Happy Aphelion Day!

Hey, Sky Fans! How’s things going today? Here in the States, Independence Day is tomorrow. July 4 is a big deal, but it’s also another holiday, an astronomical one. These two are always tied together in my mind.

Happy Aphelion Day!

What’s that? Let’s step into the ol’ science… talking… thing…

First, remember Sir Isaac Newton’s first law of planetary motion?

“In space, things travel around other things in elliptical orbits. They’re stretched out circles, right? So, the Sun’s not at the middle. Ellipses don’t really have middles. It’s off to one side. Ellipses have two foci, and the Sun is one of them. The other one’s off in space somewhere.”

I’m pretty sure that’s the exact quote, but I’m going from memory, so I might be a bit off. “Foci” is the plural form of the noun “focus.”

The more stretched out the orbit, the ellipse, is, the farther apart the two elliptical foci are. The less stretched out, the closer the two foci are. If the ellipse isn’t stretched out at all, the two foci are actually in the same place. This also means every point around the edge of this non-stretchy ellipse is the same distance from this double-foci point. We give a special name to this type of ellipse. We call it a circle, and the double-foci point is the center. This elliptical stretching-out is called eccentricity. A circle is an ellipse with 0 eccentricity.

If you can imagine this, maybe use an egg to help out, you can see that there’s a point where the Earth is at its closest to the Sun, and a point where it’s at its farthest.  Each of these extreme points are called an apsis (plural, apses). When we’re talking about these extreme points specifically about an object’s distance from the Sun, we call the nearest point perihelion, and the farthest aphelion. The prefix peri- means near, and ap- means far, while the root, helion, is something relating to the Sun. Everything orbiting the Sun has an aphelion and a perihelion.

In Earth’s case, perihelion is in January. From that moment on, the Earth gets farther and farther from the Sun until it gets to aphelion; today. While the US is getting ready for fireworks and hot dog-eating contests, today at 20:11 UTC, which translates to 4:11 PM US Eastern, Earth will reach aphelion, its farthest point from the Sun. Then it’ll start edging its way back inward toward the Sun until January’s perihelion, and around the block we go.

2017 Perihelion and Aphelion
2017 Perihelion and Aphelion

Most of us probably don’t give a lot of thought to Earth’s orbit, but what’s really interesting to me is it’s not static. Thanks to some gravitational variations from the Moon and the rest of the solar system, the Earth’s orbital shape and eccentricity changes, which means the timing and distance of these points changes slightly from one to the next.

In 2017, perihelion was on January 4, and was 147,100,997 km / 91,404,322 miles. July 3, 2017’s aphelion distance is 152,092,504 km / 94,505,901 miles. This variation is only about 5 million km / 3 million miles (about 3%) off the 150,000,000-km / 93,000,000-mile average distance we’ve all been taught since grade school. Incidentally, that average distance is called the astronomical unit or AU, and Earth reaches that exact distance twice a year: in April and October.

So, have a great day everyone, and remember, after this afternoon, each cannonball you do will be a little closer to the Sun than the last one. Thanks for stopping by, have a great  and safe aphelion today, and fourth tomorrow. Clear skies, everyone!




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