Top o' the orbit to ya!

Contributed by
Jul 4, 2011
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Today is the Fourth of July, Independence Day for us American types.

It also happens to be aphelion*, the point in Earth's ever-so-slightly elliptical orbit when it's farthest from the Sun. Perihelion -- closest approach -- happens in early January, and aphelion six months later. The dates change a bit from year to year because there aren't an even number of days in a year (that pesky extra 0.24 in the 365.24 days per year messes things up), and there are other minor factors as well.

Today though, aphelion occurs on or about 15:00 UT (11:00 Eastern US time), when the center of the Earth will be about 152,102,715 km (94,512,245 miles) from the center of the Sun -- give or take a few hundred meters. If you're curious, that's about 1.67% farther from the Sun than on average. That in turn means the Sun appears about 1.67% smaller in diameter than usual, which isn't noticeable to your eye -- and I don't recommend trying to find out -- but is pretty obvious in photographs using telescopes and heavy filtering, like this one from astrophotographer Anthony Ayiomamitis:

Cool, huh? When we're farther from the Sun we receive a bit less heat, so perhaps those of you suffering from the midwest heat wave can take consolation that it could be worse by a couple of degrees right now.

Later today, coincidentally, I'll be at a picnic with lots of solar astronomers. What do I say to them? "Hap-helion Fourth of July"? Or, "Enjoy us being at a(1+e) [where a = 1 AU and e = 0.0167] from the Sun today"?

That seems awkward. The thing is, I'm pretty sure a lot of them would get it...

Related posts:

- At the bottom of Earth's orbit
- Happy New Year again!
- Why we have leap days
- Does this perihelion make my Sun look fat?
- Does the Sun look smaller to you?

* I pronounce it app-HEEL-eeyun, if you care.