Summer happens when Earth is closer to the Sun

What actually happened

Two facts dismantle this immediately. First, the southern hemisphere has summer when the northern hemisphere has winter, and they are the same distance from the Sun on the same day. A distance explanation would make the whole planet hot or cold together. Second, perihelion, the closest approach, falls around 3 January, which is precisely backwards from the myth for anyone taught it in Europe or North America.

The orbit is barely elliptical: about 147.1 million kilometers at perihelion versus 152.1 at aphelion, roughly a 3 percent difference, giving about 7 percent variation in received energy. Tilt swamps it. At high sun angle the same beam of sunlight is concentrated on a smaller patch of ground, and the day is longer, so you get more energy per square meter for more hours. That is summer.

This one is famous because of A Private Universe, the 1987 Harvard-Smithsonian film that asked Harvard graduates at their commencement, in gowns, to explain the seasons. Twenty-one of twenty-three got it wrong, mostly giving the distance answer. It is the canonical demonstration that a persistent intuitive model can survive an elite education completely intact, and it is why this entry's change date is the year that film was made rather than any discovery.