The Solar System

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1.1. Natural satellites Formally, classified natural satellites or moons include 176 planetary satellites orbiting six of the eight planets. Of the inner planets, Mercury and Venus have no natural satellites; Earth has one large natural satellite, known as the Moon; and Mars has two tiny natural satellites, Phobos and Deimos. The large gas giant planets have extensive systems of natural satellites, including half a dozen comparable in size to Earth 's Moon. The objects in the solar system are classified foremost by their dynamics and motion, while size and composition are important for secondary classification. The solar system itself is, defined by the orbits of its constituent bodies. Classification by motion is more useful for astronomers. In fact, the dynamics of the solar system were a central issue for early astronomers such as Ptolemy in the second century and Nicholaus Copernicus in the 16th century. In what follows, we introduce different classifications of the planetary moons. 1.2. Classification of motion of natural satellites 1.2.1 Classification based on the orbits The natural satellites may be, classified into three categories according to their orbits (Newburn and Gulkis, 1973; Burns, 1986) as follows. Category 1: Regular satellites The regular satellites move prograde in nearly circular orbits in the equatorial plane of the mother planet. This sector of satellites represents the four Galilian satellites of Jupiter, the eight classical satellites of Saturn

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