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…

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

[Update: My apologies: due to a cut-and-paste error, I had mistakenly listed the perihelion distance as the average distance of the Earth to the Sun (147 versus 149 million km). To avoid confusion, I simply replaced the error with the correct value. The rest of the post is correct since this wasn't a math error but a typographical one, and I used the right value when doing my calculations below.]

Since last July, the Earth has been falling ever closer to the Sun. Every moment since then, our planet has edged closer to the nearest star in the Universe, approaching it at over 1100 kilometers per hour, 27,500 km/day, 800,000 km every month.

But don’t panic! We do this every year. And that part of it ends today anyway.

The Earth’s orbit around the Sun is not a perfect circle. It’s actually an ellipse, so sometimes we’re closer to the Sun, and sometimes farther away. Various factors change the exact date and time every year — you can get the numbers at the Naval Observatory site — but aphelion (when we’re farthest from the Sun) happens in July, and perihelion (when we’re closest) in January.

And we’re at perihelion now! Today, January 3, 2011, around 19:00 GMT (2:00 p.m. Eastern US time), the Earth reaches perihelion. At that time, we’ll be about 147,099,587 kilometers (91,245,873 miles) from the Sun. To give you an idea of how far that is, a jet traveling at a cruising speed of 800 km/hr would take over 20 years to reach the Sun.

Of course, since today is when we’re closest to the Sun this year, every day for the next six months after we’ll be a bit farther away. That reaches its peak when we’re at aphelion this year on July 4th, when we’ll be 152,096,155 km (94,507,988 miles) from the Sun.

Not that you’d notice without a telescope, but that means the Sun is slightly bigger in the sky today than it is in July. The difference is only about 3%, which would take a telescope to notice. Frequent BA Blog astrophotograph contributor Anthony Ayiomamitis took these images of the Sun at perihelion and aphelion in 2005:

This may seem a bit odd if you’re not used to the physics of orbital motion, but you can think of the Earth as moving around the Sun with two velocities: one sideways as it sweeps around its orbit, the other (much smaller) toward and away from the Sun over the course of a year. The two add together to give us our elliptical orbit. The sideways (what astronomers call tangential) velocity is about 30 kilometers (18 miles) per second, which is incredibly fast. But then, we do travel an orbit that’s nearly a billion kilometers in circumference every year!
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Does the Sun look a little brighter to you? Maybe that’s because at nine minutes after midnight (UT) tonight, January 2/3, the Earth will be at perihelion, the closest point on its elliptical orbit to the Sun.

At that moment, the Sun’s center will be 147,098,040 kilometers away from the Earth’s center (that’s 91,402,484.5 miles for you Murricans). That is, assuming the distance from the centers of the two bodies is 0.983289667 Astronomical Units, and one AU is 149,597,870.7 kilometers. You can compare that to when we reach aphelion, our most distant point from the Sun, which in 2010 will occur on July 6 at 11:30 UT, when we’ll be 1.016701958 AU or 152,096,448 km (94,508,351.3 miles) from our star.

That change in distance — about 5 million kilometers, or 3 million miles — is only a small fraction of our distance from the Sun, so it doesn’t change the Earth’s temperature very much: a few degrees Celsius, but that’s about it. So, of course, that’s not the reason we have seasons. If it were, then we’d have winter in July in the northern hemisphere! But of course, the international cabal of astronomers covers this fact up.

Still, when you think about it, the Sun is a frakkin’ long way off. Even now, at our closest point, it would take over 20 years to fly to the Sun in an airplane at 800 kph (500 mph)! And the TSA would make you sit silently with nothing in your lap for the last 3 years of the journey, too.

But my point is (in case you were wondering if I had one) that the Sun is hot, and there’s a lot of it. I’m glad it’s so far away, even when it’s at its closest.