GALILEO GALILEI, Diari di osservazioni, ms., 1609-1610
Biblioteca Nazionale Centrale di Firenze, Ms. Gal. 49, ff. 73v-74r
Facsimile

From the moment of their discovery, Galileo diligently observed the satellites of Jupiter to determine their orbital radii and periods. In his first observations he designated the satellites by the numbers 1 through 4 and measured the distances between them in units equal to the radius of the planet.