proper motion

noun

1. Astronomy. the angular motion of a star relative to a suitably defined frame of reference, expressed in seconds of arc per year.

proper motion

noun

1. the very small continuous change in the direction of motion of a star relative to the sun. It is determined from its radial and tangential motion

proper motion

1. Movement of a celestial object in the sky that is the result of the object's own motion in space rather than of how it is observed from Earth. All celestial objects are in motion with regard to each other, but because objects outside the solar system are so distant from Earth most of them seem fixed in the sky. Over long periods of time, however, their proper motions result in gradual changes in their relative positions as viewed from Earth. Measurements of these motions by modern instruments can be extrapolated forward or backward in time to produce a celestial sphere on which the stars have somewhat different positions than they have today. In general, objects nearest the Earth have the greatest proper motions and will move the farthest on the celestial sphere in such extrapolations. Extremely distant objects, although they may be moving through space at equal or higher speeds than nearby objects, will appear to move little in the sky even over thousands of years.

Word History and Origins

Origin of proper motion¹

First recorded in 1595–1605

Example Sentences

On the variations of the proper motions of Procyon and Sirius.

From Nautilus

It is notable as having a comparatively large proper motion.

From Project Gutenberg

Rollo soon learned the proper motion, so that his efforts assisted considerably in propelling the boat.

From Project Gutenberg

What we can do is to take all the stars which appear to have a proper motion, and determine the general direction of that motion.

From Project Gutenberg

As a matter of fact, the stars of largest proper motion happen to be low in the scale of magnitude.

From Project Gutenberg

It is one of the brightest stars near the Pole and has a sensible proper motion.

From Project Gutenberg