It's a line of poetry, one of many notable phrases from Yeats' The Second Coming:
Turning and turning in the widening gyre The falcon cannot hear the falconer; Things fall apart; the centre cannot hold; Mere anarchy is loosed upon the world, (etc.)
Robert B. Parker titled one of his Spenser mysteries "The Widening Gyre" back in the day, and Gene Roddenberry's Andromeda also used it for an episode title.