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The moon is not a planet.
by dal 6y ago
The moon is not a planet.
- rriepe 6y agoI'd argue Mars is still the most impressive. Rockets in no atmosphere is easy. Floating on Venus is easy because it has more atmo than any other rocky planet. Figuring out something that works well in Mars' thin (but still there) atmosphere, especially a helicopter, is really impressive. The celestial body classification is just a cherry on top.
- dotancohen 6y agoBy many definitions the Earth-Moon system is a dual planetary system. The moon has more that 1% the mass of the Earth, which is by far the largest primary-secondary "not a planet" system in the solar system. Only not-a-planet Pluto and Charon exceed it. Also, Isaac Asimov considered the system a dual-planet system as the Moon's path around the sun is at no point convex nor retrograde.
- jojobas 6y agoEarth-Moon centre of mass is inside Earth, so, it's obvious that the Moon is orbiting the Earth, not both orbiting something else. 1% is an arbitrary threshold and solar orbit is dependent on external circumstances rather than the Earth-Moon system itself.
- dotancohen 6y ago> Earth-Moon centre of mass is inside Earth, so, it's > obvious that the Moon is orbiting the Earth, not both > orbiting something else. I used to agree with that school of thought, until I came up with a small thought experiment. Consider two bodies that are right on the planet-moon / dual-planet definition threshold. Intuitively, increasing the orbital distance between them would push them in the direction of which definition: planet-moon or dual-planet? I'd tend to say that separating them further would tend more towards planet-moon. Yet doing so moves the system's center of mass outside the larger body, so would actually push the system into the dual-planet definition by that criterion.
- shawnz 6y agoThe barycentre being inside of one of the things in the system is also an arbitrary threshold, isn't it? The barycentre of the Sun and Jupiter isn't inside of the Sun.. so does that mean Jupiter doesn't orbit the Sun?
- trailbits 6y agoIt's a fun exercise to work out the Gravitational Force between the Moon and the Earth and between the Moon and our Sun. It will surprise many that the Sun exerts more force on the Moon than the Earth does. So the Earth inflicts a strong perturbation on the motion of the Moon but it's not the dominant gravitational force on the Moon.
- tempestn 6y agoTechnically the moon is a dwarf planet, as it has sufficient gravity to form itself into a spheroid shape, but it has not cleared its orbit.
- Daho0n 6y agoIt's Pluto all over again!