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

"The Court specifically reserves the right to retroactively amend or modify child support without the need for the filing of a proper motion by either party to modify support," the judge wrote.

From Fox News

It is those last two properties that are particularly edifying for astronomers: before Gaia, scientists lacked high-precision measurements of the distance to many stars, as well as what’s known as proper motion, or a star’s movement across the sky.

From Nature

He says that this extension should provide a factor-of-three improvement in the precision of Gaia’s measurement of proper motion for the stars it currently tracks.

From Nature

And although the speed at which stars move towards or away from Earth can be measured by changes in colour, sorting out their proper motion — and so their full 3D velocity — is difficult because most objects move so little across the sky on human timescales.

From Nature

This has allowed the team to measure how fast stars seem to travel across the sky — their proper motion.

From Nature