1. Don’t look to Pete Seeger’s lyric “turn, turn, turn” to explain the seasons. It’s really tilt, tilt, tilt. Earth has seasons because its axis is tilted about 23.5 degrees, exposing the Northern and Southern hemispheres to fluctuating amounts of direct sunlight during the year.
2. Contrary to popular belief, Earth’s distance from the sun, which changes along its elliptical orbit, has nothing to do with seasonal differences in temperature and sunlight. (That orbit shifts over a roughly 100,000-year cycle, however, and, at its most extreme, may increase solar radiation by up to 30 percent, affecting overall climate.)
3. Most astronomers believe that, about 4.5 billion years ago, a Mars-size body knocked into our young planet, causing the axial tilt. By the way, that tilt, or obliquity, varies over a roughly 40,000-year cycle, from 22.1 to 24.5 degrees.
