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By parallactic motion is meant the apparent displacement in consequence of the solar motion which is now known with great accuracy, and amounts to 19.5 kilometers per second.

If we ascertain the parallactic motion of a group of stars, then we can find their average distance.

From the parallactic motion of the star it is possible to deduce its distance from the sun, or its parallax.

In both cases the motion may be either a reflex of the motion of the observer, and is then called parallactic motion, or it may be caused by a real motion of the star.