Definition of prosthaphaeresis in US English:


(also prosthaphairesis, prosthaphaericis, prostaphairesis)


  • 1Astronomy and Nautical. The difference between the apparent actual position of a planet, etc., and its mean position (i.e. that which it would occupy if its angular motion in the sky were uniform); the calculation of the actual position from the mean position.

  • 2A procedure involving addition or subtraction; (Mathematics, now historical) the expression of a product of sines and cosines of two angles as a sum or difference of sines and cosines, e.g. cosx cosy = ½[cos(x + y) + cos(x − y)].


Late 16th century; earliest use found in Edward Wright (d. 1615), mathematician and cartographer. From post-classical Latin prosthaphaeresis (in astronomy) addition or subtraction as required, (in trigonometry) procedure involving addition or subtraction from Hellenistic Greek προσθαϕαίρεσις (in astronomy) addition or subtraction as required from ancient Greek πρόσθεν before (from πρό + -θεν, suffix forming adverbs indicating motion from, with inserted σ, after either ἔκτοσθεν from without, ἔντοσθεν from within or πρόσω forwards: see prosobranch) + ἀϕαίρεσις action of taking away.