I wanted us to build something together.

Here it is to live on my blog and yours forever:

Even shorn of her layers of protective clothing, Lady Sybil Ramkin was still toweringly big. Vimes knew that the barbarian hublander folk had legends about great chain-mailed, armor-bra’d, carthorse-riding maidens who swooped down on battlefields and carried off dead warriors on their cropper to a glorious roistering afterlife, while singing in a pleasing mezzo-soprano. Lady Ramkin could have been one of them. She could have led them. She could have carried off a battalion. When she spoke, every word was like a hearty slap on the back and clanged with the aristocratic self-assurance of the totally well-bred. The vowel sounds alone would have cut teak.

Vimes’s ragged forebears were used to voices like that, usually from heavily-armored people on the back of a war charger, telling them why it would be a jolly good idea, don’tcherknow, to charge the enemy and hit them for six. His legs wanted to stand to attention.

Prehistoric men would have worshiped her, and in fact had amazingly managed to carve life like statues of her thousands of years ago.

Ganymede, the biggest moon in the solar system, is 0.008% the mass of Jupiter. The Moon is a bit over 1.2% of Earth's mass and a solid 27% of its radius. There's no other planet* in the solar system with a satellite anywhere close to the kind of similarity in size that the Earth-Moon system has.

You might also say "Fine but it's literally called 'The Moon' so that's a bit silly". To which I say that I've been calling it that to be more easily understood but it would be extremely easy to switch to calling it "Luna" which is what most people do when they encounter situations where saying "The Moon" creates ambiguity - like when writing sci-fi or a nontrivial amount of astronomical research.

In conclusion, lumping Luna in with the satellites of other bodies is unhelpful because it is geophysically distinct from most of them, and orbitally distinct from all of them. Luna is a planet and it's rad that we can see one so clearly in the night sky.

Hi, I'm an astrophysicist, I am well aware of the IAU (International Astronomical Union) definition of a planet. It is a definition that is relatively controversial and doesn't really make a lot of sense - it was mostly written to prevent the list of planets from getting too long as we discovered more dwarfs. This definition does not include anything to do with barycentres - I'll get back to that.

The IAU definition of a planet has 3 parts:

1. Object in orbit around our Sun

2. Object has reached hydrostatic equilibrium (i.e. it's a sphere, not a potato)

3. Object has cleared its neighborhood around its orbit

Both Earth and Luna satisfy 2. I will accept that 1 is debatable, but the crux of my argument is that Luna is the size of a terrestrial planet (yes, ~1% of Earth's mass, but still huge and of roughly the same order of magnitude as Mercury, which is a mere ~5% of Earth's mass) and exceptionally large relative to its orbital partner compared to any other "satellite", a factor the IAU does not account for.

It's important to note that 1 literally does not permit binary planets - even if both bodies are identical, one must be the "moon". This is major criticism of the IAU definition. It also doesn't account for exoplanets or rogue planets, but that's another story.

3 is where this definition falls apart. You could easily argue that most of the planets in our solar system fail depending on how you interpret it, because it's so fuzzy. The Earth fails, because there's a planet-sized body chilling out in its orbit (Luna), Jupiter fails because it has huge quantities of asteroids trapped in its Lagrange points, etc.

In the latter case, we say that because Jupiter is determining the motion of these bodies, it still counts. For the Earth we simply ignore Luna and say that the rule is more about stray bodies than orbital partners. But by the same logic we can say that for Luna we ignore the Earth, and Luna passes.

So I would argue that the IAU definition is bad, but if you fixed it so that it allowed binary systems to exist, it would readily define Luna as a planet in my view (if you deleted the Earth from the solar system and left Luna, it would unambiguously meet the criteria).

Now let's talk about barycentres.

The barycentre of two bodies in orbit is just their overall centre of mass. People often point to the external barycentre of the Pluto-Charon system to indicate that they are binary dwarfs. However, this is a poor metric in my view, because it's highly dependent on orbital distance.

The centre of mass of the Earth-Luna system is inside the Earth at present, but if Luna simply orbited further away, the barycentre would be in the empty space between them, with nothing about the two bodies individually, or the qualitative nature of their orbits, having changed. In fact, given enough time and pretending the Sun won't consume both bodies in a few billion years, Luna would actually drift far enough away due to tidal interactions for this to happen - it would be silly for it to suddenly be a planet one day when it wasn't the day before.

It's really frustrating to me when people lash out and say "You're wrong because there's a definition, for the love of god look up the definition" about a topic where I think it's clear my point is that I KNOW the definition and I disagree with it. And that's allowed! Definitions are made up! "Planets" are made up, and I think we should make them up differently!

All the things in our solar system are just different kinds of rocks dancing to each other's gravity- they're all affected by each one's gravity, even in tiny ways. They do not fall neatly into subcategories "planet" and "not a planet" - we made this distinction up because we wanted it. We noticed that some of the bodies in our solar system seemed much more important and dominant than others and we wanted a name for that. But the planets don't know that- they don't have an inherent major distinction between them, nor are they obligated to. When we wanted to come up with a way to clearly decide which bodies were planets, we had to make something up.

What we made up is a little bit vague, and even if it were extremely clear cut, we could still debate whether it was a reasonable or intuitive or useful definition.