What actually causes Earth's seasons?

Answer

Tilt changes how directly sunlight hits each hemisphere over the year. Earth is actually closest to the Sun in early January.

Context

Earth's rotation axis is tilted about 23.4 degrees from the perpendicular to its orbital plane, and the axis keeps pointing toward nearly the same spot in the sky as the planet travels around the Sun. For half the year the Northern Hemisphere leans toward the Sun, so the Sun climbs higher, days are longer, and sunlight hits the ground at a steeper angle that concentrates its energy. For the other half the Southern Hemisphere gets that advantage, which is why the seasons are opposite on either side of the equator.

A widespread misconception is that seasons come from Earth being nearer to the Sun in summer. The orbit is only slightly elliptical: Earth is closest, at about 147 million km, in early January and farthest, at about 152 million km, in early July. So Northern Hemisphere winter occurs when Earth is nearest the Sun. The distance changes sunlight by only a few percent, much less than the tilt does.

The tilt is not fixed. Over cycles of roughly 41,000 years it varies between about 22.1 and 24.5 degrees, a pattern that figures in the timing of ice ages. Simulations by Jacques Laskar indicate the Moon's gravity helps keep this swing small, and without it the tilt could wander widely, though such modeling carries uncertainty. Other planets show the effect clearly: Mars has a similar tilt and similar seasons, while Uranus, tipped by about 98 degrees, has extreme polar day and night.