Hacker News Reader

Comment by naikrovek | Hacker News Reader

You’re probably right, but I still think there’s room for things like this.

What’s a “moon” versus a “planet”? Earth is a moon of Sol, is it not? Why is having a lot of planets a problem in the first place? Why do we have to restrict the definition at all? If 2-3 stars are at the center of a star system, are the planets in that star system planets, or something else? What if they’re small?

This whole scene is ripe for people who want to put their stamp of opinion on something to go nuts arbitrarily.

View on HN

What’s a “moon” versus a “planet”? Earth is a moon of Sol, is it not?

We already have the word "satellite" for "things that go around other things" right? I think "moon" is just "satellite of a planet" for convenience in discussing that subset.

> Why is having a lot of planets a problem in the first place?

I think keeping the number manageable is explicitly something we keep around to help kids grasp the main entities in the solar system. If we just said "there are 235 planets" it would be silly to try to teach them all, so we'd probably just settle for "The top 10 biggest planets" or something. Having a definition instead of a number to bound the set isn't much less arbitrary than teaching the "top 10," but since the long tail clearly starts after #8, "Top 8" would be the only guaranteed stable set to give special treatment to, which is what we've arrived at with the official definition.

> Earth is a moon of Sol, is it not?

No. The sun is a star, so it doesn't get to have moons. It has planets. If Jupiter started generating heat from nuclear fusion reactions, we'd call Io a planet right before we boiled to death, and with our dying breath we'd add "and also, it's no longer a moon".

Putting a leash on a cat doesn't make it a dog, and both of those creatures have four legs even if you call the tails of each a leg. A planet revolves around a star, a moon revolves around a planet (revolving around a star). There's further elements which make Ceres and Ed White's lost glove not a planet or a moon, respectively, but planets and moons are distinct and non-overlapping categories.

Why would we be boiling to death in this situation? Jupiter is much further from Earth than the sun is and Jupiter is also much smaller. Heat would increase, but probably not that much.

I would rather expect Earth to not have a stable orbit. Either ripped apart from fluctuating tidal forces, flung away or in one of suns (thus boiling would happen, briefly) or just generally a much more extreme place compared to now.

> What’s a “moon” versus a “planet”? Earth is a moon of Sol, is it not?

Planets orbit stars. Moons orbit planets. That's a clear and easy distinction. Planet vs dwarf planet isn't so clear to most.

Don't planets and moons both orbit their center of mass? The distinction only seems to make sense if the masses of the two bodies are far apart. If they have similar mass, which is the moon and which the planet?

Indeed, before the planet/dwarf planet debate, Pluto and Charon were sometimes called a “binary planet” because their center of rotation is outside the volume of either body.

What’s a moon that orbits a moon? Doesn’t that make the orbited moon a planet? Pluto has moons. But it’s not a planet? ???

If a super massive planet and two stars orbit each other in the center of a star system, all the planets that orbit those stars are moons then technically, right?

This is all super fuzzy and completely arbitrary. These concepts are constructs. Humans could make them better. Instead, everyone decided to make it all worse.

Don't forgot about moonlets!

No. A star is not a planet. The bodies orbiting the stars are planets, or dwarf planets, asteroids or comets. Bodies orbiting them are moons. Bodies orbiting the moons don't have a name.

> Bodies orbiting the moons don't have a name.

Satellites? Natural or manmade, small or big, doesn't matter.

A natural moon of a moon is called a subsatellite: https://en.m.wikipedia.org/wiki/Subsatellite

At present, purely theoretical: we don’t know of any. They are probably quite rare, but we don’t really know - maybe, in centuries to come, we’ll know of dozens of examples; maybe, there are none to find in this entire galaxy

Reply on news.ycombinator.com