Approaching the Sun

Contributed by
Jan 2, 2007

Today, January 3, on or about 20:00 Universal Time (2:00 p.m. Pacific time), the Earth will reach perihelion, its closest approach to the Sun. The distance from the Sun to the Earth will be roughly 147,093,600 kilometers (I have found several different distances on different sites, and this is an eyeball average).

Over the course of the year, the Earth's distance from the Sun changes because the orbit is not a perfect circle. It's an ellipse. The average distance of the Earth from the Sun (what astronomers call an astronomical unit) is 149,597,871 kilometers. So you can see that we will be 2.5 million kilometers closer to the Sun than average at 20:00 today, a difference of about 1.6% or so. If you had a good 'scope and measured the Sun very carefully today, then measured it again on July 7, when we reach aphelion, the farthest distance from the Sun, you'd see it would be roughly 3% bigger today.

Remember, our distance from the Sun doesn't affect our seasons (much). Because the heat we receive from the Sun depends on the square of our distance, we get 6% more heat today than we do in July. Northern hemisphere denizens may be surprised by this, but there you go. Astronomy is many things, and surprising is one of 'em.

Also, because we're closer to the Sun, we feel the effects of its gravity more. This doesn't affect us all that much, except that tides from the Sun will be stronger now than in July. It's not a big effect, since the Moon's tides are twice as strong as the Sun's to start with. But again, if you took very careful measurements of water heights, you could detect this.

I'm sure many of my readers already know this stuff to some extent. But if you didn't, then I suppose for you there is something new under the Sun. And that's one of my favorite things in the whole world... and off of it, too.

Happy perihelion!

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