Proper motion is useful to astronomers in figuring out how far away a star is from Earth because stars that are close usually have a larger proper motion (meaning they move faster across the sky) than stars that are farther away. For example, Barnard's star has the largest proper motion of any star, moving 10.3 arcseconds per year. This is equal to one quarter of a degree, or half the diameter of the Moon in the sky, in only 87 years. It is the second-closest star to the Earth at 5.98 light years away. Also, eight of the ten stars with the biggest proper motions are less than 15 light years away.