h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

Not unless someone's been by lately and towed it there. It's a moon of Saturn.

Sorry. I've put it back now.

Is the moon a planet? I really might be wrong but I could've sworn it was just a satellite of Earth, a.k.a, a moon.

Planets orbit stars (suns).
Satellites (moons) orbit planets.

And yes, Comets also orbit stars, but you can tell them from planets by their tails (comas), and their highly eccentric orbits.

The term, planet, incidentally, is from the Greek meaning "wanderer". To early observerers, the planets appeared as wandering stars, slowly crossing the night sky over a period of nights.

*Slightly* different from the moon, even for pre-Copernicans.

And what's all this about metallic hydrogen,
is that what they make Irn Bru out of?

Ganymede is Jupiter's largest moon, and in Greek mythology was abducted by Zeus (Jupiter) and taken away to do his washing up.

All the other moons of Jupiter have a direct mythological connection with Jupiter - wives, daughters, mistresses, etc., except for Pasiphae who was King Minos' wife, and mother of the minotaur. Although Zeus had a habit of turning up as animals, the white bull that had its way with Pasiphae was a gift from Neptune I think.

If you were going to pinch one of Saturn's moons for Jupiter, then Dione would be the best bet, as she was Zeus's daughter by Aphrodite. The rest of Saturn's moons are mostly Titans.

And, on the subject of odd ones out, Blur's track "Far Out" from the excellent album "Parklife" waxes lyrical over Jupiter and Saturn's moons, then sneaks in a single Shakespearean satellite

If I recall, metallic hydrogen's another one of those holy grails like cold fusion and ISP's who reply to emails.

A few years back a group at Harvard froze and squashed hydrogen and looked to see if it went shiny, but couldn't spot anything. More recently, there've been reports of it "going a bit dark" which might be a sign of phase-change, but there's been no actual observation of a transition from instulator to metal.

Modern-day alchemy anyone?

To give you some idea why it's hard to get metallic hydrogen in the lab, you need a pressure of about 4 million bar (275,000psi).

Most garage forecourt attendants complain if you try to inflate 80psi bicycle tyres with their air lines, so I can't imagine them being too happy about planetary cores.

What is going on here? I'm trying to save Pluto from being declassified as a planet..and you've made it's satellite Charon one as well Is there something you know that the rest of us don't?

Whoops. I screwed that one up. I shouldn't trust completely to memory when writing stuff, methinks.
Umm... I can't fix it myself. Oh well. At least I'm definitively innacurate.

~ Orinoco

>And yes, Comets also orbit stars, but you can tell them
>from planets by their tails (comas), and their highly
>eccentric orbits.

www.dictionary.com's great, isn't it mate?

As for the moon, I don't think it's fair to blame that sleeping satellite.

Tim

> Satellites (moons) orbit planets.

Actually, the Moon and Earth orbit each other. (Or more accurately, they both orbit a common centre of gravity.)

It depends a lot on your nonclemature. The distinction between 'planet' and 'moon' is an arbitrary human one, of course. To nature, they're just big chunks of rock. If you had two perfectly equal-sized chunks of rock locked in orbit around each other, which was orbiting a star, then that would clearly be a double planet. So, double planets are allowed. At the other end of the scale, Phobos is a fairly clear satellite of Mars.

Where do you draw the line? Phobos barely affects Mars, hence the term 'satellite' (which means 'removed', 'unimportant', 'obsequious follower') is deserved. But the moon has a fairly extreme effect on the earth.. it's more than a satellite.

Or do you go on size? In which case, the Moon is larger than Pluto, still nominally a planet.

(Of course, that's a whole 'nother story. Certain astrophysicists are trying to reclassify Pluto as a 'trans-neptonian interloper' or 'wayward comet' instead of planet. But the fact remains that the Moon is bigger than any other satellite in the solar system, and is abnormally enormous for a satellite.)

You could say "the bigger one is the planet, the smaller one is the satellite" but then we're applying a different standard than to suns, where we have "binary stars" regardless of mass. Why doesn't the smaller sun become merely "a satellite" too?

>And yes, Comets also orbit stars, but you can tell them from planets by their tails (comas), and their highly eccentric orbits.

There are comets without visible tails. Most, in fact. And many comets have nicely circular orbits out in the oort cloud. They're called what we call them, and no more.

The 'double-planet' classification I came across while doing a little research for the article. I liked it, so I'm calling us a double-planet too. If enough people do it, then it will be so. That's how things get named.

~ Orinoco

See my reasoning on the Earth/Moon thing. Here's a reference:
http://www.seds.org/nineplanets/nineplanets/pluto.html
"Charon is unusual in that it is the largest moon with respect to its primary planet in the Solar System (a distinction once held by Earth's Moon). Some prefer to think of Pluto/Charon as a double planet rather than a planet and a moon."

I've thought long and hard about where I stand on the issue of Pluto's nonclemanture. I've decided this:

If it's a big thing that I can stand on and not fall off too much, then it's a planet. Unless someone bigger and nastier than me wants to call it something else.

Thus, since most astrophysicists aren't bodybuilders and few know where I live, it's a planet. And so is Charon.

~ Orinoco

FAAAAAR to intellectual for me, all this discussion on circumnavigational issues through the circumnication of an apex via a distention of an emotional barage of intradictions and post stress disorder.

I've removed the reference to Titan from the entry.

cheers, i think!

Whys that then?

Oh no..not all reference surely!!

> Actually, the Moon and Earth orbit each other.
> (Or more accurately, they both orbit a common centre of gravity.)

Fascinating. Can you point me at a reference source for this gem?

Sean.

Yeah. It's a shame more people don't use it while RESEARCHING (clue's in the name, guys) their articles.

Since the only reference was 'the biggest being Titan' when talking about Jupiter's moons, I don't think it's a real loss to the article.