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Sun just too far away for comfort

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The blistering days of summer are fully upon us, but don't blame it on closeness to the sun.

Through some cosmic whimsy, Earth is nearly at its farthest point from the sun. According to the U.S. Naval Observatory in Washington, D.C., the planet reaches aphelion — the most distant point in its orbit — at 5 p.m. MDT on July 3.

At that time Earth will be 94.6 million miles from the sun. During the parhelion, the closest point, reached on Jan. 2, Earth was 91.45 million miles from the central nuclear furnace.

Those who want to remember the difference between these terms can keep them straight by the ancient trick of associating aphelion with away, both starting with the letter A.

Many may be surprised to learn that we in the Northern Hemisphere are hottest when farthest from the fire and coldest when closest to the sun, said Patrick Wiggins of Hansen Planetarium.

"However, our overall temperature changes have little to do with our proximity to the sun. Rather, the changes are caused by the tilt of the Earth," he said.

Two factors are involved in these events.

Earth and the other planets of the solar system orbit the sun not in circles but in ellipses, as the German mathematician Johannes Kepler demonstrated early in the 17th century. An ellipse is an oval shape.

Earth does not stand perfectly upright on its axis as it orbits, but is tilted at 23.5 degrees.

The first fact means that Earth varies in its distance from the sun, depending on where it is in the orbit. The second means the angle of sunlight reaching Earth changes as it goes around the sun.

The tilt makes sunlight fall most directly on the Northern Hemisphere during the summer, so the days are long and warm. Sunlight falls most obliquely during the winter, and the opposite is true about length of day and the ability of the sun to heat our atmosphere.

In the summer, Wiggins said, the tilt puts the sun high in our skies and permits its warming rays to hit us more directly and for more hours per day.

During the colder months, the Northern Hemisphere is tilted away from the sun, putting the sun lower in the sky and letting it warm this part of the world for fewer hours of daylight.

The combination of the direction of the sunlight and length of day is much more important to our climate than is the relatively small difference in distance from the sun.

Residents of the Southern Hemisphere experience no such confusion about seasons vs. distance from Sol. They are warmest when Earth is tilted toward the sun, which happens to be during the closest approach of the planet to the celestial furnace. But their warmer weather in January still isn't due to increased proximity, but because of the angle of sunlight.


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