Parallactic

par·al·lax  (pr-lks) n. An apparent change in the direction of an object, caused by a change in observational position that provides a new line of sight. [French parallaxe, from Greek parallaxis, from parallassein, to change : para-, among; see para-1 + allassein, to exchange (from allos, other; see al-1 in Indo-European roots).] paral·lactic (-lktk) adj.

parallax The apparent position of a star among more distant stars changes when viewed from opposite sides of the earth's orbit.