Stellar Velocity Page 4

Transverse or Angular velocity

The across motion of celestial objects in the sky as we observe that, is expressed in an angular measure per time, usually milli-arcsec per year (mas/yr). It is due to a combination of two effects:

  1. the actual across motion of the star with respect to us (called proper motion) and
  2. the periodic apparent motion with respect to the stellar background, due to annual parallax caused by the motion of the Earth around the Sun (and some other smaller apparent effects). Stars that are relatively nearby will exhibit significant parallax.


Parallax is the apparent shift of a relatively nearby object with respect to a background of objects that are much further away, when the observer changes position. We have all experienced that many times, e.g. when we drive along a road and see the nearby trees or power poles flashing by, while objects in the distance move much slower.

The objects are not moving, it is the observer who changes the viewpoint which causes the apparent motion of the objects.

Image Natejunk2004,

In astronomy parallax can be seen when we look at nearby stars against a background of stars that are much further away while we on Earth are moving.

We move because of the rotation of the Earth (diurnal parallax) and because the Earth revolves around the Sun in a year (annual parallax). For the purpose of our discussion here, diurnal parallax is negligibly small even for the nearest star, but annual parallax cannot be ignored. Proxima Centauri which is the nearest star to the Solar System, has an annual parallax of less than 800 mas/yr.


Drag the slider to change the distance to the nearby star.
Origin of this movie unknown.


More about parallax and how that is used for measuring distance can be found in our Module "Stellar Distance".