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Since the Moon's orbit is always concave towards the Sun, one could even argue that the Moon is a planet, not a satellite, it just happens to have an orbit that
by pdonis 1mo ago
Since the Moon's orbit is always concave towards the Sun, one could even argue that the Moon is a planet, not a satellite, it just happens to have an orbit that is closely coupled to that of another planet, Earth.
As I understand it, in fact, there was discussion about defining systems like Earth-Moon as double planets, but that didn't make it into the final definition.
- nephihaha 1mo agoI have heard people argue that we are in fact in a kind of binary planetary system. No other planet has such a large satellite relative to its size, until you get to the likes of Pluto etc.
- Dylan16807 1mo agoThe Earth is closer but I don't think there's any good argument to say it's actually a binary system. Notably the Earth-Moon barycenter stays inside the Earth, and the Sun-Jupiter barycenter doesn't stay inside the Sun. But barycenters are too easy to get outside of objects anyway.
- pdonis 1mo agoAs I understand it, one suggested "double planet" definition was that the barycenter would have to be outside both objects. Earth-Moon doesn't (quite) satisfy that condition, but for example, Pluto-Charon does.
- nephihaha 1mo agoThe Earth-Moon system is certainly anomalous within our Solar System. There is no such equivalent among the three other rocky planets, and while there are large moons around the other planets, the planets themselves are far larger.
- pdonis 1mo agoI assume you're not including Pluto-Charon here (presumably since Pluto is now demoted to "dwarf planet")? Their mass ratio is only about 8:1, which is far smaller than Earth-Moon, 81:1.
- fluoridation 1mo ago>the Moon's orbit is always concave towards the Sun Huh?
- bspammer 1mo agoHere’s a visualisation https://www.youtube.com/watch?v=KBcxuM-qXec https://www.youtube.com/watch?v=KBcxuM-qXec
- pdonis 1mo agoThe Sun's gravitational pull on the Moon is always stronger than the Earth's. (Do the math and see--on average the Sun's pull is more than twice as strong as the Earth's.) So the net acceleration of the Moon is always towards the Sun, never towards the Earth.