What is the Vedāṁga Jyotiṣa’s concept of a “yuga”? It is a period of time that starts and ends whenever the beginning of the year coincides with the beginning of a month.
What does Vedāṁga Jyotiṣa consider the beginning of the year? When the Sun ceases moving southward and begins moving north; the “Winter Solstice.”
What does Vedāṁga Jyotiṣa consider the beginning of a month? When the Moon ceases waning and begins to wax; the “New Moon.”
When the New Moon occurs in close proximity to the Winter Solstice, a new Yuga begins. This Yuga will last 5 years, because after that much time there will be another New Moon near the Winter Solstice.
The Vedāṁga Jyotiṣa gives details about the exact duration of the Yuga and how to keep it synchronized from solar and lunar perspectives. It also gives details about the stars and phases the Moon will be in when the yuga goes through all 20 of its important milestones: 5 Winter Solstices, 5 Summer Solstices, 5 Vernal Equinoxes, and 5 Autumnal Equinoxes.
The specific details reveal that the content of Vedāṁga Jyotiṣa really is very old, because it says the winter solstice occurs when the stars of “Dhaniṣṭhā” rise with the Sun. The last time that was true was in roughly the third millennium BCE.
Except for one text very obviously added at a later time, Vedāṁga Jyotiṣa does not divide the sky into the 12 divisions we are familiar with today – which are anchored to the solstices and equinoxes. Rather it divides the sky into 27 divisions anchored to the stars. The stars that rise with the Sun on the winter solstice gradually change over the centuries. Thus the data given in Vedāṁga Jyotiṣa regarding the stars at which the New Moon occurs to begin a yuga are about 5,000 years out of date. We could update them, but it is simpler to use the classical 12-sign tropical system and measure as the start of a yuga the New Moon occurring near the first degrees of Capricorn (the tropical marker for the winter solstice).
It is also rather unusual for readers familiar with later Sanskrit literature to learn that the Vedāṁga’s yuga is only five years long! We are used to hearing from the Purana that yugas are thousands, if not hundreds of thousands of years long. The very different definition we encounter in Vedāṁga Jyotiṣa should help us understand that “yuga” is a general concept for measuring a span of time longer than a year. There are different types of yuga for different spans of time.
Since “yuga” literally means “coupling”, and since Vedāṁga Jyotiṣa provides the oldest definition of yuga, it is safe to say that the five year yuga it defines – based on the coupling of the Sun and Moon with the Winter Equinox – is the original idea, and that a different type and duration are altogether different, and probably later as an expansion on the original idea.
– Vic DiCara