Seasons are not caused by Earth moving closer to and farther from the Sun. The orbit is a slight ellipse, but that is not what makes winter and summer. The axis of rotation is tilted about 23.5 degrees relative to the plane of the orbit. As the planet goes around the Sun, that tilt stays pointed the same way in space, roughly toward Polaris. For part of the year the Northern Hemisphere leans toward the Sun: days lengthen, sunlight strikes more directly, and summer arrives. Six months later that hemisphere leans away: the Sun sits lower, days shorten, and winter comes. The Southern Hemisphere has the opposite season at the same time.

Near the equator the tilt barely changes day length, so wet and dry periods often matter more than hot and cold. Near the poles, summer can mean months of light and winter months of dark. The Arctic and Antarctic take turns pointing at the Sun.

If the axis were upright, every latitude would have a steadier year, and the climate belts would not swing. Ice ages and ice-free worlds also depend on slow changes in tilt and orbit over tens of thousands of years, a set of rhythms called Milankovitch cycles. The calendar of planting and harvest is, at root, geometry: a spinning ball, tipped, circling a lamp.
