'Parallax' is an object's apparent shift when you change your viewpoint — the key to measuring cosmic distances.
The apparent displacement of an object when viewed from two different positions; in astronomy, the angular difference in the apparent position of a star as observed from opposite sides of the Earth's orbit, used to calculate stellar distances.
From Greek 'parallaxis' (a change, an alternation), from 'parallassein' (to alter, to change slightly, to make things alternate), from 'para-' (beside, alongside, beyond) + 'allassein' (to change, to exchange), from 'allos' (other, another), from PIE *al- (beyond, other). The word describes the shift in apparent position caused by looking at the same thing from two different vantage points — the 'othering' of perspective that occurs when the observer moves beside themselves. Key roots: para- (Greek: "beside, alongside, beyond"), allos (Greek: "other, another"), *al- (Proto-Indo-European: "beyond, other").
Hold your thumb up and close one eye, then the other — your thumb appears to shift against the background. That shift is parallax. Astronomers use the same principle on a cosmic scale: as the Earth orbits the Sun, nearby stars appear to shift slightly against distant ones. The first stellar parallax was