What do sunflowers, Salvador Dali’s painting The Sacrament of the Last Supper, and black holes have in common? They share characteristics described by a curious irrational number, which approximates to 1.618. Known since antiquity as the Golden Ratio, or Phi, it has come to represent the proportions of ideally pleasing geometrical structures. Astrophysicist Mario Livio separates the myth from the math to bring this remarkable number to life.