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##### Wiktionary
proper motion

n. (context astronomy English) The change in apparent position of a celestial body due to that body's velocity relative to the Sun.

##### Wikipedia
Proper motion

Proper motion is the astronomical measure of the observed changes in apparent positions of stars in the sky as seen from the center of mass of the Solar System compared to the imaginary fixed background of the more distant stars.

The components of proper motion in the Equatorial coordinate system are measured in seconds of time for right ascension (RA or α) and seconds of arc in declination (Dec. or δ). Their combined value is computed as the total proper motion (μ), which is expressed in seconds of arc per year (arcsec/yr) or per century (arcsec/100yr), where 3600 arcseconds equal one degree. Because most proper motions are much less than one arcsec per year, most modern catalogues, like the Hipparcos Index Catalogue (HIP), now express proper motion in terms of milliarcseconds per year (mas/yr), where 1000 mas equals one arcsecond. This transverse sky motion is separate from the radial velocity, being the velocity moving toward or away from the observer in kilometres per second (km/s), as obtained from the Doppler shifts in starlight seen with a spectroscope. Knowledge of the proper motion, distance, and radial velocity allow approximate calculations of a star's true motion in space in respect to the Sun.

Proper motion is not entirely "proper" (that is, intrinsic to the star), because it includes a component due to the motion of the Solar System itself.

#### Usage examples of "proper motion".

By now the distance was very great due to the velocity the corvette had built up in the Matrix, but the two ships' proper motion was nearly zero.

We're setting up a proper motion study to see if the source is well behind our line of sight to Vega, and we should have an answer in -- what?

We're setting up a proper motion study to see if the source is well behind our line of sight to Vega, and we should have an answer in-- what?

That is to say, every human being is intrinsically an independent individual with his own proper character and proper motion.

They are moving in three dimensions and we see the proper motion only in a two-dimensional projection.

I do have a few older pictures of that part of the sky and I could detect an appreciable proper motion.

I do have a few older pictures of that part of the sky and I could detect an appreciable proper motion, That would need more work.

The third component of 61 Cygni was discovered over a hundred years ago when they noticed some irregularities in the proper motion of the binary.

It was just hanging there, showing no proper motion and not growing, either.

Infrared and proper motion windows showed more conventional features: Beyond the rings lay a massive asteroid belt, and far beyond that a single jovian planet, its own million-klick ring system a puny afterthought.

The third component of 61 Cygni was discovered over a hundred years ago when they noticed some irregularities in the proper motion, of the binary.