What Actually Causes the Seasons

• The cause is a 23.4° axial tilt that stays pointed in a fixed direction as Earth orbits.
• The tilt does two things: it changes the angle sunlight strikes the ground, and it changes the length of the day.
• Earth's distance from the Sun varies by only about 3%, and the timing is backwards for the distance explanation.
• The hottest weeks lag the solstice by a month or two because of thermal inertia, not because of orbital position.
• Milankovitch cycles — slow wobbles in tilt, orbit shape and axis direction — are what turn seasons into ice ages.

Killing the distance explanation

Start with the numbers, because they settle it quickly.

Earth's orbit is an ellipse, but a very nearly circular one — its eccentricity is about 0.0167. In practical terms:

	Distance	Date
Perihelion (closest)	~147.1 million km	Around 3 January
Aphelion (farthest)	~152.1 million km	Around 4 July

That is a difference of roughly 3.3% in distance, which works out to about 7% in the intensity of sunlight arriving at the top of the atmosphere. Not nothing — but small, and pointing the wrong way for the northern hemisphere, which contains most of the world's population and most of the people who invented the intuition.

There is a second, even more decisive objection. If distance caused seasons, both hemispheres would have summer at the same time. They obviously do not. December is midsummer in Sydney and midwinter in Stockholm, on the same day, at the same distance from the Sun. No distance-based explanation survives that fact.

The tilt, and the two things it does

Earth's rotational axis is tilted about 23.4° away from perpendicular to its orbital plane. The critical detail — the one that makes the whole mechanism work — is that the axis keeps pointing in the same direction in space as Earth travels around the Sun. It does not swivel to keep facing the Sun. The north pole points at Polaris in June and at Polaris in December.

So there is a point in the orbit where the northern end of the axis leans toward the Sun, and, six months later on the far side, the same fixed axis now leans away. Nothing about the Earth changed. Only its position relative to the Sun did.

That geometry then produces two separate heating effects, and most explanations mention only the first.

1. The angle of the light

When the Sun is high in your sky, its light strikes the ground close to head-on and a given beam is concentrated into a small patch. When the Sun is low, the same beam hits at a slant and spreads across a much larger patch — the same energy, thinner on the ground.

Anyone who has aimed a torch at a wall has seen this: perpendicular gives a small bright circle, tilted gives a big dim ellipse.

The effect is larger than people expect. At mid-latitudes — say 45° north — the noon Sun sits about 47° higher in the sky at the June solstice than at the December solstice. That is nearly half the way from horizon to zenith, and it dramatically changes how concentrated the sunlight is.

A slanted path through the atmosphere adds a secondary penalty: low-angle light travels through more air, so more of it is scattered and absorbed before it reaches you. This is the same effect that makes a sunset dim enough to look at directly.

2. The length of the day

The second effect is more intuitive but often skipped: when your hemisphere is tilted toward the Sun, you spend more hours per rotation on the sunlit side.

Picture the terminator — the line dividing day from night. On a tilted planet, that line does not run pole to pole. It cuts across at an angle, so high northern latitudes sit almost entirely on the lit side in June and almost entirely on the dark side in December.

At the extreme this becomes total. Above the Arctic Circle at 66.6°N, there is at least one day when the Sun never sets. That latitude is not a convention: it is 90° − 23.4°, precisely the point at which the tilt is enough to keep a location in sunlight through a full rotation.

The two effects reinforce each other rather than trading off — summer gives you more hours of stronger sunlight, winter gives you fewer hours of weaker sunlight. That compounding is why the seasonal temperature swing at mid-latitudes is so much larger than a mere 7% change in solar intensity could ever produce.

Why the hottest month is not June

If the June solstice is the day of peak solar energy in the northern hemisphere, why is late July usually hotter?

Because temperature is not set by how much energy arrives — it is set by the balance between energy arriving and energy radiating away, and land and ocean have enormous heat capacity. After the solstice, incoming energy declines but still exceeds outgoing energy for weeks. The surface keeps net-accumulating heat, and temperature keeps climbing, until the two curves cross. Only then does it start to fall.

This is thermal lag, and it typically runs about four to eight weeks at mid-latitudes — longer near oceans, which store heat far better than land, and shorter in continental interiors. The same physics on a shorter cycle explains why the warmest part of the day is mid-afternoon rather than local noon, when the Sun is actually highest.

The solstices and equinoxes

Four moments in the orbit have names, and each is a specific geometric event rather than a season boundary by convention.

Event	Approximate date	What is happening
March equinox	20 March	Axis tilted neither toward nor away; day and night near-equal everywhere
June solstice	21 June	North pole maximally tilted toward the Sun
September equinox	22 September	Tilt again side-on
December solstice	21 December	North pole maximally tilted away

"Solstice" comes from Latin for sun stands still — around those dates, the Sun's noon height stops changing and reverses direction, so its apparent drift pauses. "Equinox" means equal night, though it is only approximately true: atmospheric refraction bends sunlight over the horizon, and sunrise and sunset are defined by the Sun's upper edge rather than its centre, so the day of exactly equal daylight falls a few days off the equinox.

Between the Tropic of Cancer (23.4°N) and the Tropic of Capricorn (23.4°S), the Sun passes directly overhead twice a year. Those lines mark the furthest the overhead Sun ever reaches — which is, once again, just the tilt written onto the map.

A footnote the southern hemisphere gets

Because southern summer coincides with perihelion, the southern hemisphere receives its summer sunlight at slightly higher intensity than the north does. And by Kepler's second law, Earth moves fastest when closest to the Sun, so the southern summer is also slightly shorter — by around five days.

In principle that should make southern summers marginally hotter and winters marginally colder. In practice the effect is swamped, because the southern hemisphere is overwhelmingly ocean, and ocean moderates temperature swings far more powerfully than a 7% difference in insolation enhances them. Geography beats orbital mechanics at this scale.

When the tilt itself moves

None of these parameters is truly fixed. Over tens of thousands of years, three of them cycle:

• Obliquity — the tilt itself oscillates between roughly 22.1° and 24.5° on a ~41,000-year cycle.
• Eccentricity — the orbit's shape stretches and relaxes on cycles of roughly 100,000 and 400,000 years.
• Precession — the axis traces a slow cone, like a spinning top, over about 26,000 years, changing which season coincides with perihelion.

These are the Milankovitch cycles, and their combined effect on how sunlight is distributed across latitudes and seasons is the leading explanation for the timing of glacial and interglacial periods over the last few million years. The seasons you experience are one turn of a much slower set of gears.

One last thing worth appreciating: our tilt is unusually stable, and the Moon is why. A large moon acts as a gyroscopic anchor on Earth's axis. Mars, with only two tiny moons, has an obliquity that wanders chaotically over tens of degrees across millions of years — which means its seasons, and any climate depending on them, have no long-term consistency at all. Ours do, and that stability is part of the backdrop to everything that has lived here.

Further reading: NASA Space Place has a clear illustrated explainer on what causes the seasons. The Harvard commencement interviews are from the 1987 documentary A Private Universe, produced by the Harvard-Smithsonian Center for Astrophysics and still one of the best pieces of evidence that a confident explanation is not the same as a correct one.