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

You could have changed it to "the biggest being Ganymede". That's the point of the attached fora, surely?

Happy now?

Pending a reply from Patrick Moore, I've popped the word 'secondary' between 'amazing' and 'planet'.

I would say an honest method of defining whether or not a system is a planet-satellite system or a double-planet system would be by way of examining the orbital centre between the two components.

A planet orbits Sol (using our own system as a reference point) while a satellite would orbit the planet AS the planet orbits Sol. The centre of orbit for the planet is Sol while the centre of orbit for the satellite is the planet. The orbit of the satellite, depending on just how much mass it has, would cause the planet to 'wobble' slightly, during each orbital period of the satellite, in its orbit around Sol. However - the planet is still basically orbiting Sol and not its satellite.

In a Double-planet system the mass of each would be strong enough to create a gravitational situation where the orbital centre between the two is neither planet but actually between the two worlds. In this situation the planets are actually orbiting EACH OTHER while the dual-system as a whole orbits Sol. You could argue that both in this case would be satellites and not planets as they both orbit each other but as neither world has an actual planetary body as it's centre of orbit I personally would consider they were a Double-Planet system both orbiting the Primary Star. (I know I'm gonna get an argument here ... C'mon - Bite... Bite...)

I actually don't know which category Pluto and Charon fits so I can't offer an opinion on the truth of it. The truth would depend on which above description suits.

HOWEVER

RE Earth and Luna ( As our sun has its own name, SOL, the Moon also has it's own name, LUNA - the term Moon is a description as meaning Satellite ie Phobos is a Moon, or Satellite, of Mars)

Even though Luna is the largest Satellite in the Sol System it is still a satellite - a Moon.

Earth and Luna are definitley NOT a double planet system. We would probably not be in existance as a race if it were as the tidal forces and constant climatic upheavals would be enormous and destructive.

Earth orbits Sol and Luna orbits Earth. The Earth definitely does 'wobble'in its orbit around Sol due to Luna's orbit but the Orbital Centre between Earth and Luna is still definitely the Earth. Luna orbits Earth but the Earth definitely does NOT orbit Luna. Luna is, thankfully for us, simply not big enough and subequently doesn't have a large enough gravitational field of it's own to create such an effect.

Therefore Luna is a satellite, a Moon, of Earth and is not a planet.

If Luna wasn't in Earth orbit (as in the series Space 1999 - God rest it's hopelessly illogical but yet entertainingly original concept) and had it's own orbit around Sol then it definitely WOULD be a planet.

The terms Planet and Satellite relate to the orbital situation of the subject body in question - not it's size or mass.

A planet orbits it's PRIMARY (star) while a satellite orbits a SECONDARY (Planet).

IE if Jupiter had a satellite the size of Earth it would still be classed as a satellite, or moon in spite of being planet sized.

Heh .. If moons were named after animals at all then they could be referred to as 'Pets' of the planet they orbit. That would raise some interesting imagery come feeding time, wouldn't it - snigger.

I have some young mates I amusingly refer to as 'pets'- they almost are too.

Hey - c'mon - I'm big but not yet planet-sized - maybe after a few more cheesecakes... but then there was that letter I got from NASA announcing they had assigned me an Orbital Reference of my very own...

So what would you consider a satelite of a satelite? That is to say, something the size of the moon, orbiting something the size of a small planet, in turn orbiting something the size of jupiter, in turn orbiting a star.

Zakko seems to be somewhat of an astrophysicist, and he is quite build (so he claims, and in case he's not lying, I'll accept his claim as the truth)

Still a satellite but what an interesting thought. The concept deserves its own title I'm sure.

A planet could still be considered a satellite of it's star so therefore a moon, in that case, could then be a satellite of a satellite.

Anyone out there actually heard of any known satellites orbiting known planets that have satellites of their own.

As Luna is the largest satellite of any planet orbiting Sol and doesn't I would guess not but it must be possible - like I mean - why not.

Surely Luna could be capable of holding a moon like say Phobos in it's orbit if it were passing by slowly enough to be captured into an orbit (and not hit Earth - Aaarrggghhhhh)

As I recall, the Appollo missions sent up little Tin Cans carrying people to orbit the moon. I guess for those few brief moments, Luna did have it's own satelite.

Not an Astrophysicist but probably have been called an Astro-nut and some people I know reckon I'm permanently in orbit but they just can't figure out exactly where that orbit is.

I have an deep interest in Stellar-Science but I don't know everything. I consider all my opinions to be right as per my own sense of logic and welcome being proved wrong as this would obviously improve my life-education. Hopefully I'm not as 'Lost in Space' as some of my mates like to think.

Maybe the Guide would categorize me as some kinda unexplained semi-evolved maybe-avatar of self-absorbed observational thought-processes that may or may not be either rational or irrational but merely an individual repository of opinions that simply reflect the experiences of one thought-centre within our infinite universe and therefore qualifies to be worth listening to and evaluated as such anyway.

Shit - gotta catch my breath after that one - now - did that all REALLY make any kinda sense at all - ahhhhh - who knows. Maybe I am truly the only one that knows that I do think and realize therefore I subsequently do in fact exist and so qualify to breathe and think about anything at all anyway.

Maybe I'm just scared that if I fart I'll disappear altogether.

Sure - they gotta count - in an infinite universe there is no smallest

We all love a good run-on sentence.
I believe 'found in space' is a term you may prefer to use.

It was Will Robinson - I saw him do it - I really did. He was helped by Lister when they were playing Space Pool together last continuum, as you recall.

I've been found.. I've been found... YAHOOOOOO - I do exist after all. Cya Will, bye Robot.

Don't get ahead of yourself.
Many people have "found" god but still can't prove that he exists.
Let's get out of here and go somewhere that loads quicker. (how about that hi there thing I left you)

Anycase - someone told me Pluto was gay - so, if this is so, why would he want Charon hanging around him anyways.

Sorry to pint this out but Luna is not the biggest satelite in the Sol system. I am sure that you will find Ganymede, the 3rd galilean satelite of jupiter, is in fact the biggest satelite in Sol

I can tell you it is a planet. The moon has a significant effect on the centre of gravity of rotation placing it somewhere just below (I believe) the Earth's surface. Its still within the Earth but its significant enough. That is why it is a double planet.

Also the moon did get a satellite for a while. An asteroid got pulled into a figure of eight loop around the moon and the Earth (Lorenz Attractor). It may still be there.

>> Satellites (moons) orbit planets.

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

The planet/moon thing is much more complex than you might think. Though Earth and Moon indeed orbit one another (with the center of mass of the system located about 1000 km beneath Earth's crust), the gravitational attraction of the Sun on the Moon is TWICE that of the Earth on the Moon (this is not true of the moons of the outer planets, including Pluto/Charon).

If you were to look at the Moon's path around the Sun, ignoring the presence of the Earth, it would be almost impossible to distinguish it from the orbit of an independent planet. Contrary to popular belief, there is no generally accepted definition of "planet" or "moon."

You know I didn't have a clue,i'm so glad your around to ask,yours moneywort.