Proper motion

Definition

Proper motion is the apparent angular movement of a star across the sky relative to distant background objects, measured in arcseconds per year. It is a component of the star's overall motion through space.

5 Must Know Facts For Your Next Test

1. Proper motion is typically measured in two perpendicular directions on the celestial sphere: right ascension and declination.
2. Proper motion does not include radial velocity, which is the motion of a star toward or away from us.
3. The total proper motion can be determined by combining its components in right ascension and declination using the Pythagorean theorem.
4. High proper motion indicates that a star is relatively close to Earth.
5. Barnard's Star has one of the highest known proper motions among stars.

Review Questions

• What units are used to measure proper motion?
• What are the two directions in which proper motion is typically measured?
• How does high proper motion relate to a star's distance from Earth?

Related terms

Radial Velocity:

The component of a star's velocity that moves it toward or away from Earth, typically measured using Doppler shifts.

Parallax:

The apparent shift in position of a nearby star against distant background objects due to Earth's orbit around the Sun, used to measure stellar distances.

Stellar Parallax:

A method for measuring distances to nearby stars by observing their apparent shift against more distant stars over six months.