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The Moon is unique among natural satellites in that it experiences a stronger gravitational attraction to the Sun than to its primary, Earth. As a consequence, its path is always concave to the Sun. It can be argued that this makes the Moon a planet, orbiting the Sun, rather than a satellite of Earth. Usually, it is considered to be in orbit around Earth, but its orbit is substantially distorted from a simple elliptical shape by the gravity of the Sun, which includes a tidal gradient that causes the Moon to be attracted less strongly in the direction of Earth at full and new moon than at the quarter phases (in a frame of reference in which Earth is stationary). This perturbs the orbit so as to make its curvature more acute in the directions of the quarter phases than elsewhere. If the orbit were otherwise circular, this perturbation would make it approximately elliptical, with its major axis lying along the direction of Earth's motion around the Sun. Earth would be at the centre of this ellipse, rather than at one of its foci. In reality, this perturbation is superimposed on the elliptical orbit of the Moon, rotating with the seasons. The Moon's motion is therefore quite complex, and can be calculated only very approximately by assuming the orbit to be an ellipse. Likewise, the parameters of the orbit, eccentricity, semimajor axis, etc., can be stated only as approximate averages.--><!-- Better sources than this exist; see the article --><!--[[File:Máni and Sól by Lorenz Frølich.jpg|thumb|The gods [[Máni]] (left) and [[Sól (Sun)|Sól]] (right), the personified Moon and Sun in [[Norse mythology]], as depicted in an illustration by [[Lorenz Frølich]] (1895)]]--><!--(end {{notes}})--> |
The Moon is unique among natural satellites in that it experiences a stronger gravitational attraction to the Sun than to its primary, Earth. As a consequence, its path is always concave to the Sun. It can be argued that this makes the Moon a planet, orbiting the Sun, rather than a satellite of Earth. Usually, it is considered to be in orbit around Earth, but its orbit is substantially distorted from a simple elliptical shape by the gravity of the Sun, which includes a tidal gradient that causes the Moon to be attracted less strongly in the direction of Earth at full and new moon than at the quarter phases (in a frame of reference in which Earth is stationary). This perturbs the orbit so as to make its curvature more acute in the directions of the quarter phases than elsewhere. If the orbit were otherwise circular, this perturbation would make it approximately elliptical, with its major axis lying along the direction of Earth's motion around the Sun. Earth would be at the centre of this ellipse, rather than at one of its foci. In reality, this perturbation is superimposed on the elliptical orbit of the Moon, rotating with the seasons. The Moon's motion is therefore quite complex, and can be calculated only very approximately by assuming the orbit to be an ellipse. Likewise, the parameters of the orbit, eccentricity, semimajor axis, etc., can be stated only as approximate averages.--><!-- Better sources than this exist; see the article --><!--[[File:Máni and Sól by Lorenz Frølich.jpg|thumb|The gods [[Máni]] (left) and [[Sól (Sun)|Sól]] (right), the personified Moon and Sun in [[Norse mythology]], as depicted in an illustration by [[Lorenz Frølich]] (1895)]]--><!--(end {{notes}})--> |
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The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. |
The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite. |
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Revision as of 22:45, 9 August 2018
The Moon is an astronomical body that orbits planet Earth and is Earth's only permanent natural satellite.