The number phi - 1.6180339887, to be approximate - recurs throughout nature from mollusk shells and rose petals to the shape of the galaxy. It was discovered by Euclid over 2,000 years ago in the measurements of the mystical pentagram. The followers of Pythagoras believed it revealed the hand of God. In telling the story of phi, astronomer Mario Livio tells their stories along with those of other phi-obsessed individuals like Kepler, Goethe, Cezanne, Bartok and physicist Roger Penrose.