What You Should Do Tonight

A week and a half ago, when posting about the Henry Mountains, I mentioned observing the moons of Jupiter through binoculars. At that time, as Jupiter rose in the sky, 3 of the 4 Galilean moons appeared to the right, and 1 to the left.

Saturday I checked out Jupiter again from my driveway. This time 3 were to the left and 1 was to the right. Tuesday night I looked again, and this time 2 were to the left and 2 to the right. And last night (Wednesday 9/23/09) it was 3 to the left, 1 to the right again.

Most of us know a bit about planets and moons and stuff because we heard something about them in grade school, or maybe watched a NOVA episode or maybe even read something in the paper when that probe landed on Titan a few years back. But in our day-to-day lives it’s easy to forget all that stuff because correlating that knowledge with the little dots of light up in the sky requires all kinds of time, attention and maybe even a telescope.

But Jupiter is different. Right now Jupiter is visible in the continental US in the East/Southeastern sky shortly after sunset, and over the span of just a few days you can enjoy a great show of a planet and moons moving around it, and all you need to do to check it out is read the rest of this post.

How To Find Jupiter

Eat dinner. If your spouse cooked the meal, be sure to compliment him/her on the meal*. Do the dishes. Hang out for a bit, drinking coffee or helping your kids do their homework or prepping your bike for tomorrow morning’s ride, or just sitting around with your spouse at the table, belching and complaining about the government- or whatever else it is you like to do after dinner- until it gets dark. In Northern Utah this week this’ll be at around 7:50PM this week. Then go outside with a pair of half-decent** binoculars.

*What? Your wife cooks dinner every night and you never thank or compliment her? Newsflash: This is why she bitches about you to all her friends and is having an affair with the guy who built your deck.

**Mine are 10x42, specifically these.

Look up to the East/Southeast, maybe ~25-30 degrees up in the sky. The stars will just be coming out, and if you live in a light-polluted area (like here in the Salt Lake Valley) there won’t be many others visible anyway. The big, honking “star” you see, bigger/brighter than anything else in that part of the sky, is Jupiter.

Extra Help: Remember, all the planets orbit the sun on more or less the same plane, and that plane is very roughly parallel (+/- 23 degrees) to the earth’s equator. That means that planets in the night sky are always going to appear in the Southern half of the sky, assuming you’re in the Northern Hemisphere. (So if you’re looking at the Big Dipper or Cassiopeia, you need to turn your ass around.) At this time of year (close to the Equinox) the path the planets follow across the night sky is almost exactly the same as that followed by the Sun over the course of the day. (In the winter it’s higher, and in the summer it’s lower.)

There’s only 1 planet that’s ever brighter than Jupiter, and that’s Venus, which is the brightest “star” anywhere in the sky. But Venus’ orbit is closer to the Sun than ours, so it’ll always appear on the same “side” of the sky as the setting or rising sun. So in other words, it can’t be anywhere to the East shortly after sunset*.

*Venus is actually preceding the Sun across the sky right now, so it’s the honking big star to the East just before sunrise.

All About Jupiter

Everyone knows Jupiter is the biggest planet in the solar system, but you may not know how weird it is. Its mass is more than 300 that of Earth’s, but it’s volume is 1300 times as great, which means that on average, it’s less than 1/4 as dense. Surprisingly (for me anyway) we still don’t fully understand the structure of the planet- what’s liquid, what’s solid, or even whether it really has a “core”. But we know that it’s about 89% hydrogen, 10% helium, and 1% other stuff. Which is way, way different than the stuff that the Earth* is made out of. In fact, its composition is much more similar to the Sun**.

*The Earth is made mostly out of iron, oxygen, silicon and magnesium, with healthy helpings of sulfur, nickel, calcium and aluminum.

**The Sun is about 75% hydrogen,24% helium and 1% other stuff.

This is why you may have heard Jupiter referred to as a “failed star”- because if it were bigger its hydrogen would start to fuse and it would shine/”burn*” as the sun does, but the moniker is probably a bit overly dramatic; Jupiter would have to be something like 75 times more massive to achieve stellar ignition.

*Stars don’t actually “burn”. The energy released by the sun is the result of large-scale fusion of hydrogen atoms into helium atoms, the same mechanism at work in a thermonuclear** warhead.

**Which is very different than the mechanism at work in a fission warhead, such as we dropped on Japan, or Kim Jong Il & co. recently detonated in a tunnel.

The Moons

Jupiter has at least 63 moons, and more are discovered every couple of years or so. But nearly 50 of these are little rinky-dink things a mile or so across, some number of which were probably asteroids that got “caught”* in the planet’s gravitational field.

*Astronomers can tell this because the orbits are usually weird: wrong direction, or an oddly inclined orbital plane, or highly elliptical orbit.

Only 4 are more or less “moon-sized” by which I mean “roughly the same size as our own moon”, and these are- in order of closest to furthest from Jupiter- Io, Europa, Ganymede and Callisto*.

*From smallest to largest it’s Io, Europa, Callisto, Ganymede.

Side Note: Think about this for a sec- Jupiter’s biggest moon is only twice as massive as ours, but Jupiter is more than 300 times as massive as Earth. That’s why I keep going on about how our moon is so incredibly-freak-show-huge.

These 4 moons are called the Galilean moons in honor of their discoverer, Galileo Galilei, whose accomplishment was of course chronicled in the Queen song “Bohemian Rhapsody.”

Tangent: Does anyone know what the hell that song is about? I must have heard/sung along to it about 1,000 times and I have absolutely no idea.

Nested Tangent: In fairness, I should point out that I pretty much never know what any song is about, even with my favorite bands. For example I’m a long-time Pixies fan, and with the possible exception of “Palace of the Brine*”, I can’t tell you what any of their songs are actually about.

*And that’s only because of the plethora of Utah references.

For something like 15 years I’ve also been a fan of the Undertones. About a year ago my brother- let’s call him “Phil”- mentioned in passing their focus on “The Troubles” in Northern Ireland. I was like, “What?”, whereupon Phil painstakingly walked me through how their music is all about The Troubles… Really, I was clueless*.

*I was like, “Really? But I thought their songs were all about Chocolate and Girls…”

I’m embarrassed to admit I don’t even know what “Stairway to Heaven” is about. Drugs? That’s always my fallback for any ballad from the 60’s or 70’s. “Oh yeah,” I’ll say, nodding thoughtfully, “That song had a real poignant message, you know with what it was saying about drugs and all…” Hey Jude? American Pie? Drugs? Drugs? Really, I don’t know. In fact the only bands/artists whose lyrics I think I understand are The Queers (telling off parents, ex-girlfriends), Lily Allen (snarking at ex-boyfriends) and the Dead Milkmen (automobiles, religion and Reagan.)

I really am a total idiot savant.

When you look at Jupiter through binoculars, you’ll see immediately that it’s not a star, but a little round disk. Right next to the disk, on either or both sides, nicely lined up in a row, will be the 4 Galilean moons. If you don’t see all 4, it’s because 1 or more is either blocked by, or in front of the planet. So first off that’s way cool- you’re seeing a planet and its moons.

But even cooler is if you check it out say 2 nights later- because the moons will be in different positions. And this is the way cool thing about watching Jupiter’s moons, that their orbital periods- their months, as it were- are way, way short.

Our moon has a month of roughly 27 days. That’s how long it takes the moon go completely around the Earth one time. But check out the “months” of the Galileans moons:

This means that over a period of 2 days, the positions of the 4 moons- particularly the inner 2- will be way different, and if you look for just a minute on 2 different nights, and pay just a teensy-weensy bit of attention, you can actually see the moons orbiting around the planet.

All About Orbital Resonance

There’s something else interesting about these orbital periods. Did you notice it? The “months” of Io, Europa and Ganymede have a ratio of 1:2:4. This is an example of a common astronomical phenomenon call orbital resonance, in which two or more orbiting bodies influence each other gravitationally. Such resonances can be either stabilizing or destabilizing. The Galilean resonance (which is technically a mean motion orbital resonance) is stabilizing and self-correcting, keeping the 3 moons in resonance.

Side Note: Most orbital resonances are destabilizing, which can result in such things as changed orbits or gaps in planetary rings.

Callisto isn’t part of the resonance, probably because it’s a ways further out. All 4 of these moons BTW are tidally locked, like our own moon, meaning that they always present the same face to the planet they orbit.

All About Tidal Locking

Geeky-Astro-Tangent: Tidal Locking is way freaky and way cool. Most moons in the solar system are tidally locked. Tidal locking occurs because the gravity of the planet actually deforms the shape of the moon, creating slight bulges in its crust. These bulges act as a slight brake (because the host planet’s gravity acts more strongly on them) with every rotation, gradually slowing down the rotation of the moon until the bulge is always pointed toward the host-planet.

There are (at least) 2 wild things about tidal locking. The first is that angular momentum is conserved, so that the moon orbits farther out from the planet as its rotation slows and eventually locks.

The second is that the same bulging*/braking effect (diagram above, not mine) works on the host-planet, though at a much slower rate. The bigger the planet, the more gradual/weaker the braking effect. So Pluto for example is tidally-locked with its moon Charon**; both always present the same face to each other, and you can only see Charon*** from one side of Pluto (and vice versa.)

*The bulging in Earth’s case is primarily in the oceans, which we notice as tides.

**Technically Pluto and Charon are twin planets, since they orbit a common point exterior to the surface of Pluto. Yes, yes, I know Pluto isn’t officially a planet anymore, but that’s BS. As far as I’m concerned Pluto still is a planet, along with Eris, Sedna, Makemake and Haumea.

*** So the spell-checker doesn’t recognize “Charon”, which suggests that the folks at Microsoft are sadly ignorant of both astronomy and Greek mythology. Oo- that reminds me- I have a Greek mythology-tangent brewing…

So what does all this have to do with us? 2 things. First, the moon is getting higher. Each year, the moon is just a bit farther from the Earth, and a month is getting a teensy bit longer. And second, the Earth’s rotation is slowing down, meaning that our days are getting longer. Eventually, someday in the far, far future, billions of years from now*, the Earth will be tidally locked with the moon, meaning that only half the planet will ever see the moon, and it will always be at the same position in the sky, and it will appear smaller than it does today. A day will be a whole month long, and it’ll be a longer month- something like 55 (of our present-24 hour) days long*.

*Whole bunch of qualifiers for this tangent. First, I had a hell of a time digging up figures for a) at what distance the Moon’s orbit would stabilize (anywhere from 1.25 to 1.6x present-day distance), b) when the Earth would tidally lock (anywhere from “a few billion” to 15 billion years from now) and c) exactly how long the eventual day/month would be. (My kiddie-math skills are nowhere near taking these calcs on myself.) It’s also worth noting that if these events happen later than ~5 billion years from now , then the Sun would go nova first, possibly engulfing both Earth and moon**.

**Though possibly not. The Earth’s orbit might get “higher” relative to the Sun.

Excepting the phases of the moon or a lunar eclipse, this is hands-down the easiest astronomy-in-action thing you can see in the night sky. Planets (including Earth) change positions all the time of course, so it’s not always so easy to see Jupiter, and to see it at such a convenient, early hour. So check it out already. And please comment if you can tell me what Stairway to Heaven is about.