From Halopedia, the Halo wiki
The Circinius system is a UNSC-controlled star system. It has at least one known colonized planet; Circinius IV, which is the location of the Corbulo Academy of Military Science, a school for future UNSC officers from prestigious backgrounds. In mid-2526, the academy became the target of an attack by the alien hegemony known as the Covenant.
In the real world, there is no star by the name "Circinius". The system's name may have been derived from the constellation Circinus.