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Sun heats up Earth in long distance relationship

(CNN) -- The Earth moved to its greatest distance from the sun this year on Wednesday. But summer heat in the Northern Hemisphere could actually intensify rather than diminish, scientists said.

Like all our neighbors in the solar system, the Earth travels in an elliptical path around the sun. In the summer, our planet reaches the most distant point of its orbit. In the winter, it swoops to its nearest.

"We make our closest approach to the Sun in January. That's called perihelion. And we're farthest from the Sun in July. That's aphelion. This year, aphelion falls on Independence Day in the United States," NASA astronomer George Lebo said.

 Earth-sun distance during:
Aphelion: 94.8 million miles (152.6 million kilometers)Perihelion: 91.7 million miles (147.5 million kilometers)

Compared to other planets, the Earth's orbit is almost circular. The nearest and farthest points differ by less than 2 percent from the mean distance between the Earth and sun. On a page-sized map, one could hardly distinguish our planet's orbit from an exact circle.

Then what drives weather on our planet, rather than the mild eccentricity of its orbit? The 23.5-degree tilt of Earth's axis.

"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," Lebo said.

 Which is which?
To remember the difference between perihelion and aphelion, try a mnemonic memory trick. "Aphelion" and "away" both start with the letter "A."

Aphelion does bring a miniscule amount of relief to the planet. Averaged around the entire globe, sunlight is about 7 percent less intense during aphelion compared to perihelion, according to Roy Spencer, a climate researcher in Huntsville, Alabama.

But the Earth is actually warmer when it is farther from the sun. The average temperature on the planet is about 4 degrees F (2.3 degrees C) higher during aphelion than during perihelion. What gives?

Space exploration  

The Earth, in a sense, is unbalanced. Continents and oceans are sprinkled unevenly around the planet. The north has more land and the south more water. Confronted by the sun, land cooks quickly compared to the oceans, which can absorb great amounts of heat.

Earth's average temperature is slightly higher in July because the sun is shining on all the northern continents, which heat up rather easily. In January, the situation is reversed. The southern waters face the sun.

Therefore, the Earth is cooler during perihelion than aphelion. "We're closer to the sun in January. But the extra sunlight gets spread throughout the oceans," Spencer said.

• Interactive Planetary Orbits
• Solar Dynamics Observatory
• The Johns Hopkins University
• Living With a Star Program

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