The proper motions of stars are:
A) easy to measure for most stars on photographs taken six months apart.
B) noticeable to the unaided eye as a gradual shift of the constellations westward over the course of a month or two.
C) easy to observe with the unaided eye over the course of an hour or two.
D) difficult to measure, even on photographs taken several years apart for any but the nearest stars.

Question

Quizplus · Accepted Answer

Proper motions are difficult to measure, even on photographs taken several years apart for any but the nearest stars. This is because the apparent motion of a star in the sky due to its proper motion is very small, typically only a few tenths of an arcsecond per year. Therefore, special techniques and high-precision measurements are required to detect and measure proper motions accurately.

The Proper Motions of Stars Are