Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is know as a [[light-year]], which is about 6 million million (6x10<sup>12</sup>) miles. For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away. | Since the speed of light in a vacuum is constant, it can be used to define distances as well. The distance that light travels in one year is know as a [[light-year]], which is about 6 million million (6x10<sup>12</sup>) miles. For instance, the nearest large galaxy to our own [[Milky Way Galaxy]] is the [[Andromeda Galaxy]], which astronomers have measured to be about 2.5 million light years away. |