Astronomy is a skill, described in Basic Set, p. 179.
It is referenced in GURPS Banestorm™, p. 186, on astronomy in Yrth; in GURPS Low-Tech™, p. 10, on the Observational specialty and use of Mathematics at low TLs; and in GURPS Low-Tech Companion 1: Philosophers and Kings™, p. 20, on creating calendars, and p. 22, on placing standing stones for astronomical observation, and using orreries and telescopes.