Q: What is a “double planet”? 什?是双行星? A: A pair of objects, which each independently satisfy the definition of “planet” are considered a “double planet” if they orbit each other around a common point in space that is technically known as the “barycentre”. In addition, the definition of “double planet” requires that this “barycentre” point must not be located within the interior of either body. 在一对天体中，两颗星皆符合行星的定义，如果它们的轨道环绕在彼此的质心，且质心不位在任一星球的内部，两星就称作”双行星”。 Q: What is a “satellite” of a planet? 什麽是行星的卫星？ A: For a body that is large enough (massive enough) to satisfy the definition of “planet”, an object in orbit around the planet is called a “satellite” of the planet if the point that represents their common centre of gravity (called the “barycentre”) is located inside the surface of the planet. 对於一个满足行星的定义天体来说，如有一个绕着行星的物体与行星彼此的质心位在行星表面之下，则此物体就称作”卫星”。 Q: The Earth’s moon is spherical. Is the Moon now eligible to be called a “planet”? 地球的卫星月球是个球体，那月球有具备行星的条件吗？ A: No. The Moon is a satellite of the Earth. The reason the Moon is called a “satellite” instead of a “planet” is because the common centre of gravity between the Earth and Moon (called the “barycentre”) resides below the surface of th e Earth. 没有，月球只是地球的一个卫星，月球被称作卫星而非行星的原因是因为月地系统的质心位在地球的表面之下。 Q: Why is Pluto-Charon a “double planet” and not a “planet with a satellite”? 为何冥王星与夏隆是双行星，而不是行星与卫星呢？ A: Both Pluto and Charon each are large enough (massive enough) to be spherical. Both bodies independently satisfy the definition of “planet”. The reason they are called a “double planet” is that their common centre of gravity is a point that is located in free space outside the surface of Pluto. Because both conditions are met: each body is “planet-like” and each body orbits around a point in free space that is not inside one of them, the system qualifies to be called a “double planet.” 冥王星与夏隆都为够大的球体，两星球也都符合行星的定义。它们被称作双行星的原因是由於两星的质心位在冥王星表面之外的太空，因此两星球的轨道都环绕着太空中的一点，而非公转於另一颗星球，这个系统就称作”双行星”。 -- Because the Kocmoc is so ~ I will explain everything one day ! By iLyuSha -- ※ Origin: 星空之子 <starkids.twbbs.org> ※ From : 218-168-36-44.dynamic.hinet.net