Hey, sky fans! Can you believe I almost forgot what today is? What? Did you, too? It’s January 4!
See, here’s what happened. I was writing this week’s Word of the Week post when I remembered about today and hit the brakes on that one. I’ll do it in a week or two. The suspense is crushing isn’t it? Wait until you find out who shot J.R. Ewing.
Today, my friends, is another astronomical holiday. Just in case you haven’t read the title of this post, it’s Perihelion Day! Good thing it falls on a Wednesday this year because it gives me chance to be lazy and only write one post today for both the word of the week and perihelion, and then spend tomorrow throwing a tennis ball at the ceiling. Let’s get to it.
As we’ve talked about before, the Earth’s orbit around the Sun isn’t a circle; it’s slightly elongated, slightly egg-shaped. It’s an ellipse. This is Kepler’s first law of planetary motion. All orbits, all planets’, all moons’, all of them, are all ellipses. Now, some might happen to be circles, it’s true, but circles are just ellipses with no eccentricity.
This brings us to the word of the week. This week, it’s apsis. Picture an egg, and imagine its outside edge is the Earth’s orbit around the Sun, or if you prefer, Neso’s orbit around Neptune. See how there’s what looks like a near point and a far point? In a real orbit, each of those two points, the extremes, are an apsis. The plural of apsis is absissessessses. No, it’s not. It’s absides.
There are two absides in each orbit. To describe each apsis, we use the Greek-rooted prefixes peri- for the near one, and ap- or apo- for the far one. The general terms for any object’s nearest and farthest point around whatever object it orbits are periapsis and apoapsis.
We also have have specific terms for these absides depending on what’s being orbited. For instance, the near and far points of an object around the Earth are its perigee and apogee. You might remember these words from our talk about supermoons last year. A supermoon is a perigee full Moon, a full Moon that happens around when the Moon is at perigee; the closest point in its orbit around the Earth.
There’s loads of these, too, for example pericynthion & apocynthion (apsides around the Moon, which was handy during the Apollo days) and perijove & apojove (Jupiter). One of my favorite words in the whole wide world is, and it’s so good it needs to be in all capitals and said with a deep, echoing voice, APOGALACTICON. Any idea what that is?
No, not the name of a bunch of alien robots with a morale problem. Get a load of this. It’s the word that describes the farthest point in a star’s orbit around its galaxy’s center. Our solar system, as carried by the Sun, is cooking around the center of the Milky Way at something like 500,000 mph (800,000 kmh). The galaxy is so big that it takes our solar system around 250 million years to make it all the way around once. The apogalacticon is the farthest point in that orbit. This makes the nearest point its perigalacticon, which is still a good word, but come on, not as good. By the by, the Sun’s about four and a half billion Earth years old, which means its about 18 galactic years old (4.5 billion years ÷ 250 million years per orbit = 18); ready to get its driver’s license and vote in the galactic election.
What does all of this have to do with today, you ask? Well, today’s the big day here on Earth. At 14:17 UTC today, so 9:17 A.M. US Eastern, 6:17 A.M. US Pacific, and, let’s see… 4:17 P.M. in Cairo (EET), the Earth reached nearest point, 91,404,322 miles (147,100,998 km) from the Sun. This is about 1.5 million miles closer than the Earth’s 92.9 million-mile average distance, which is called one astronomical unit (AU). Today, the Earth is 0.98 AU from the Sun. From here, we’ll be making out way farther outward until apogee, as we’re contemplating cannonballs and hot dogs, in July. We’ll talk about seasons again another time, but you read that right: The Earth is at its closest point to the Sun in early January, when it’s winter in the northern hemisphere, and its farthest point in the north’s summer.
So, for today, look up, and for a moment, enjoy being at periapsis, perihelion, our closest point to the Sun. Happy Perihelion Day, and clear skies, everyone!