The component of a object's velocity that is perpendicular to our line of sight.
Transverse velocities vT, are very difficult to measure in astronomy without determining the distance D, and proper motion μ, of the stars. These are related by the following formula where the proper motion is in natural units:
vT = 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:
vT = 0.21 (μ/D) km/s