Word of the Week: Opposition!

Hey, Sky Fans! With so much happening, it’s hard to keep up. If you’re a fan of orbital geometry, and who isn’t (am I right?), tonight’s the night for you. Overnight tonight, June 14 into June 15 (US time) the planet Saturn will be at…

OPPOSITION!

Saturn’s been making its way into the evening’s skies over the last couple of months, and will be at opposition tonight. As some other stories go, this means that it’ll be brighter, bigger, and therefore easier to see than when it’s not at opposition. Many don’t spend much time talking about what opposition is.

Imagine the solar system, with the planets running around in their orbits. Let’s keep things simple, and imagine our solar system is just the Sun in the middle. Earth is a ways out and Saturn out… where Saturn is.

Now, hit pause. Where are the planets? Maybe Earth is off to the left of the Sun, and maybe Saturn is to the right. Or, maybe, they’re in a perfectly straight line: Sun – Earth – Saturn. In this, Saturn is directly opposite Earth from the Sun.

Can you picture it?

From Earth, since we’re in the middle of that line, we’d see the Sun at one end of the sky, and need to turn around to see Saturn directly opposite it the other. This is kind of like when you’re standing in line at the supermarket. You’re chatting about the weather with Lisa, in front of you, and need to turn all the way around to talk about these new mint M&Ms with Larry, behind you. I’m as disgusted by Larry’s taste in M&Ms as you are.

Opposition is the point in when one object (Saturn) is directly opposite another object in the sky (the Sun) as seen from a given viewpoint (Earth). To be more precise,  it’s where two things are exactly 180° apart in the sky.

Word of the Week: Opposition!
Word of the Week: Opposition!

There are a couple of practical side stories here. First, since objects at opposition are 180° apart in the sky — you know, half of a sphere’s 360° — one rises just as the other sets. Tonight, Saturn will rise at sunset.

Also, opposition brings things to their closest points to each other. So, objects at opposition are generally at their brightest. Plus, since the lineup is straight, we’re seeing Saturn’s fully lit day side. Closer, more fully lit things are brighter, right? After tonight, we’ll go our separate ways and move father apart. The line between us and Saturn will lengthen, and we’ll gradually see less and less of Saturn’s fully lit day side. This means less light gets to us, and it’ll dim in our sky. In about a year, relatively speedy Earth will come back around and catch back up with the much slower-moving Saturn. Right! another opposition!

If this all sounds kind of familiar, you’re right. This is the same arrangement the Sun, Earth, and Moon are in when the Moon is full. The full Moon is directly opposite the Sun, and rises at sunset. It’d be a little weird (though not as weird as saying Lisa and Larry are at opposition) but you could say “The Moon is at opposition” when there’s a full Moon. The only real difference is how far from Earth the Moon and Saturn are.

Incidentally, this is another astronomy thing that’s a matter of perspective, just like so much of life. While we’re talking about Saturn at opposition, the good people there are talking about Earth being at inferior conjunction. That’s when an object (Earth) is between the viewer (Saturn) and a third object (the Sun). Earth’s nighttime side faces Saturn, and is up and invisible in Saturn’s daytime sky. This is the same thing as when there’s a new Moon. If you want, maybe we’ll come back to this another time.

But wait, there’s more!!! You can’t see Saturn’s famous rings with the naked eye, but they’re also going to reflect a bunch of extra light toward us. They’re tilted at about 27°, around their widest, toward us. Even more brightness! If you’re good with a telescope, this is a great time to have a look.

Head out tonight and look toward the southeast once it’s good and dark; maybe around 10:00. You’ll see Saturn alongside Scorpius, the scorpion, and its brightest star, Antares. I’ve said this before and will again, but remember, the reflected sunlight we’re seeing has traveled almost two billion miles, so it’s a dimmer and more subtle than you probably want it to be. It’s worth it the hunt, though.

Saturn near Scorpius, June 14, 2017 (From Stellarium)
Saturn near Scorpius, June 14, 2017 (From Stellarium)

Thanks for stopping by. I hope you’ll head out and have a look. Clear skies, everyone!

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5 thoughts on “Word of the Week: Opposition!

  1. I used to get really confused about opposition. It seemed to me that it should mean a planet is on the opposite side of the Sun. I had to adopt a more geocentric way of thinking for this term to make sense to me, with the Sun and some planet on opposite sides of Earth. Still feels weird to me, but I’m mostly used to it now.

    Also, I was sort of surprised the other night by how bright Saturn was. I kept thinking “that can’t be Saturn,” but of course it was. And now I know why it was so bright!

    Liked by 1 person

    1. Yeah, I thought the same thing about Saturn, too. I always think of it (and write about it) being mellow and fussy, but there it was, plain as day. It was great to see, yeah.

      Hm… interesting. I never got confused by opposition, but I can see that, yeah. You’re right, though, it only makes sense from a geocentric view.

      Liked by 1 person

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