What Is Aphelion? Earth Reaches Its Greatest Distance From The Sun On Friday

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It's summer in the Northern Hemisphere. But while you indulge in long, balmy days at the beach or elsewhere in nature, you may be surprised to learn that our planet is creeping toward its greatest distance from the sun, a point known as aphelion.

That Earth has an aphelion is a result of its orbit being elliptical, rather than circular. According to Kirby Runyon, a geologist at the Planetary Science Institute, all planets in the solar system travel in elongated circles around the sun, rather than perfect ones. And it's most likely true for worlds around other stars, too.

"All the planets tend to jostle each other around," pulling their orbits from perfect circles, Dr. Runyon said. "It's literally this chaotic tug of war between small amounts of gravitational influence that the planets have on each other."

Jupiter exerts the most influence because it is the most massive planet in our solar system, he added.

How much an orbit deviates from a perfect circle is measured by its eccentricity. The higher the eccentricity, the more elliptical the orbit. For some bodies in the solar system, this is quite pronounced: Mars, with an eccentricity of 0.094 , ranges from 129 to 155 million miles away from the sun. Pluto, whose distance from the sun varies from 2.8 to 4.5 billion miles, is even more eccentric at 0.244 .

On the other hand, our home planet has an eccentricity of only 0.017 . "Earth's orbit is fairly circular," said Larry Wasserman, an astronomer at Lowell Observatory in Flagstaff, Ariz. "If you drew it on a piece of paper to scale, you probably wouldn't notice it was slightly flattened."

From the ground, three million miles may seem like a lot, but it doesn't amount to much on astronomical scales. The size of the sun in the sky appears about 4 percent smaller at aphelion than at perihelion, an effect that is too small to be noticed without precise instruments, Dr. Wasserman said.

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