When two celestial bodies are close, the inertia of motion makes them deviate from each other, and gravity makes them close to each other. In the end, they all revolve around each other, that is, they both revolve around their own concentric centers. The mass of the earth is much larger than that of the moon, and their concentric centers are all close to the center of the earth, so it is generally believed that the moon revolves around the earth.

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Celestial bodies refer to the physical form of the universe. The condensation of celestial bodies forms the research objects of various astronomical States. The sun, planets, satellites, asteroids, comets, meteoroids and interplanetary substances in the solar system, stars, clusters, nebulae and interstellar substances in the Milky Way, and extragalactic galaxies, clusters, superclusters and intergalactic substances. Infrared source and ultraviolet light sources were discovered by radio detection and space detection.

Radio sources, X-ray sources and gamma-ray sources are also celestial bodies.

The law of universal gravitation was published by isaac newton in Mathematical Principles of Natural Philosophy on 1687. Newton's law of universal gravitation is expressed as follows:

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Any two particles are attracted to each other by the force in the direction of the line. This gravity is directly proportional to the product of their masses and inversely proportional to the square of their distance, and has nothing to do with the chemical composition of two objects and the type of medium between them.