An astronomer, examining two photographs of a small area of sky taken 20 years apart, notices that some of the stars have moved slightly across the sky, relative to most of the other stars in the photographs. This displacement of the stars across the sky is an example of
A) proper motion.
B) Doppler shift.
C) precession.
D) radial velocity.

Question

Quizplus · Accepted Answer

Proper motion refers to the apparent angular movement of a star across the sky relative to distant background stars, as observed from Earth, over time. This is exactly what the astronomer is observing in the photographs taken 20 years apart.

An Astronomer, Examining Two Photographs of a Small Area of Sky