Definition of planet Earth's own orbit around the Sun—looking down on the ecliptic, the Moon never actually loops back on itself, and in essence it orbits the Sun in its own right. Also many moons, even those that do not orbit the Sun directly, often exhibit features in common with true planets. There are 19 moons in the Solar System that have achieved hydrostatic equilibrium and would be considered planets if only the physical parameters are considered. Both Jupiter's moon Ganymede and Saturn's moon Titan are larger than Mercury, and Titan even has a substantial atmosphere, thicker than the Earth's. Moons such as
Sun-synchronous orbit Sun-synchronous orbit A Sun-synchronous orbit (SSO, also called a heliosynchronous orbit) is a nearly polar orbit around a planet, in which the satellite passes over any given point of the planet's surface at the same local mean solar time. More technically, it is an orbit arranged so that it precesses through one complete revolution each year, so it always maintains the same relationship with the Sun. A Sun-synchronous orbit can place a satellite in constant sunlight, which allows the solar panels to work continually. This orbit is also useful for imaging, spy, and weather satellites, because every time that the