What is proper motion?
Q: What is proper motion?
A: Proper motion is the name for the way stars seem to slowly move relative to each other when seen from Earth.
Q: How does proper motion occur?
A: Proper motion occurs because all stars (including the Sun) are moving through space at hundreds of kilometres per second, though it takes a long time for us to see that they have moved.
Q: When was proper motion first discovered?
A: Edmond Halley first noticed the stars Sirius, Arcturus and Aldebaran had moved from their positions in star charts drawn by Hipparchus around 130 BC in 1718.
Q: How far had these stars moved after 1,800 years?
A: After 1,800 years, these stars had only moved less than half a degree.
Q: What is proper motion useful for?
A: 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 those farther away.
Q: Which star has the largest proper motion?
A: Barnard's star has the largest proper motion of any star, moving 10.3 arcseconds per year which is equal to one quarter of a degree or half the diameter of the Moon in 87 years.
