Skywatch

  • January 4, 2007

    Episode 76: What Causes Seasons?

    Download this episode

    Shownotes

    On Jan. 4, 2006, the Earth reached perihelion, or the closest point in its elliptical orbit to the Sun. Yet the Northern Hemisphere isn't exactly toasty in the beginning of January. What gives? It's actually the tilt of the Earth that causes the changes in the seasons, not the proximity of the Earth to the Sun. Right now the Northern Hemisphere is tilted away from the Sun, meaning less sunlight hits the surface. The less sunlight, the colder it is. How sunlight affects the Earth depends on where it hits. Ground heats more quickly than water, so continents react more swiftly to changes in the amount of sunlight than oceans.