# Peculiar velocity

Peculiar motion or peculiar velocity refers to the true velocity of an object, relative to a rest frame.

## Galactic astronomy

In galactic astronomy, the term peculiar motion (or peculiar velocity) refers to the motion of an object (usually a star) relative to a local standard of rest.

Local objects are usually related in terms of proper motion and radial velocity, that is the amount an object appears to move across the sphere of the sky, and the measured speed at which it travels towards or away from us, respectively. Proper motion and radial velocity can be combined through vector addition to produce the object's motion relative to the Sun. For "peculiar" motion, the local standard of rest used is not usually the Sun but instead the average motion of a chosen set of local stars.

## Cosmology

In physical cosmology, the term peculiar velocity (or peculiar motion) refers to the components of a receding galaxy's velocity that cannot be explained by Hubble's law.

According to Hubble, and as verified by many astronomers, a galaxy is receding from us at a speed proportional to its distance. The relationship between speed and distance would be exact in the absence of other effects.

Galaxies are not distributed evenly throughout observable space, but typically found in groups or clusters, ranging in size from fewer than a dozen to several thousands. All these nearby galaxies have a gravitational effect, to the extent that the original galaxy can have a velocity of over 1,000 km/s in an apparently random direction. This velocity will therefore add to, or subtract from, the radial velocity that one would expect from Hubble's law.

The main consequence is that, in determining the distance of a single galaxy, a possible error must be assumed. This error becomes smaller, relative to the total speed, as the distance increases.

A more accurate estimate can be made by taking the average velocity of a group of galaxies: the peculiar velocities, assumed to be essentially random, will cancel each other, leaving a much more accurate measurement.