What actually causes the seasons?

The seasons are caused by the tilt of Earth's rotational axis, about twenty-three and a half degrees relative to its orbital plane. They are not caused by distance from the sun, which is the most persistent misconception on the subject.

The distance explanation fails immediately on two counts. Earth's orbit is very nearly circular, so the variation is small. And the northern hemisphere summer occurs when Earth is at its furthest from the sun, which is the opposite of what the theory predicts. The hemispheres experiencing opposite seasons simultaneously is the clearest evidence that distance cannot be the mechanism.

The tilt produces seasons through two effects working together. First, the angle at which sunlight strikes the surface. When a hemisphere tilts toward the sun, light arrives closer to vertical and the same energy is concentrated onto a smaller area. When tilted away, the same beam is spread across more surface, delivering less energy per square metre. Second, day length: the tilted-toward hemisphere spends more of each rotation in sunlight, so it receives energy for longer.

The axis maintains roughly the same orientation in space throughout the orbit, which is why the tilt points toward the sun at one point in the year and away from it six months later.

Solstices mark the extremes of tilt and equinoxes the points where the axis is side-on, giving roughly equal day and night everywhere.

One further detail explains a common puzzle: the hottest weather lags the summer solstice by weeks, because land and ocean take time to warm. The same lag applies in winter, which is why the coldest period follows the shortest day rather than coinciding with it.