Earthís tilt creates seasons

By Jack Williams,

The reason for changes in Earth's seasons is the Earth's tilt, not its distance from the sun.

In the Northern Hemisphere summer, the land north of the equator is tilted towards the sun, allowing more of the sunís energy to heat the Northern Hemisphere.

Conversely, during the Northern Hemisphere winter, the land north of the equator is tilted away from the sun, which lowers the amount of the sunís energy warming the Northern Hemisphere.

The Earth is actually closer to the sun during the Northern Hemisphere winter, but since the hemisphere is tilted away from the sun, it still feels like winter.

In fact, the Earth's average temperature is actually higher in July, when it's the farthest from the sun.

"Averaged over the globe, sunlight falling on Earth in July (aphelion) is indeed about 7% less intense than it is in January (perihelion)," says Roy Spencer of the Global Hydrology and Climate Center in Huntsville, Alabama. Still, "the average temperature of Earth at aphelion is about 4 degrees F (2.3 degrees C) higher than it is at perihelion."

Why is the Earth warmer when we're farther from the Sun?

It's because there's more land in the northern hemisphere and more water in the south. During July the land-crowded northern half of our planet is tilted toward the Sun. Land warms faster than the water. "Earth's temperature (averaged over the entire globe) is slightly higher in July because the Sun is shining down on all that land, which heats up rather easily," says Spencer.

This is explained in more detail in a story, Aphelion Away, on the Science@NASA Web site.