Earth reaches aphelion on the 4th of July. Curiously, our planet is warmest when we are farthest from the Sun.

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You learned it in school. Astronomers say it all the time. It's the Truth: "Earth circles the Sun."

Well... almost.

Earth does travel around the Sun, but the path is not a perfect circle. It's an ellipse, slightly lopsided. One end is a bit closer to the Sun than the other.

On July 4th, 2003, our planet will reach the distant end -- a point astronomers call "aphelion." We'll be farther from the Sun than we are at any other time of the year.

Above: The sun setting behind the Statue of Liberty is a little more distant than usual on the 4th of July.

"All planets in our solar system travel around the Sun in elliptical orbits. It's Kepler's 1st Law," says 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. This weekend we will be 152.6 million km away--a five million kilometer difference."

Editor's Note: Do you have trouble remembering the difference between perihelion and aphelion? An old astronomer's trick is to recall that the words "away" and "aphelion" both begin with the letter "A".

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 4oF or 2.3oC higher than it is at perihelion." (See the global temperature data at the GHCC web site.) Our planet is actually warmer when we're farther from the Sun. Strange but true.

Above: Earth's land-masses are found more north of the equator than south. But it wasn't always that way. Image credit and copyright: the PALEOMAP Project.

This happens because continents and oceans aren't distributed evenly around the globe. There's more land in the northern hemisphere and more water in the south. During the month of July the land-crowded northern half of our planet is tilted toward the Sun. "Earth's temperature (averaged over both hemispheres) is slightly higher in July because the Sun is shining down on all that land, which heats up rather easily," says Spencer.



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Physicists would say that continents have low heat capacity. "Consider the desert," says Bill Patzert, an oceanographer at NASA's Jet Propulsion Laboratory. "At night the desert is cold, perhaps only 60F (16C). When the Sun rises in the morning the temperature might jump to 100F (38C) or more." Such mercurial behavior is characteristic of materials like rocks and soil with low heat capacity. It doesn't take much sunlight to substantially elevate their temperature.

Water is different. It has high heat capacity. "Let's say you went sailing off Malibu Beach at noon," continues Patzert. "The offshore temperature might be 75o F (24o C) -- pretty pleasant!" What happens after sunset? "The temperature drops, but only a few degrees because the heat capacity of the ocean is so high."

All this explains why July is our planet's warmest month: Northern continents baked by the aphelion Sun elevate the average temperature of the entire globe. January, on the other hand, is the coolest month because that's when our planet presents its water-dominated hemisphere to the Sun. "We're closer to the Sun in January," says Spencer, "but the extra sunlight gets spread throughout the oceans." Southern summer in January (perihelion) is therefore cooler than northern summer in July (aphelion).

: Earth's orbit is eccentric but not nearly so much as the orbits of Mars or Mercury. In this diagram solid lines trace each planet's elliptical path around the Sun. The dotted lines show circular orbits with the same mean radius. For more information, please visit Bridgewater College's Interactive Planetary Orbits web site.

"Another notable difference between summers in the two hemispheres is their duration," adds Lebo. According to Kepler's 2nd Law, planets move more slowly at aphelion than they do at perihelion. As a result, Northern summer on Earth is 2 to 3 days longer than southern summer -- which gives the Sun even more time to bake the northern continents.

If you're feeling baked during the 4th of July holiday weekend and wish that aphelion brought more relief, there is something you can do: Take a hint from the watery southern hemisphere. Locate the nearest swimming pool and dive in; feel the water's high heat capacity. A little physics can be refreshing ....

Earth's Seasons -- (US Naval Observatory) dates and times of equinoxes, solstices, perihelia, and aphelia

Daily Earth Temperatures from Satellites -View global atmospheric temperature trends at different layers of the atmosphere, courtesy of the Global Hydrology and Climate Center.

Kepler's Laws: animated (by physics teacher Bill Drennon); a mathematical introduction (NASA/GSFC)

Shaq's Solstice Shadows -- (Science@NASA) Shaquille O'Neal's stubby shadow is proof that our planet is tipped on its side--and summer has arrived

Seasons: The Movie -- (NASAKids.com) a too-cool movie explaining the seasons.

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