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Origin: astro.club.nthu.edu.tw

Q: What is a “double planet”?
什?是雙行星?

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.
在一對天體中，兩顆星皆符合行星的定義，如果它們的軌道環繞在彼此的質心，且質心不位在任一星球的內部，兩星就稱作”雙行星”。

Q: What is a “satellite” of a planet?
什麼是行星的衛星？

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.
對於一個滿足行星的定義天體來說，如有一個繞著行星的物體與行星彼此的質心位在行星表面之下，則此物體就稱作”衛星”。

Q: The Earth’s moon is spherical. Is the Moon now eligible to be called a “planet”?
地球的衛星月球是個球體，那月球有具備行星的條件嗎？

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.
沒有，月球只是地球的一個衛星，月球被稱作衛星而非行星的原因是因為月地系統的質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”?
為何冥王星與夏隆是雙行星，而不是行星與衛星呢？

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.”
冥王星與夏隆都為夠大的球體，兩星球也都符合行星的定義。它們被稱作雙行星的原因是由於兩星的質心位在冥王星表面之外的太空，因此兩星球的軌道都環繞著太空中的一點，而非公轉於另一顆星球，這個系統就稱作”雙行星”。


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