Lunar month circular, the speed of Earth's progression around the Sun varies during the year. Thus, the angular rate is faster nearer periapsis and slower near apoapsis. The same is so for the Moon's orbit around the Earth. Because of these variations in angular rate, the actual time between lunations may vary from about 29.18 to about 29.93 days. The long-term average duration is days (29 d 12 h 44 min 2.8016 s). The synodic month is used to calculate eclipse cycles. It is customary to specify positions of celestial bodies with respect to the vernal equinox. Because of Earth's precession of
Vṛṣabha the Tamil Hindu calendar. The ancient and medieval era Sanskrit texts of India vary in their calculations about the duration of Vrsabha, just like they do with other months. For example, the "Surya Siddhanta" calculates the duration of Vrishabha to be 31 days, 10 hours, 5 minutes and 12 seconds. In contrast, the "Arya Siddhanta" calculates the duration of Vrsabha to be 31 days, 9 hours, 37 minutes and 36 seconds. The Indian solar month names are significant in epigraphical studies of South Asia. For example, "Vrsabha" month, along with other solar months, are found inscribed in medieval era Hindu