July 4th is Earth at Aphelion

You learned it in school, astronomers say it all the time, it's The Truth: "Earth circles the Sun." Well... almost.

Earth does go around the Sun, but not in a circle. Earth's orbit is an ellipse, a lopsided curve with one end closer to the Sun than the other.

On July 4, our planet is at the distant end--a point astronomers call "aphelion." This puts us farther from the Sun than we are at any other time of year.

"All planets in our solar system travel around the Sun in elliptical orbits. It's Kepler's 1st Law," explains University of Florida astronomy professor George Lebo. "The eccentricity of Earth's orbit is 1.7%. In January when we're closest to the Sun (perihelion), the distance is 147.5 million km. In July we're 152.6 million km away--a five million kilometer difference."

A distant sun means less sunlight for our planet. "Averaged over the globe, sunlight falling on Earth at aphelion is about 7% less intense than it is at perihelion," says Roy Spencer of NASA's Global Hydrology and Climate Center (GHCC).

Then why is it so warm outside?

"Seasonal weather patterns are shaped primarily by the 23.5 degree tilt of our planet's spin axis, not by aphelion or perihelion," continues Lebo. "During northern summer the north pole is tilted toward the Sun. The Sun climbs high in the sky, and days are long. That's what makes July so hot." (Note: seasons are reversed in the two hemispheres, north and south. So July is generally cold in the southern hemisphere.)

But there's more to the story: Says Spencer, "the average temperature of the whole earth at aphelion is about 4F or 2.3C higher than it is at perihelion." Our planet is actually warmer when we're farther from the Sun: plot it!