Earth is closest to the Sun in January, which is awkward for the usual explanation

Most explanations of what causes the seasons stop at the point where it starts to matter. This one carries on.

The intuition and why it is reasonable

Stand near a fire and it gets warmer, so a planet closer to the Sun getting warmer is a perfectly sensible inference from ordinary experience. Filmed interviews with graduating students at a leading university famously found that most of them reached for exactly this explanation. Those were not careless people, which is the point, and the persistence of the answer says something about how intuitive models survive formal teaching.

The distance model also predicts something specific and checkable, which is more than many folk explanations manage. Its problem is that the check comes out wrong, in a way that is very hard to explain away.

The fact that breaks it

Earth's orbit is very slightly elliptical, and the closest approach happens in the first days of January each year. That is midwinter in the northern hemisphere, where most of the world's population and most of its textbook writers happen to live.

Checked against the record, meanwhile it is high summer in Argentina, Australia and South Africa, at the same distance from the Sun on the same day. One planet cannot be simultaneously too far away and too close, so distance cannot be doing the work. The variation in distance across the year is also small in proportional terms, far too small to produce the swings people experience.

What the tilt actually does

Earth's rotational axis is tilted by a little over twenty-three degrees relative to the plane of its orbit, and that tilt keeps a fixed direction in space. As the planet goes round, each hemisphere spends part of the year leaning towards the Sun and part leaning away. Leaning towards the Sun raises the Sun higher in the sky, which concentrates the same amount of energy onto a smaller patch of ground.

Checked against the record, it also lengthens the day, so the same patch is being heated for more hours and cooling for fewer. Those two consequences of a single geometric fact are more than enough to produce every seasonal pattern on the planet.

Why the classroom diagram misleads

Orbital diagrams are almost always drawn with a badly exaggerated ellipse, because a true-to-scale orbit looks like a circle and makes a dull picture. A reader looking at a squashed oval with the Sun off to one side will naturally conclude that distance is the variable being illustrated.

The tilt, meanwhile, is often shown as a small line through the globe that is easy to overlook or to read as decoration. The diagram is not lying, but it is emphasising the wrong feature, and readers take their explanation from what a picture emphasises.

A diagram drawn to scale with a prominent axis and a nearly circular path would teach the right lesson without a word of text.

The lag nobody predicts

The longest day arrives in June in the north, yet the hottest weeks usually come in July or August, which puzzles people who have just learned about tilt. The reason is thermal inertia, because land and especially ocean take time to warm up and continue absorbing more than they release for weeks afterwards.

The same lag runs in reverse at the other end, which is why the coldest period follows rather than coincides with the shortest day. Coastal regions show a longer lag than continental interiors, because water stores heat far more effectively than rock and soil. Seasonal timing is therefore set by astronomy but delivered by geography, which is why the same latitude can feel very different in different places.

Testing the explanation

The strongest test of any seasonal model is whether it predicts the southern hemisphere correctly, and only the tilt model does. A second test is the equator, where seasons in the temperate sense largely disappear and are replaced by wet and dry periods driven by circulation.

Tested properly, a third is the polar regions, where the tilt produces continuous daylight and continuous night, an outcome distance cannot generate at all. Any explanation that handles all three is doing real work, and the distance model fails every one of them. It is a rare case where a single well-chosen fact, the January perihelion, does the entire job of demolition.

The takeaway

The Sun is nearest when the northern hemisphere is coldest, and that single sentence ends the argument.

The satisfying version of a story is the one that travels, which is the whole problem.