Wikipedia:Reference desk/Archives/Science/2023 November 22

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November 22

Moon made of cheese

'The Moon being made of cheese' was a recurrent trope in children's stories and folklore. People did not really believe in it, it was just a way of mocking a foolish belief. (Personally I think it kind of looks like Wensleydale cheese, but that might be just me.) However, until the 20th century there was no direct empirical evidence against this belief. So, why did rational people dismiss the belief? How could they know or assume it was not made of cheese when directly experiencing it was not possible? Bumptump (talk) 18:18, 22 November 2023 (UTC)

• 19th, not 20th. See Astronomical spectroscopy. --142.112.221.156 (talk) 05:05, 23 November 2023 (UTC)

People tend to apply some form of Occam's razor. --Wrongfilter (talk) 18:26, 22 November 2023 (UTC)

It was as likely to be made of cheese (green cheese, specifically) as was the cow to have jumped over it. ←Baseball Bugs ^{What's up, Doc?} carrots→ 21:28, 22 November 2023 (UTC)

Of course, Wikipedia has an article on everything; see The Moon is made of green cheese. Curiously, the European Space Agency also addresses the issue here. Alansplodge (talk) 21:53, 22 November 2023 (UTC)

According to A Grand Day Out, which seems like a fairly accurate documentary on the subject, it is indeed Wensleydale. DMacks (talk) 01:26, 23 November 2023 (UTC)

A little known fact: the science aspects of the Moon landings were a dressing to hide the covert objective of replenishing the strategic Wensleydale reserves. --Lambiam 10:09, 24 November 2023 (UTC)

Is it possible to have a binary planet system under IAU's definition of a planet?

I was watching a YouTube video regarding the Pluto-Charon system where it was suggested that it should be regarded as binary dwarf system due to the fact that its center of gravity lies outside either body. But my question isn't about Pluto-Charon, but planets in general (or at least ones in our solar system). Given that the third criteria of IAU's definition of planet requires that a planet must have "cleared the neighborhood" around its orbit, is it even possible to have a binary planet system? In other words, does the mere fact that there's a second body in a two-body system mean that it hasn't cleared it's neighborhood and therefore isn't a planet? For example, if there was a second Jupiter-sized body in the same orbit of Jupiter where both orbited a common center of gravity between them, does that mean that Jupiter would no longer be considered a planet under the IAU definition? Or what if the Moon were the same size as the Earth, would the Earth cease to be a planet under the IAU definition? Pealarther (talk) 21:30, 22 November 2023 (UTC)

The IAU definition explicitely covers only the Solar System (I've always found this a bit odd, but that's the way it is). None of the eight known planets is binary, so the question is moot. That might change if Planet Nine exists and turns to be binary. It may well be that the definition might be amended in this case if people think it necessary. --Wrongfilter (talk) 21:53, 22 November 2023 (UTC)

According to our article "Clearing the neighbourhood" (or dynamical dominance) around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence." I would say that "under its gravitational influence" covers a binary planet. The point isn't that the orbit is completely empty of all other objects, but that it is regulated by the planet in question.--User:Khajidha (talk) (contributions) 22:16, 22 November 2023 (UTC)

Your best bet would be to write to the IAU, and maybe pose the question of whether the Earth and the Moon could constitute a binary planet. ←Baseball Bugs ^{What's up, Doc?} carrots→ 00:22, 23 November 2023 (UTC)

There are professional astronomers who argue this. With regard to Pluto–Charon, it's a live debate, as the vote that adopted the IAU's Solar system 'planet' definition (there had not actually been one previous to this) was taken in a conference meeting at which, deliberately or accidentally, many planetary geologists were not present and orbital dynamicists were in the majority.

Humans like 'rules' to be neat. Nature is often messy. {The poster formerly known as 87.81.230.195} 2.218.227.124 (talk) 00:52, 23 November 2023 (UTC)

The planetary-geology definition is equally messy. :) Vesta is not round now, but over a Hubble time it will probably round itself again. Should that count? What about Tethys, which looks round but has a density so low that it is probably porous? Scientists have different opinions about such objects. Double sharp (talk) 08:13, 25 November 2023 (UTC)

Clearing the neighbourhood sounds vague, but our article mentions three ways to quantify this and the gap between planets and dwarf planets is at least three orders of magnitude in each of them, so it's pretty clear. The Moon is in the Earth's own back yard, that is gravitationally bound to the Earth, not an object to be cleared from the neighbourhood, so it doesn't disqualify Earth as a planet.

No formal definition of binary planets exists. We'll make one when we find some object that looks like it could be one. People do talk about binary asteroids. I don't see why the position of the centre of mass would matter. Why would the Pluto–Charon system be a binary dwarf planet, but wouldn't be one if their separation had been a little smaller, and why would the Earth–Moon system not be a binary planet, but would be one if their separation had been a little larger? PiusImpavidus (talk) 10:34, 23 November 2023 (UTC)

Isaac Asimov, in his essay "Just mooning around" in Asimov on Astronomy, argues that the Moon is not a satellite of the Earth, because the effect of the Sun's gravity on the Moon is twice as strong as the pull of the Earth. He suggests that the Moon is therefore a planet in its own right, orbiting the Sun in tandem with the Earth. He points out that the Moon's orbit around the Sun is everywhere concave: the Earth never causes the Moon to fall 'away' from the Sun. -- Verbarson  ^talk_edits 13:43, 23 November 2023 (UTC)

References

The Sun's gravitational effect on the Moon is more than twice that of Earth's on the Moon; consequently, the Moon's trajectory is always convex (as seen when looking Sunward at the entire Sun–Earth–Moon system from a great distance outside Earth–Moon solar orbit), and is nowhere concave (from the same perspective) or looped. That is, the region enclosed by the Moon's orbit of the Sun is a convex set. See Orbit_of_the_Moon#Path of Earth and Moon around Sun. Philvoids (talk) 19:24, 23 November 2023 (UTC)

On the other hand, irregular satellites in general are very perturbed by the Sun. Neptune's moons Psamathe and Neso would actually satisfy Asimov's "tug-of-war" criterion, but obviously Neptune–Psamathe–Neso is too lopsided to reasonably be called a triple planet. Double sharp (talk) 08:07, 25 November 2023 (UTC)

The question is moot, since the IAU does not define what a binary planet is. Jean-Luc Margot mentioned in 2015 a possible extension that would call systems "double planets" if each object would clear the neighbourhood by itself. The Earth–Moon system would qualify. But Saturn–Titan would barely qualify as a very lopsided double planet too by that definition, and Jupiter–Io–Europa–Ganymede–Callisto would become a very lopsided quintuple planet. (These largest moons are huge, being bigger than any of the dwarf planets.) In any case, this isn't official.

It's likely that the gap between planets and dwarf planets only looks clear because we haven't seen enough weird exoplanets yet. A Mars-sized object in the Oort cloud would fail to clear its orbit, but it would be larger in size than two planets. Conversely, around red dwarf stars the mass needed to clear the neighbourhood can actually be less than the mass needed for a body to round itself under gravity (like a scaled-up version of Pan around Saturn; it is far from round, but clears a channel in the rings). Double sharp (talk) 07:57, 25 November 2023 (UTC)