Sankranthi (Pongal) is one of the biggest festivals in India. Hindus celebrate this festival day as the dawn of auspicious days based on the movement of the Sun. The earth rotates around its own axis tilted 23.5 Degrees with respect to ecliptic plane (the Sun’s path across the sky). We all know, that Earth’s rotation causes day and night and its revolution around the Sun causes seasons. The seasons are not created merely by the revolution of the Earth but because of the Earth’s axial tilt due to which angular direction of Sun rays differ on the globe across different latitudes, causing different seasons across Northern and Southern hemispheres. As the Earth revolves round the Sun, it appears to the observer on the Earth that the Sun is moving on the celestial sphere. The points of intersection of celestial equator and ecliptic plane are called equinoxes (Day light and Night will have same length on the day). Solstice is an event occurring when the Sun appears to reach its most northerly or southerly excursion relative to the celestial equator on the celestial plane. Two solstices occur annually, on about 21 June and 21 December. The seasons of the year are directly connected to both the solstices and the equinoxes. The concepts of celestial sphere, Celestial equator, Ecliptic plane, positions of equinoxes and solstices on the celestial plane can be better understood in the pictures shown below.
Pics obtained from the internet for better understanding of the readers only.
Apart from rotation and revolution of the Earth, there is one subtle and important movement of earth, it is called precession. The detailed explanation about this subtle movement was dealt in my previous post The Precession of the Earth . The gravitational pull of the Sun and the Moon on the gyroscopic Earth bulge produces torque on its rotational axis causing it to drift periodically from East to West completing a turn in 25,800 years. This woobling movement of Earth’s rotational axis is called the Precession. Shifting of Equinoxes and Solstices is one of the important phenomenon, apart from shift of pole star, that is caused by the Earth’s precession. As the Earth revolves round the Sun, the position of occurrence of Equinoxes and Solstices will not be same every year because of the precession of the rotational axis. By the time, the Earth reaches the same point around the Sun in its orbit, the rotational axis orientation would be slightly shifted due to precession. The equinoxes are nothing but the intersection points of ecliptic plane and celestial equator on the celestial sphere, as the Earth precession goes on, the intersection points are also shifted and hence the Equinoxes and Solstices. The shift in Equinoxes and Solstices changes the occurrence of seasons for the given position of the Earth in its orbit. Approximately, the Equinoxes and Solstices will be shifted by 1 day for every 70.6 years. The seasonal cycle is exactly repeated after 25,800 years again. This cycle is also called the Great Year in Astronomy.
The ages in which spring equinox occurs in different zodiacs mentioned in this table
|Zodiac ( Raasi)||Spring Equinox period in Zodiac ( Raasi)|
|BC 6350 – BC 4200|
|Taurus (Vrushabham)||BC 4200 – BC 2050|
|Aries (Mesham)||BC 2050 – AD 100|
|Pisces (Meenam)||AD 100 – AD 2250|
AD 2250 – AD 4400
Note : The western zodiac and Indian Raasi system have many similarities, still, both are not exactly same. Western zodiac system follows position of the Sun and Indian Raasi system follows position of the moon. Both terminologies are mentioned here only to indicate the corresponding terms for the convenience of readers.
Astronomically, Sankranthi festival is celebrated on the starting day of the year, when the Sun enters Capricorn zodiac. It is traditionally believed that on the same day, auspicious Winter Solstice (Sanskrit:Uttarayana Punayakalam) begin. On the day of winter solstice, the Sun ends its southward journey (Sanskrit:Dakshinaayan) touching tropic of Capricorn (Sanskrit:Makar Sankramanam) and begins northward journey (Sanskrit:Uttaraayan) towards tropic of Cancer (Sanskrit:Karkata Rekha). Traditionally, this day is celebrated as Sankranthi festival from the early ages in India at which time the Sun entering in Capricorn and Winter Solstice coincided i.e at the age where spring equinox occurred Aries (Mesha) constellation in BC 2050 – AD 100. But, due to earth’s precession, equinoxes and solstices shifted and now Winter Solstice happens on Dec 22 (in northeren hemisphere) when spring equinox occurs in Sagittarius (Dhanu Raasi). That means, Makar Sankranthi celebrated on January 14th or 15th of the year is about 23 days after Winter Solstice (Sanskrit:Uttarayana Punayakalam) happened. It is true that on Jan 14th, the Sun enters Makar Raasi (Italian:Capricorn) but Winter Solstice will not happen on Jan 14th every year. This discrepancy is because in Panchangams the Earth’s precession movement affect is not considered, which is very subtle, about 1 degree in 72 years !!!
Astonishingly, there is a reference about this subtle movement in ancient Hindu texts. In one of the interesting story in Hindu mythology, Lord Vishnu appears and blesses his devotee, Dhruva, who he will be remained as constant star in the space and all the stars (Saptha Rishis) will revolve around him for 26,000 years! Hindu astronomers called the ,pole star’ as ‘Dhruva Tara’. Do they really know about the periodicity and the Precession of the Earth axis? If not, how Hindus referred this period exactly in their age old mythological story is a mystery! Probably, some texts were lost and later pundits who prepared Panchangams missed this subtle concept. Whatever may be the Astronomical dates accuracy, the basic recognition that Sun and Earth movements are critical for existence of life and observing it as festival in practical life, decorating animals in villages, observing as farmers festival, all together shows Hindus way of living is intertwined harmoniously with Nature.