National Geographic News
The sun sets behind smoke from a wildfire in New Mexico on June 29.

The sun sets over Los Alamos National Laboratory in New Mexico on Wednesday.

Photograph by Eric Draper, Reuters

Ker Than

for National Geographic News

Published June 30, 2011

If the sun looks a little smaller than usual as you're barbecuing this Fourth of July, it's not your imagination: Earth will be farther from the sun on Monday than on any other day this year.

That's because the orbits of all the planets in our solar system—including Earth's—are not perfectly circular, a phenomenon that was first explained in mathematical detail by the 17th-century German astronomer Johannes Kepler.

Kepler "figured out the orbits of the planets were elliptical in shape and that the sun was offset from the center," explained Mark Hammergren, an astronomer at the Adler Planetarium in Chicago, Illinois.

Earth's elliptical orbit means there will be a point each year when the planet is closest to the sun, called perihelion, and a point when it is farthest away, known as aphelion.

(Related: "Sun Headed Into Hibernation, Solar Studies Predict.")

On July 4 our planet will be at aphelion—94,511,923 miles (152,102,196 kilometers) from the sun. This year's perihelion was on January 3, when Earth was 92,955,807 miles (149,597,870 kilometers) from the sun.

On average, Earth is about three million miles—or about 3 percent—farther from the sun at aphelion than at perihelion.

As a result, the apparent size of the sun in the sky will be about 3 percent smaller, but you may not notice the difference without a telescope.

"It's probably not noticeable, but it's certainly measurable," Hammergren said.

(Related: "'Supermoon': Biggest Full Moon in 18 Years Saturday.")

Distance Won't Dampen Fourth of July Heat

So if Earth is at its maximum distance from the sun on the Fourth of July, why are those of us in the Northern Hemisphere sweltering in the heat of summer?

Because it's the tilt of the Earth and not our distance from the sun that determines the seasons, Hammergren said.

Earth's north-south axis is tilted by about 23.4 degrees, so during its orbit, the poles point in different directions from the sun.

By coincidence, Earth reaches aphelion when the North Pole is tilted more toward the sun than the South Pole.

"Because the Earth has a tilt, it means that in the summer months [the Northern Hemisphere] receives a longer duration of sunshine—so the day is longer and the night is shorter—but also the sunlight hits the ground more vertically," Hammergren said.

"Those two things together contribute to the difference in heating for the seasons."

Also see "Summer Solstice 2011: Why It's the First Day of Summer" >>

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