### projected separation

(minimal distance between two distant objects close to each other)

The **projected separation** is an estimate of the minimum distance
between two astronomical objects (separation) that are presumed to
be are near each other, i.e., roughly the same distance from
Earth, and are close to each other on the celestial sphere
(i.e., not a large angle between them). It is simply a calculation of
the distance between them if they were exactly equally distant from
Earth, given the angle and their presumed distance. The separation
estimate is no better than the estimate of distance to the objects.

Even with a good estimate of the distance to the objects, one of
the objects may be closer than the other, so the actual separation
could be an order-of-magnitude or more different. If the distance to the
objects is known, it is a minimum possible distance between them.

Objects for which *projected separations* are of interest include
visual binaries, galaxies in 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.

(*astronomy,measure*)
https://en.wikipedia.org/wiki/Glossary_of_astronomy#P

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