7. Due to the gravitational effects of Sun and Moon (and to a lesser extent, Venus, Jupiter and Saturn), the Earth wobbles on its axis, and completes a non-uniform cycle in about 25,771 years, referred to as Precession of Equinox. Due to this wobble, the celestial North Pole (and South Pole) appears to change over time, and the Rashis appear to drift slowly over the years. More than 2500 years ago, ancient Indians had observed and measured the wobble at a degree for every 100 years. This translates to a measure of 36,000 years, a figure repeated by Hipparchus around 150 BCE. One of the best estimates of Precession was made by Bhaskara II of Ujjain in the 12th century, to 25,461 years, and not improved upon till modern times. It is very interesting that ancient Indians had noted a time when Abhijit (the star Vega) was once the pole-star, and also a time when it was no longer the pole-star. Abhijit was at the Celestial North Pole approximately 14,000 years ago. Around 7000 years ago, it would have appeared to have “fallen” in the sky, as noted by Dr. P.V.Vartak (in Scientific Dating of Ramayana and the Vedas), calling out a reference to a passage in the Mahabharata.
We now define Makar Sankranti as the date when from an Earth-bound observation point, the Sun enters the Makar Rashi, also called Capricorn.
Ancient Indians noted the Winter Solstice as the start of the auspicious Uttarayana. At some point in the past, Uttarayana coincided with Makar Sankranti, and constitutes our first point of synchrony. We can determine the time period when the two coincided by considering the effects of Precession. Prior to that, it is instructive to note how ancient Indians and Europeans recorded the passage of time.
Subhash Kak notes that even before Vedanga Jyotish, ancient Indians’ 27-Nakshatra and 12 Rashi system used a luni-solar calendar where every 5 years, an additional month called Adhika Masa was added, synchronizing the lunar and solar years. Ancient Indians also estimated the tropical year, defined as the period when the Sun enters the same seasonal point – say, a solstice point.
Aryabhata and Bhaskara II had estimated the tropical year at 365 days, 6 hours, 12 minutes, and 30 seconds, the same figure as estimated in the ancient Indian text, Surya Siddhanta. The modern figure for the tropical year is approximately 365 days, 5 hours, 48 minutes and 45 seconds.
In the Western system, Julius Caesar instituted the Julian calendar in 46 BCE, dividing the year of 365 days to 12 months, and adding a day every 4th year, thus averaging to 365 days, 6 hours – a figure less accurate than the Surya Siddhanta. Due to this approximation, this calendar accumulated errors over the years, causing a “slip” in the dates of the equinoxes and solstices. The modern Gregorian calendar introduced in 1582, introduced a correction, where if a year is integer-divisible by 4, it is considered a leap year, except for those centurial years that are integer-divisible by 100, and with further overruling exception to those centurial years that are integer-divisible by 400, which were considered as leap years. With the modern Gregorian calendar, the equinoxes and solstices occur on approximately the same date each year, and considering Precession, has an error of about 1 day every 7700 years.
Considering the first synchrony, the Winter solstice today coincides with the Dhanus Sankranti – one Rashi away from Makar. This slip has happened due to the Precession noted earlier.