The Hindu calendar is based on a geocentric model of the solar system. A geocentric model describes the solar system as seen by an observer on the surface of the earth. The Hindu calendar defines nine measures of time (Sanskrit: मान; IAST:māna): 1. * brāhma māna 2. * divya māna 3. * pitraya māna 4. * prājāpatya māna 5. * guror māna 6. * saura māna 7. * sāvana māna 8. * cāndra māna 9. * nākṣatra māna Of these, only the last four are in active use and are explained here.