There are three clocks of nature; the diurnal clock that measures the days, the lunar clock that measures the months and the solar clock that measures the seasons. The calendar is man's frustrated attempt(s) to synchronise these three clocks. Different cultures have attempted, in different ways, to synchronise these clocks or formulate a stable calendar. The calendar of the Vedic period has confounded historians of science for over hundred years. The foundational elements of a calendar (season, year, month and day) are noted in Rgveda Samhita, the oldest surviving text in Sanskrit. However, the nineteenth- and twentieth- century scholars failed to identify in the Vedic texts, any accurate procedure to synchronise the three clocks of nature. The consensus has been that the Vedic ritualists had only the crudest of calendar and they had failed to adequately synchronise the three clocks of nature. This book shows that this conclusion is false. The Vaidikas had developed unique and accurate schemes to synchronise these clocks and had formulated a stable calendar. Vedanga Jyotisa is a continuation of the development of calendric science of the Vedic texts. Vedanga Jyotisa expresses the calendric concepts of the Vedic texts in mathematical form and introduces algorithms to compute calendric parameters significant to the Vedic ritualists. It also describes a (partial) ecliptic co-ordinate system to define the position of the moon and the sun. This coordinate system is not irregular divisions of the naksatras, as has been assumed up to now. With this co-ordinate system the algorithms of Vedanga Jyotisa can be interpreted unambiguously, without it they are meaningless.