.
Key note
The Earth’s axis is tilted about 23.4° and keeps pointing the same way in space throughout the year. As the Earth orbits, first one hemisphere and then the other leans towards the Sun. The leaning hemisphere gets more hours of daylight and a higher Sun at noon, so sunlight arrives more concentrated and for longer, and that is summer. The other hemisphere has winter at the same moment. Distance from the Sun plays no part: the Earth is nearest in January, during northern winter.
Going further
The hottest weeks of the year come well after midsummer — usually late July in Britain, a month after the longest day. Land, sea and air take time to warm through, and while the Sun is still delivering more energy each day than the ground is losing, the temperature keeps climbing. The same lag runs the other way in winter, which is why February is often colder than December. It is called seasonal lag, and the same effect makes mid-afternoon warmer than noon.
The tilt is not permanent. Over about 41 000 years it drifts between roughly 22.1° and 24.5°, the orbit’s shape varies over about 100 000 years, and the direction of the axis swings round a full circle every 26 000. Together these Milankovitch cycles change how sunlight is shared between the hemispheres and across the year, and their rhythm shows up clearly in the timing of ice ages.
Before this lesson
Connects to
At GCSE this becomes
- The Earth–Sun geometry behind insolation, energy balance and the greenhouse effect, and orbital forcing of long-term climate change.
Where to next
Ask Mr Badmus AI
Want to check why the two hemispheres disagree about the season?
The bench is a teaching model. Solar declination is taken as 0° at the equinoxes and ±23.44° at the solstices, and daylight length is calculated from the standard sunrise equation for those declinations. That calculation ignores atmospheric refraction and the finite width of the Sun’s disc, both of which lengthen real daylight by several minutes, and it takes no account of altitude or local horizon. Noon Sun altitude is 90° minus the difference between latitude and declination. Energy per square metre is the sine of that altitude, which ignores absorption and scattering in the atmosphere.
Lesson content © MrBadmusAI.