Most states in India follow some version of a lunisolar calendar. Those that settled on their calendar relatively late, like Kerala, tend to have solar calendars.
So, what is a solar calendar, and how is it different from a lunisolar one?
Solar Years
The progress of seasons are a good way to tell time. One good way to define a solar year is to use the Sun's position in the cycle of seasons, as we see it from Earth. The Spring Equinox, when day isequal in length to night is one possible point of reference. The mean “Tropical Year”, as this is called, is approximately 365.2422 days long.
This leads to the well known Gregorian Calendar, with 365 days, a leap day once in four years (+0.25), minus a leap day once in 100 years (-0.01) plus a leap day every 400 years (+0.0025)
In the Gregorian calendar, the familiar month lengths are arbitrary.
Sidereal Years
An alternate way of tracking solar years is to look at the Sun's transit on the background of fixed stars, as we see it from Earth. As seen from the Earth, a year later, the position of the Sun should be the same. This measurement results in what is known as a “Sidereal Year”.
Would this be identical to the “Tropical Year” ? Unfortunately, not quite. The Earth's axis is tilted 23 degrees to the plane of its orbit around the sun, as we learn in school.
Now think of what happens if a top spins at a non vertical angle? Yes, and the Earth's axis does the same!
This results in an astronomical phenomenon known as “precession of the equinoxes”. In a period of 26000 years, the position of the Sun at Spring Equinox undergoes a full cycle on the background of fixed stars. As a result of this phenomenon, each Sidereal Year is longer than the Tropical Year by 20 minutes. This seems like a short time, but can add up to substantial differences over centuries.
Indian Solar Calendar
Indian solar calendars are Sidereal, and start with the Sun moving into the “first point of Aries” — the beginning of the constellation मेष. Thus, the Solar New Year (Vishu, for example) coincides with this transit.
In the Kerala version of the solar calendar, the solar months— मेष, वृषभ, मिथुन, कटक, सिंह, कन्या, वृश्चिक, धनु, मकर, कुम्भ, मीन — are named after constellations, unlike the lunisolar month names. Some other solar calendars use the lunisolar month names as they are. The path of the sun on the background of stars is divided into 12 equal parts, and each part is named after a constellation, making all of them the same width in the sky.
This does not, however, imply that the months are of equal length. The earth's orbital speed changes depending on where it is in its orbit, as Kepler proved. This makes months where the Earth is near perihelion shorter.
Years and month lengths are self-aligning, since they are derived from orbital calculations, and therefore do not require mechanical corrections by adding or subtracting days as in the Gregorian year. However, the Sidereal and Tropical years slowly diverge thanks to precession.
Some states known to use a Sidereal Solar Calendar are Kerala, Tamil Nadu, Orissa, Bengal, Tripura and Assam.
Alignment between Lunisolar and Solar months
Since शुद्ध lunisolar months require a solar transit, this results in self-alignment without specific correction between the solar and lunisolar years, with their start points being off by a maximum of the length of a lunar month.
Solar calendars do not require intercalary month additions, which makes them a bit easier to follow than lunisolar ones.
Marking days
While lunisolar calendars mark days names with moon phases (तिथि), solar calendars mark day names with the नक्षत्र (asterism) near the moon. This leads to minor divergences in birthdays for those who follow different calendars.
Major festivals in the solar calendar
- Solar New Year (Vishu, Tamil New Year, Vaisakhi, Bengali New Year etc.). These are marked by the transit of the Sun into मेष(Aries).
- Makara Sankranti. Marked by thte transit of the Sun into मकर (Capricorn)
- Onam: full moon of the सिंह month. Technically, the Malayalam (Kollam) calendar begins with this month, not मेष, but new year rituals are still performed on Vishu.
States that have Solar calendars tend to have some festivals on different days from those that follow lunisolar calendars. Most parts of India celebrate Janmashtami on the अष्टमी (eighth day) of कृष्णपक्ष (the dark half) of the भाद्रपद (bhaadrapada) month. Kerala and TN celebrate the same festival in the month of siMha when the moon is near रोहिणी (Rohini).
Since the Sidereal and Tropical years diverge by 20 minutes each year, festivals like Baisakhi/Vishu slowly drift to later dates in the Gregorian calendar over centuries. Spring Equinox happens around March 21st, and the Indian Solar New Year is on April 14th in 2017, three weeks later.
