Number 4: Neso

Hey, Sky Fans! Wednesday already? Tonight’s the fifth night of Hanukkah already? I hope you’re having a great week, and are enjoying your holidays. It became a sad day yesterday when we learned that Carried Fisher had passed.

Here to start the second half of our list, we find ourselves at Neptune. I’ve always been a big fan of the other blue planet. In fact, other than the blue planet that my couch and coffee are on as I write this, it might be my favorite planet of them all. It’s so far, and so… so difficult to see, and so far (plus, those jokes about its name!) that it’s always interesting.

As a bit of a nifty aside, there’s lots of talk that Uranus, whose discovery is credited to William Herschel in 1781, is bright enough to have been seen by the ancients, as the other planets were. If you know where to look, it’s actually surprisingly easy to see even in suburban skies with a pair of binoculars. Neptune, though, is tough. In January 2017, it’ll be really close to Mars in the sky, hidden by its own distance and the weakness of reflected sunlight. I’m sure I’ll fail, but I’m going to try to see it in binoculars if I can.

Neptune also has the great story, which I wrote about a while ago, that it was found in 1846 by Urbain LeVerrier and, separately, by John Couch Adams using math and science, not by observation, like the others were. Between that and Voyager 2’s visit there in 1989, it’s even more proof of the amazing things people can accomplish.

#4: Neso

There wasn’t a lot of news from Neptune or its 14 moons this year, but even so, two of its moons are high up on my list of all-around favorites. So, it was harder to pick between those two than to pick whether one should be on the list at all.

Triton is the biggest of all of Neptune’s moons, by far. Like I mentioned the other day, it’s one of that group of three with really similar names: Triton (Neptune), Titania (Uranus), and Titan (Saturn). It’s bit smaller than our own Moon, while all of the rest of Neptune’s moons fall in behind it, and it’s the biggest moon in the solar system with a retrograde orbit, backward relative to the rotation of its planet. There’s a lot of thought that it was a dwarf planet that wandered too close to Neptune and got trapped by its gravity.

It’s Neso, though, that edged Triton out this time. Maybe if I’d written this post later today or tomorrow Triton would have been the one. Neso’s a small pile of rocks, less than 40 miles (65 km) across, but what makes it really interesting is its orbit. Neso is so small and so far that I couldn’t find any photos of it. So, Triton wins a consolation prize.

Neptune & Triton from Voyager 2, 1989
Neptune & Triton from Voyager 2, 1989 (from NASA)

Neso has the distinction and honor of being the most distant moon in the solar system. “What?” you ask, “What about Dysnomia, which we talked about just yesterday?” you say, shaking your fist and remembering that there are kids around, “Isn’t that the most distant?” It is, but Neso’s the one whose orbit is the most distant from it’s primary (the thing it orbits directly).

Its in such a long, weird, and again, retrograde, orbit that its farthest point from Neptune is about 45 million miles (72 million km), and takes over 29 years to make it around once. If you remember back to the astronomy text book sitting over there on your bookshelf, the planet Mercury averages 36 million miles from the Sun, takes only 88 Earth days to orbit, and its aphelion, farthest point in its orbit around the Sun, is about 44 million miles (70 million km). Twenty-nine years. Wow. Imagine if The Moon took 29 years, not 29 days, to orbit?

How ’bout that? The farthest planet from the Sun has a moon that, itself, is farther from the planet than the nearest planet to the Sun is. That gives you a bit of a feel for how big things are out there. To keep things easy, Neptune’s orbit is about a billion miles in one direction from Uranus’s and another billionthat’s billion with a bin the other direction from Pluto’s, so there’s plenty of room for a moon that’s 40 million miles away.

If Neso were orbiting Earth, and able to not get thrown off by other gravitational problems, soon or later it’d smash into Venus. The line of Venus’s orbit is only about 25 million miles away from us.

Things are actually more complicated than whether the orbits themselves are near each other, if for no other reason than the planets aren’t all lined up along those orbits at the same time. They’re also inclined, and… just… it’s not easy.

So, here’s to number 4 on our list, Neso for sticking around even though it’s is farther away from Neptune than Mercury is from the Sun!

Clear skies, everyone!

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