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The moon is not that small, it is 27% of earth's size. Any other satellite in the solar system is way smaller compared to the planet they orbit. I dare to say t

The moon is not that small, it is 27% of earth's size. Any other satellite in the solar system is way smaller compared to the planet they orbit. I dare to say that earth and moon are twin planets instead of a planet and a satellite.

Mtinie 10y ago
@ConceptJunkie > The Moon is about 2% of Earth's mass, not 27%. 2% of mass, yes, but also 27% of the diameter. You are both right.
- mirimir 10y ago
  My first reaction was WTF? Then 0.27^3 = 0.0197 And then, only 60% as dense (no core) so 0.0119 :)
  - saalweachter 10y ago
    One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth. And only just barely! You get to around 99% of the Earth's mass with the rest of the inner solar system. All of the other rocky bodies in the solar system give you another 10% of Earth's mass out to Pluto and another 10% beyond.
    - mirimir 10y ago
      You just blew my mind ;) What do we know about the gas giants' cores?
      - saalweachter 10y ago
        Gas giant cores are weird. Based on models of solar system formation, we're like, pretty sure they started with rocky (or icy) cores that were dozens of times bigger than Earth. But we're not sure they're still there. Due to the intense conditions inside a gas giant, we're not sure if they still have a rocky core, or if the cores have liquified or even convected away to mix with the gasses. So the answer could be "there are dozens of times the Earth's mass worth of rocky material inside gas giants" or "there is no rocky material inside gas giants", or anywhere in-between. We need more detailed study of the gas giants before we can answer that question. Models of solar system formation also suggest the Kuiper belt started with dozens of times Earth's mass, but we only see about 10% there now, and we only have pretty good guesses about what happened. (I mean, it almost certainly got ejected, but the "why" is harder.)
        mirimir 10y ago
        Thanks. I asked because searching seemed to be telling me that we have only theoretical arguments about gas giants' cores. That surprised me at first, but then I realized how little data we probably have. Basically, mass and its distribution, plus spectroscopic data about surface composition, right?
        Sharlin 10y ago
        Juno is hopefully going to tell us more :) https://en.wikipedia.org/wiki/Juno_(spacecraft)#Scientific_objectives https://en.wikipedia.org/wiki/Juno_(spacecraft)#Scientific_o...
    - Arizhel 10y ago
      >One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth. Is that correct? Venus alone is about 90% the size of Earth, and Mars roughly 33%. It really seems like all those bodies put together should be greater than 1 Earth mass (but less than 2 Earth masses).
      - saalweachter 10y ago
        That's size, not mass. Venus is about 81% Earth's mass, and Mars is only a little over 10%. Mercury gets you another 5.5%, the Moon (as above) another 1.2%, and the asteroids are negligible. In the outer solar system, it's basically all the Galilean Moons + Titan. ETA: Hmm, or maybe those are surface gravity numbers you're thinking of?
        Arizhel 10y ago
        Yep, I was roughly equating mass with surface gravity, which I see now is incorrect. I do find it really interesting and a little mind-boggling that the Moon, as huge as it is, with a good 1/6 of Earth's gravity, only has 1.2% of Earth's mass. Same with Mars: a good 1/3 of Earth's gravity, but a mere 10% of its mass. I really thought there'd be more direct correlation between mass and gravity than that. Of course, surface gravity is related to mass with both density and radius, but still, I would have assumed that the densities of these bodies would have been rather similar, as they're all small, rocky worlds.
        saalweachter 10y ago
        The densities do vary by almost a factor of 2 (~3 g/cc for the Moon, 5.5 g/cc for the Earth), but the larger problem is that surface gravity is a linear function of the radius, and mass is the cube of the radius. Consequently, the mass of an object with a given surface gravity is inversely proportional to the square of the density. If the Moon were the same density as the Earth, and had the same surface gravity as it does now, it'd actually mass less than it does now -- and Uranus, despite have 15x the mass of Earth, has a lower surface gravity, because its density is so low. The rule of thumb for space is "any way your intuition can be wrong, it will be wrong". :-)
angry_octet 10y ago
Radius 27%, mass 1.2%. http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html
bergoid 10y ago
Unless you are excluding Pluto deliberately for not being "a planet", its moon Charon has half the diameter of Pluto. They are even both orbiting a common center of gravity outside Pluto. https://en.wikipedia.org/wiki/Charon_(moon) https://en.wikipedia.org/wiki/Charon_(moon)

Mtinie 10y ago

@ConceptJunkie > The Moon is about 2% of Earth's mass, not 27%. 2% of mass, yes, but also 27% of the diameter. You are both right.

mirimir 10y ago
My first reaction was WTF? Then 0.27^3 = 0.0197 And then, only 60% as dense (no core) so 0.0119 :)
- saalweachter 10y ago
  One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth. And only just barely! You get to around 99% of the Earth's mass with the rest of the inner solar system. All of the other rocky bodies in the solar system give you another 10% of Earth's mass out to Pluto and another 10% beyond.
  - mirimir 10y ago
    You just blew my mind ;) What do we know about the gas giants' cores?
    - saalweachter 10y ago
      Gas giant cores are weird. Based on models of solar system formation, we're like, pretty sure they started with rocky (or icy) cores that were dozens of times bigger than Earth. But we're not sure they're still there. Due to the intense conditions inside a gas giant, we're not sure if they still have a rocky core, or if the cores have liquified or even convected away to mix with the gasses. So the answer could be "there are dozens of times the Earth's mass worth of rocky material inside gas giants" or "there is no rocky material inside gas giants", or anywhere in-between. We need more detailed study of the gas giants before we can answer that question. Models of solar system formation also suggest the Kuiper belt started with dozens of times Earth's mass, but we only see about 10% there now, and we only have pretty good guesses about what happened. (I mean, it almost certainly got ejected, but the "why" is harder.)
      - mirimir 10y ago
        Thanks. I asked because searching seemed to be telling me that we have only theoretical arguments about gas giants' cores. That surprised me at first, but then I realized how little data we probably have. Basically, mass and its distribution, plus spectroscopic data about surface composition, right?
        Sharlin 10y ago
        Juno is hopefully going to tell us more :) https://en.wikipedia.org/wiki/Juno_(spacecraft)#Scientific_objectives https://en.wikipedia.org/wiki/Juno_(spacecraft)#Scientific_o...
  - Arizhel 10y ago
    >One of my scale numbers is, everything between Jupiter and the Sun masses less than 2 Earths -- Venus, Mars, Mercury, the Moon, and all the asteroids mass less than the Earth. Is that correct? Venus alone is about 90% the size of Earth, and Mars roughly 33%. It really seems like all those bodies put together should be greater than 1 Earth mass (but less than 2 Earth masses).
    - saalweachter 10y ago
      That's size, not mass. Venus is about 81% Earth's mass, and Mars is only a little over 10%. Mercury gets you another 5.5%, the Moon (as above) another 1.2%, and the asteroids are negligible. In the outer solar system, it's basically all the Galilean Moons + Titan. ETA: Hmm, or maybe those are surface gravity numbers you're thinking of?
      - Arizhel 10y ago
        Yep, I was roughly equating mass with surface gravity, which I see now is incorrect. I do find it really interesting and a little mind-boggling that the Moon, as huge as it is, with a good 1/6 of Earth's gravity, only has 1.2% of Earth's mass. Same with Mars: a good 1/3 of Earth's gravity, but a mere 10% of its mass. I really thought there'd be more direct correlation between mass and gravity than that. Of course, surface gravity is related to mass with both density and radius, but still, I would have assumed that the densities of these bodies would have been rather similar, as they're all small, rocky worlds.
        saalweachter 10y ago
        The densities do vary by almost a factor of 2 (~3 g/cc for the Moon, 5.5 g/cc for the Earth), but the larger problem is that surface gravity is a linear function of the radius, and mass is the cube of the radius. Consequently, the mass of an object with a given surface gravity is inversely proportional to the square of the density. If the Moon were the same density as the Earth, and had the same surface gravity as it does now, it'd actually mass less than it does now -- and Uranus, despite have 15x the mass of Earth, has a lower surface gravity, because its density is so low. The rule of thumb for space is "any way your intuition can be wrong, it will be wrong". :-)

angry_octet 10y ago

Radius 27%, mass 1.2%. http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html http://nssdc.gsfc.nasa.gov/planetary/factsheet/moonfact.html

bergoid 10y ago

Unless you are excluding Pluto deliberately for not being "a planet", its moon Charon has half the diameter of Pluto. They are even both orbiting a common center of gravity outside Pluto. https://en.wikipedia.org/wiki/Charon_(moon) https://en.wikipedia.org/wiki/Charon_(moon)