The speed of a star in km/s, measured in a direction perpendicular to the line of sight from Earth to the star, is known as:

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Quizplus · Accepted Answer

Tangential velocity is the speed of the star in a direction perpendicular to the line of sight from Earth to the star. Proper motion is the apparent motion of a star across the sky, and radial velocity is the speed of the star in the line of sight from Earth, towards or away from us. Retrograde motion is the apparent motion of a planet moving in the opposite direction to the other celestial bodies in its system, as seen from Earth.

The Speed of a Star in Km/s, Measured in a Direction