Hey, hey, everyone! Thanks for stopping by.

Well it’s that fantastic time again. Fall? October? No, well, okay, yes, but today’s a big day for another reason. It’s another astronomical holiday! Break out the silly hats and the fancy soup; it’s the second half of the year’s 1 AU day! What’s that, you say? 1 AU day?

It breaks down like this.

Somewhere along the way you probably learned that the Earth is about 93 million miles (150 million km) from the Sun. That’s generally true, but it’s not so simple.

If the Earth’s orbit were a circle, every point in its orbit would be the same distance from the Sun. Every spot, every minute, every day, for billions of years, the Earth would unfailingly be 93 million miles from the Sun.

The truth, though, is there are times when it’s more than 93 million, and times when it’s less. See, the Earth’s orbit around the Sun isn’t a perfect circle. It’s pretty close, but it’s a slight ellipse, stretched out a bit so it’s a little longer than it is wide. Also, the Sun isn’t at the middle. It’s a bit off to one side. That’s Kepler’s first law of planetary motion in action, right there. The 93 million-mile distance is the average. Half of the time it’s farther and half the time it’s nearer.

The farthest point in our orbit, just before the Earth turns and starts edging closer in toward the Sun is called *aphelion*, is about 94.5 million miles, (153 million km) and happens in July. At the other end of the orbit, where we’re nearest the Sun, is *perihelion*. That’s about 91.4 million miles (147 million km), and happens in January. If we do a little grade school math, 94.5 million + 91.5 million = 185.6 million, then divide that by two and, blammo, 92.95 million miles; just under 93 million. In kilometers, it’s 153 million + 147 million = 300, by 2 is 150 million. That’s not the real astronomers’ way of figuring out the exact average distance, but it’s close enough for us.

If you think of trying to go for a pizza some afternoon, you can’t go from your house to the pizza place two miles away without being exactly 974 feet, 9.375 inches from your couch at some point. In order for the Earth to get from perihelion, it’s nearest distance, to aphelion, it’s farthest, it needs to go through every distance between. One of those distances is the 93 million-mile average, and it hits that point twice: once on the way out from perihelion to aphelion, and once on the way back in.

But what’s an AU?

Everything in space is really far apart. Think about it, the nearest thing to us is the Moon, which is, on average 239,000 miles away. Venus and Mars, which are the closest planets to us are tens of millions of miles away on average. The stars, even the nearest ones to us, are so far away that their light takes years to get to your eye. Because of this, it becomes really cumbersome to try to keep measuring distances in kilometers and miles. Really, imagine reading something that says “Sirius is the brightest star in the entire night sky, and it’s one of the closest at about 48,000,000,000,000 miles (78,000,000,000,000 km).”

Because of this, we’ve come up with other units of measurement to be used in astronomy that make it easier to talk about these huge distances. The light year, which is about 6 trillion miles or 9.5 trillion km is the distance light travels in a year, is probably the most famous of these.

Another one of these astronomical units of measurement is called… well… the astronomical unit, or the AU. The AU represents the average Earth’s average distance from the Sun, the 93 million miles/150 million km. Geocentric as it is, it’s really handy for measuring distances within solar systems. How far is Saturn from the Sun? About 10 AU. How far is Mercury? A bit more than a third of an AU. How about Voyager 1? 118 AU!

Since 93 million miles is the average distance, the Earth spends half its time more than 1 AU from the Sun, and half less than 1 AU, but it’s only exactly 1 AU twice a year. Well, Sky Fans, depending on where you are, today’s the day! Just past midnight, 12:06am US Eastern time tomorrow, October 5, 2016, the Earth will reach that spot in the outward part its orbit, and be exactly 1 AU from the Sun. From then until the inward average point, which is in April, you can say the distance from the Earth to the Sun is more than distance from the Earth to the Sun! I love that joke.

Happy 1 AU day, and a happy new year if you’re celebrating, and clear skies everyone!

Happy AU to U.

I have one minor point. A circle is an ellipse. It’s an ellipse with no eccentricity. My former math-centric students made sure I knew that when I taught about orbits. Now I can’t help myself. 🙂

LikeLiked by 1 person

…and a happy AU to you, too, good sir. 🙂

You’re absolutely right about circles and their eccentricity. I spent a bunch of time trying to figure a way to talk about that point without getting too deep into the woods, as they say, but got a little lost, myself. It’s a good point, maybe I’ll come back to it for another post. Thanks for the tip.

LikeLiked by 1 person