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Sun farther away but Earth hotter

Our planet warms up in July, despite receiving less sunlight.
Our planet warms up in July, despite receiving less sunlight.  

(CNN) -- On Saturday the Earth reaches its farthest point from the sun during its orbit, but the average global temperature actually goes up this time of year. What gives?

Like the other planets in the solar system, the Earth moves in a lopsided orbit around the sun. In July, our planet reaches the most distant point of its annual revolution, called aphelion. In January, it dips to its nearest position, or perihelion.

Compared to other planets, the Earth's orbit is only slightly askew. It has an eccentricity of 1.7 percent, while Mars' is more than 9 percent and Mercury's more than 20 percent.

Looking at a page-sized map, an observer might mistake the Earth's path around the sun for an exact circle. Scientists know better.

"When we're closest the sun, the distance is 147.8 million kilometers [91.8 million miles]. This weekend we will be 152.6 million kilometers [94.8 million miles] away, a 5 million kilometer (3.1 million mile) difference," University of Florida astronomer George Lebo said.

The greater the distance, the less intense the rays of the sun.

"Averaged over the globe, sunlight falling on Earth at aphelion is about 7 percent less intense than it is at perihelion," NASA climate researcher Roy Spencer said.

Earth's orbit is much less elliptical or lopsided than other planets' orbits.
Earth's orbit is much less elliptical or lopsided than other planets' orbits.  

Then why is it so warm now in the Northern Hemisphere? The reason is that the Earth tilts 23.5 degrees on its axis, and because of where it is in its orbit, the northern half is the portion most exposed to the sun during the summer months.

The conditions are reversed in the Southern Hemisphere, which basks in the solar rays half a year later, when the South Pole tips more toward the sun.

Overall, the entire globe averages higher temperatures when the Earth is farther from the sun. The planet is about 4 degrees F (2.3 degrees C) warmer during aphelion than perihelion, NASA scientists said.

The reason? Continents and oceans are distributed unevenly. The north has more land and the south more water. When sunlight strikes the former, it heats up more quickly than the latter, which can absorb more heat before rising in temperature.

In other words, Earth is slightly warmer in July because the sun is shining on all the northern continents, which heat up rather easily. In January, the sun focuses its rays on the southern oceans, which have higher heat capacity.

Another quirk of physics contributes to even more planetary heat this time of year. During aphelion, planets move in their orbits more slowly than during perihelion.

As a consequence, the summer in the north is several days longer than in the south, affording the sun more time to cook the landmasses in the Northern Hemisphere.




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