Shouldn’t Makar Sankranti/Uttarayana fall on Dec 21?
Uttarayana (????????) is the “northward movement” of the sun and marks the winter solstice in the northern hemisphere.
This “northern movement of the sun” is due to the Earth’s axial tilt and the reason the days start to get longer. Makar sankranti (??? ?????????) also celebrates the entry of the Sun into the Makar rashi (Capricorn).
The question then is why did Uttarayana fall on Jan 14 this year instead of Dec 21 last year - the true winter solstice?
Is the Indian calendar inaccurate? Did it work with insufficient information? Does it need to be updated? Turns out it's as good as any - and some would argue even better. To understand why, we will need a working knowledge of astronomy.
Basic Astronomy
a) Earth’s motions. Earth moves thru space in multiple ways -
1. Axial Rotation. This is the familiar concept of the earth day - approximately 24 hours.
2. Sidereal Year. Earth’s "truly complete" motion around the sun with respect to the stars (assumed to be stationary in this frame of reference). This is the sidereal year. Approximately 365 days, 6 hours, 9 minutes, 10 seconds.
3. Tropical Year. One revolution of the earth around the sun such that the sun returns to the same seasonal start (such as an equinox or solstice). This is called the tropical year and is approximately 365 days, 5 hours, 48 minutes, 45 seconds.
4. Precession of equinoxes. The reason for the 20 minute per year difference between the sidereal and tropical years in how the earth’s axis wobbles like a spinning top. This wobble takes 26,000 years to complete one cycle. This 20 minute difference over the course of the past two millennia has caused Uttarayana to shift by nearly a month.
5. Axial tilt. The earth also has a “nodding” motion where the axial tilt (currently about 23°) changes over a 41,000 year period.
6. Orbital eccentricity. The shape of the Earth’s orbit around the sun undergoes a change from more eliptical to more circular over a course of 100,000 years.
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7. There are many more but the above will suffice for the purposes of this discussion.
b) Nakshatras. These are 27 divisions of the celestial sphere along the ecliptic, each spanning 13° 20' of the zodiac. These divisions are based on the apparent path of the Moon as it orbits Earth. The Nakshatras correspond to star clusters or constellations in the sky, and the Moon spends approximately one day in each Nakshatra during its monthly cycle. The system of Nakshatras is closely linked to the precession of the equinoxes, which causes the positions of stars and constellations to shift gradually over time.
c) Rashi. Refers to the 12 divisions of the ecliptic plane, each covering 30° of the Earth's orbit around the Sun. These divisions correspond to the zodiac signs in both Western and Indian astronomy, with each sign representing a segment of the celestial sphere along the path the Sun appears to travel over the course of a year. The 12 Rashis are used to track the Sun's position relative to the Earth, marking key points in the annual cycle and forming the foundation of the zodiac in astrology. Each Rashi is associated with a specific region of stars, but the signs themselves are primarily defined by the Sun's apparent position within the sky. Makar is one of the rashis.
Two different Calendars
The modern Gregorian calendar follows the tropical year and therefore "forces" the start of the seasons to be exactly on the same day each year - and thereby sacrificing the fact that the earth didn't really complete a full solar revolution. This is the key reason for the difference. That’s why the seasons always start at the same time of the calendar year, winter solstice is always on Dec 21, vernal equinox always falls on March 21 and so on. This is both convenient and practical. But this calendar "cheats" by subtracting 20 minutes from the "true" year to accommodate this convenience.
Indian calendar on the other hand is a luni-solar calendar that incorporates lunar cycles for determining the start of each month and takes into account the nakshatras (the observable-from-earth celestial sphere). That’s why unlike the Gregorian calendar, each month has the same number of days and is aligned with the lunar phases with the two parts - krishna (?????) and shukla (?????) paksh. In order to track the year this calendar uses the sidereal year - taking into perspective the broader constellation system that remains virtually static with respect to the earth-sun system. In other words - a sidereal year is a complete circle around the sun.
While from the purist's perspective this is the true year since it marks one full revolution, disregarding the local phenomenon of precession. This view has it's own problems, however - it makes the calendar less practical. If you want the crop sow/harvest cycle to align with the calendar the seasons must be consistent - and hence you must go with the tropical year.
So that's really the gist of it - Uttarayana did align with the winter solstice in the year 285 CE and will do so again in the year 26,057 CE - in sync with the precession of equinoxes. Bit it will continue to drift with respect to the Gregorian calendar since it's based on the "true" sidereal year.
As a footnote, no calendar can be static/perfect - simply because the celestial bodies are not static and their motions and time periods do not align with each other. We have leap years, unevenly sized months, and lots of religious concepts built into the Gregorian calendar. Similarly the Indian calendar has, among other things, the adhik mas (???? ???) that needs periodic adjustments.
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