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Each planet revolves around the Sun in an elliptical path, with the Sun occupying one of the foci of the ellipse. -- Kepler's 1st Law of Planetary Motion
"Like all planets in our solar system, Earth travels around the Sun in an elliptical orbit," explains astronomy professor George Lebo, a Summer Faculty Fellow at the NASA Marshall Space Flight Center. "We make our closest approach to the Sun (147.5 million km) in January, that's called perihelion, and we're farthest from the Sun (152.6 million km) in July, that's aphelion. This year, aphelion falls on Independence Day in the USA."
Above: The sun setting behind the Statue of Liberty is a little more distant than usual on the 4th of July. Image Credit and copyright: Corbis.com, all rights reserved.
The eccentricity of our planet's orbit is mild; aphelion and perihelion differ from the mean Sun-Earth distance by less than 2%. In fact, if you drew Earth's orbit on a sheet of paper it would be difficult to distinguish from a perfect circle. "Seasonal weather patterns are shaped primarily by the 23.5 degree tilt of our planet's spin axis, not by the mild eccentricity of Earth's orbit," notes Lebo. "During northern summer the north pole is tilted toward the Sun. Days are long and the Sun is shining more nearly straight down -- that's what makes July so warm."
"Averaged over the globe, sunlight falling on Earth in July (aphelion) is indeed about 7% less intense than it is in January (perihelion)." That's the good news. The bad news is it's still hot. "In fact," says Spencer, "the average temperature of Earth at aphelion is about 4o F (2.3o C) higher than it is at perihelion." Earth is actually warmer when we're farther from the Sun!
How can that be? It's because our planet is --in a sense-- lopsided. 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 --near the start of northern summer-- the land-crowded northern half of our planet is tilted toward the Sun. "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.
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.
Solar heating raises the temperature of continents more than water because the heat capacity of land is lower. Bill Patzert, an oceanographer at NASA's Jet Propulsion Laboratory explains: "The temperature of land changes easily, which is what we mean by low heat capacity. Consider the desert: At night the desert is cold, perhaps only 60o F (16o C). When the Sun rises in the morning the temperature might jump to 100o F (38o C) or more." Such mercurial behavior is characteristic of materials like rocks and soil with little thermal inertia. It doesn't take much sunlight to substantially elevate their temperature.
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. Six months later, in January, the situation is reversed as 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). Strange but true!
"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 on the 4th of July holiday 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 sample first hand the benefits of water's high heat capacity. A little physics can be refreshing after all!
presented by ThursdaysClassroom.com
- Discussion Questions: These questions are as cool as southern summer! [lesson plan] [activity sheet]
- Season Myths: Throughout history humans have explained the seasons in many different ways -- a recent misconception is that Earth's eccentric orbit causes them. In this activity students read a Native American myth about seasons. Then they will write a myth of their own accounting for the seasons or for day & night. [lesson plan] [activity sheet]
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Earth's Seasons - a table of aphelia, perihelia, solstices and equinoxes from the US Naval Observatory
Kepler's Laws of Planetary Motion - find out more about how planets move around our star.
Daily Earth Temperatures from Satellites -View global atmospheric temperature trends at different layers of the atmosphere, courtesy of the Global Hydrology and Climate Center.
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