Tilta

Meet Tilta, Earth's tilted spin axis, the real reason we have seasons.

Earth spins on an axis tilted about 23.4° from the perpendicular to its orbit. The axis keeps pointing in nearly the same direction in space all year. So for part of the year the Northern Hemisphere leans toward the Sun and for part it leans away.

Leaning toward the Sun does two things. Sunlight arrives more directly, concentrating its energy on each square metre of ground, and the days are longer. Both warm the summer hemisphere. Six months later, the same hemisphere leans away and gets winter. That is why the seasons are reversed between the hemispheres.

Distance is not the cause. Earth is closest to the Sun in early January and farthest in early July, during northern summer. The difference is only about 3 percent.

The tilt is not fixed forever. It varies between about 22.1° and 24.5° over a cycle of about 41,000 years, and the axis also slowly wobbles. These changes are among the slow orbital cycles linked to ice ages.