Galileo called his discovery the Cosmica Sidera ( ` Cosimo 's stars ' ) , in honor of Cosimo II de ' Medici ( 1590 -- 1621 ) , grand-duke of Tuscany , whose patronage he wanted . Galileo initially called his discovery the Cosmica Sidera ( `` Cosimo 's stars '' ) , in honor of Cosimo II de ' Medici ( 1590 â `` 1621 ) .