What is a Yuga?

Our ancestors had a concept of a Yuga as a block of time. I am not clear whether there is a definite statement as to how large is a Yuga. There is a concept of a cycle of Four Yugas, viz. Kruta, Treta, Dvapara and Kali. Are they all equal? Why four yugas?

There are four Cardinal points of the Sun’s annual travel through the ecliptic, viz. the two equinoxes and the two solstices. Our ancestors had become aware of this and must have observed that the four points move backwards along the ecliptic, although only very slowly. Is the concept of four Yugas somehow connected with this phenomenon? Brahmadeva formulated the system of time counting with a starting point when Sun moves into Dhanishtha and was at Summer Solstice. At least, that appears the rational explanation of धनिष्ठादि तदा कालो ब्रह्मणा परिनिर्मित:

My speculation is whether a Yuga is the period from that point to the next point in time when Autumnal Equinox starts occurring when Sun is in Dhanishtha! It would mean that a yuga is a block of time needed for one cardinal point to move back to such an extent that the next cardinal point takes its place. There would thus be a cycle of four yugas before the starting point is again reached. We now know that total time of this cycle is 26000 years and so each yuga would have a size of 6500 years! I am of course quite aware that I have no basis to support this speculation.

This year, Summer Solstice occurred when Sun just entered Ardra. Between start of Ardra and end of Dhanishtha there are 9 clear nakshatra. Thus if Summer Solstice is assumed to have been at end of Dhanishtha when Bhrahma started his ‘system’ the Summer Solstice has moved backwards by full 18 nakshatra till now or about 17300 years have gone by. In other words, the year at that time was 15300 BCE! If Sun was at the start of Dhanishtha at that time then the year was 14350 BCE (960 years forward). We may take a mean year or 14850 BCE as the year when the Yuga started. That yuga would then end 6500 years later or in 8350 BCE and the next yuga would end in 1850 BCE! Was this the year when Dwapar yuga ended and Kaliyuga started?

An alternative concept for a Yuga could be the period during which, a Cardinal point occurs within a particular Nakshatra. This will make a Yuga much smaller, about 960 years and there would be no specific significance for four Yugas. There should then be a cycle of 27 Yugas. There is an Arati of Lord Vitthala in Marathi, the first line of which is 'युगे अठ्ठावीस विटेवरी उभा' Any connection?