Which Planet Is Closest?
There are some decent arguments that the Moon should be considered to be a planet, which would make the Earth/Moon two planets sharing an orbit rather than a planet and its satellite.<p>The Moon, if ever recognized as a planet, will of course be the closest one.<p>Here are some of the ways the Moon is more like a planet than a satellite [1].<p>I also remember an essay by Isaac Asimov that brought up another one. For all the other satellites in the solar system, if you compute the ratio of gravitation force on them from the planet they orbit to the gravitation force from the Sun on them, that ratio is greater than one.<p>For the Moon, it is less than one! The Sun exerts more force on the Moon than the Earth does.<p>Related to this, if you look at the path traced out by all the other satellites in the solar system as their planet makes one orbit around the Sun, that path is sometimes concave and sometimes convex, as the satellite circles the planet. The planet takes a convex path around the Sun, and the satellite makes a little circle around it.<p>An observer on the Sun would see the planet moving always moving forward, and see the satellite sometimes moving forward and sometimes moving backward.<p>For the Moon, that path is convex. It's basically a rounded dodecagon, as is the path of Earth. They are out of phase with each other, so that when one is near a vertex of the rounded dodecagon, the other is in the middle of a side, and so can slip past the other.<p>An observer on the Sun would see the Earth and Moon both always moving in the same direction, with them speeding up and slowing down, so that about 12 times in a year they trade the lead, and the overtaking one always passing on the same side.<p>[1] <a href="https://blogs.nasa.gov/Watch_the_Skies/2009/07/10/post_1247240047519/" rel="nofollow">https://blogs.nasa.gov/Watch_the_Skies/2009/07/10/post_12472...</a>