This article expands a traditional pedagogic interpretation of Plato’s reasons for urging trainee rulers to study astronomy. It argues, primarily, that they need to become familiar with astronomy because it teaches them about cosmic harmony. This harmony indeed models a “personal harmony,” which will prevent them from becoming tyrants, and informs them about the analogous social harmony— which it will be their special duty to create and maintain. In Plato’s view, indeed, astronomy shows that social harmony requires obedience on the part of the lower orders and that social goods are best distributed in accordance with a special antidemocratic principle of equality.