A helen is a humorous unit of measurement based on the concept that Helen of Troy, from the Iliad, had a "face that launched a thousand ships". The helen is thus used to measure quantities of beauty in terms of the theoretical action that could be accomplished by the wielder of such beauty. The classic reference to Helen's beauty is Marlowe's lines from the 1592 play The Tragical History of Doctor Faustus, "Was this the face that launched a thousand ships / And burnt the topless towers of Ilium?" In the tradition of humorous pseudounits, then, 1 millihelen is the amount of beauty needed to launch a single ship. According to The Rebel Angels, a 1981 novel by Robertson Davies, this system was invented by Cambridge mathematician W.A.H. Rushton. However, the term was possibly first suggested by Isaac Asimov.

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