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Although the Moon is Earth's only natural satellite, there are a number of near-Earth objects (NEOs) with orbits that are in resonance with Earth. These have been called "second" moons of Earth or "minimoons". [2] [3] 469219 Kamoʻoalewa, an asteroid discovered on 27 April 2016, is possibly the most stable quasi-satellite of Earth. [4]
Earth also has more than 20 known co-orbitals, including the asteroids 3753 Cruithne and 469219 Kamoʻoalewa, and the occasional temporary satellite, like 2020 CD 3; however, since they do not permanently orbit Earth, they are not considered moons. (See Other moons of Earth and Quasi-satellite.)
Several objects farther from the Sun also have satellite systems consisting of multiple moons, including the complex Plutonian system where multiple objects orbit a common center of mass, as well as many asteroids and plutinos. Apart from the Earth-Moon system and Mars' system of two tiny natural satellites, the other terrestrial planets are ...
Some 4.5 billion years ago, when Earth was only 100 million years old or so, a Mars-sized protoplanet named Theia smashed into our planet, ejecting loads that eventually returned to the Earth’s ...
Considering the Earth–Moon system as a binary planet, its centre of gravity is within Earth, about 4,671 km (2,902 miles) [24] or 73.3% of the Earth's radius from the centre of the Earth. This centre of gravity remains on the line between the centres of the Earth and Moon as the Earth completes its diurnal rotation.
The Moon is Earth's only natural satellite. It orbits at an average distance of 384,400 km (238,900 mi), about 30 times the diameter of Earth. Tidal forces between Earth and the Moon have synchronized the Moon's orbital period (lunar month) with its rotation period at 29.5 Earth days, causing the same side of the Moon to always face Earth.
For precise applications, many coordinate systems have been defined for the Moon, each with a slightly different prime meridian. The IAU recommends the "mean Earth/polar axis" system, [ 1 ] in which the prime meridian is the average direction (from the Moon's center) of the Earth's center.
Some, however, do and become so-called mini-moons. An asteroid called 2020 CD3 was bound to Earth for several years before leaving the planet's orbit in 2020 and another called 2022 NX1 became a ...