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January 03, 2007

Happy perihelion

photo of the sun

Maybe it's because the solstice was just less than two weeks ago, or maybe it's because I just spent several days at a much higher latitude, or maybe because last year I took a class that covered the basics of navigating a spacecraft to other planets, but-- whatever the reason-- these days I often think about the grand whirl and motion of the Earth, Sun, Moon and planets...the giant clockwork in the sky.

Another notable event in the celestial clockwork happens this afternoon, at about 2 PM Pacific time. At that point the Earth reaches perihelion, the point where we are closest to the Sun. As you may remember from your high school science class, the Earth's orbit around the Sun isn't a perfect circle, it's a slightly squished ellipse and as a result the distance between the Earth and the Sun is constantly changing. This afternoon the distance reaches a minimum...about 147,093,600 kilometers according to the Bad Astronomy blog.

The Sun's distance doesn't vary all of that much over the year, only about 1.5%, so you don't really notice it. If you have precise instruments you can measure the change in the Sun's size in the sky, and you can record slight differences in the Sun's effect on tides, but that's about it. Oh, we also get a bit (about 6%) more energy from the Sun right now than when the Sun is at its' greatest distance, but you don't really notice that either. (It's the Earth's tilt, not changes in the distance to the Sun, that makes the seasons).

Happy perihelion!

Posted by Chris Spurgeon at January 3, 2007 06:01 AM

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