The component of a object’s velocity that is perpendicular to our line of sight.

Aa star’s transverse velocity *v*_{T} can be determined if the distance *D* and proper motion *μ* are known. These are related by the following formula where the proper motion is in natural units (radians per unit time):

*v _{T} = μD*

A common problem when calculating the transverse velocity of a star occurs when people mix the units of proper motion, velocity and distance. When the distance *D*, to a star is in kiloparsecs and the proper motion *μ*, in milliarcseconds per year, the formula becomes:

*v _{T} = 4.74 (μD)* km/s

The constant of 4.74 arises from the combination of the conversions of distance (kpc to km), angle (from milliarcseconds to radians), and time (from years to seconds).

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