Newton described his Principia as a work of ‘experimental philosophy’, in which theories were supposed to be deduced from phenomena.  To this end, Newton presented six ‘phenomena’: propositions that describe patterns of motion, generalised from observations of the planets, earth and moon.  However, these don’t seem to fit any standard definition of ‘phenomenon’ – not even the one Newton toyed with in drafts of Principia.  Some commentators have argued that Newton’s labelling was mistaken, while others have argued that Newton was using the label ‘phenomenon’ in an attempt to pass off a mathematical treatise as a work of experimental philosophy.
Was Newton making a mistake?  Or being disingenuous?  Neither.  Drawing on Bogen & Woodward’s (1988) distinction between data, phenomena and theories, I argue that Newton’s choice of label was appropriate.  In the Opticks, Newton isolated his explanatory targets by making observations under controlled, experimental conditions.  In Principia, Newton isolated his explanatory targets mathematically: from astronomical data, he calculated the motions of bodies with respect to an ideal observer.  Viewed in this way, the phenomena of the Principia and experiments from the Opticks are different routes to the same end: isolating explananda.
Newton’s ‘phenomena’, then, are explanatory targets drawn from raw data.  Newton was not in error, nor using experimentalist rhetoric simply for political reasons.  He was, however, bending the meaning of commonly used terms to his own needs.