Earthís tilt creates seasons
By Jack Williams, USATODAY.com
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
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
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.