Top o' the orbit to ya!

The entire universe in blog form
July 4 2011 5:00 AM

Top o' the orbit to ya!

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.

Phil Plait Phil Plait

Phil Plait writes Slate’s Bad Astronomy blog and is an astronomer, public speaker, science evangelizer, and author of Death from the Skies!  

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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...



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