Astrophysics (Index)About

projected separation

(lower limit on distance between two objects in same part of the sky)

The projected separation is a calculated distance between two astronomical objects (their separation) assuming they were exactly equally distant from Earth, given the angle between them and their presumed distance. It can be thought of as the minimum distance between them. The concept is used for objects close to each other on the celestial sphere, i.e., the angle between them is small. As an estimate of their actual separation, it is no better than the accuracy of the determination of the distance to the objects. Furthermore, a small relative difference in the distance to each (e.g., a percent) can still result in an actual separation that is an order-of-magnitude or more larger than the projected separation. Objects for which projected separations are of interest include visual binaries, galaxies within the same galaxy group or galaxy cluster, extra-solar planets from their host stars, and other objects presumed to be (possibly) near each other, such as if they appear gravitationally bound.

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