地球质心(简称地心)是指包括海洋大气等的整个地球的质量中心，地心运动是地球质量中心相对于地球参考框架原点的位移。
Geocenter motion is the relative displacement from the center of mass of Earth to the origin of the Terrestrial Reference Frame.
在我们地球上，重力会把我们拉向星球的质量中心，因此在平坦的地表，我们自然站得很直。
On our spherical Earth, gravity pulls "down" us toward the planet's center of mass. So on a flat surface, we naturally stand up straight.
据推算，NGC3115中心的超大质量黑洞，距地球3200万光年，质量相当于太阳质量的20亿倍。这也使它成为距地球最近的“10亿太阳质量级别”黑洞。
The supermassive black hole at the center of NGC 3115, they calculate, has a mass of about 2 billion Suns, making it the nearest billion-solar-mass black hole to Earth at 32 million light-years away.
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