Seasons from the Sun
Kamal Lodaya, Bengaluru

JM will carry a series of articles about calendar makers from history. Both astrologers and astronomers, and most people, are interested in the calendar. Why?
Calendars are a very simple form of forecasting. You can look up a calendar and find out which day of the week your birthday will fall on this year (if it hasn't happened already). That is a forecast. You will find on your birthday that the forecast was correct!
The earlier article discussed summer and winter and explained that these happened because the Sun's daily arc in the sky, from sunrise to noon to sunset, shifts northwards (uttarayana) from December 21 to June 21, and then southwards (dakshinayana) from June 21 to December 21.
Puzzled by this, ancient people who did not know that Earth goes around the Sun tried to see which stars were behind the Sun in the sky. But the Sun is so bright that we cannot see anything near it in the sky. So let us start wth an easier question. Which stars are behind the Moon in the sky?

Months come from Moon
Traditional calendars are centred around the Moon, because the changing phases of the Moon make it easy to count the days in a month. The word "calend" refers to the practice of calling out in public announcements that the "new" Moon has been seen, two or three days after an amavasya. Since it is easier to see, we can go from Full Moon (purnima) to Full Moon.
When the "new" Moon is seen a day or two after amavasya, it is in the West after sunset and sets soon after the Sun. If you think about it, this means it would have risen in the East soon after sunrise, and was not seen in the brightness of the Sun. Only at sunset can we see it.
The next day it is a little higher in the West and sets later. The day after that it sets later still. Keeping on going like this, when purnima arrives, the Moon rises soon after sunset and sets at dawn. You may have already noticed this about Full Moons. The day after that, it rises later. And the day after that, it rises later still.
This can be explained very simply. The Moon is at a different position in the sky every day. Why? Because it is going around Earth. The month, from Full Moon to Full Moon, takes 29 or 30 days. 
Skywatchers saw from very ancient times that the Moon's path in the sky, which remains more or less fixed, can be divided into lunar nakshatras (one star or a few stars) which mark the Moon's daily movement. These nakshatras were named Ashwini, Bharani, Krittika, Rohini, ..., ending with Revathi. The picture shows a crescent moon in Krittika (Pleiades).
Lunar nakshatras are very old, and may have originated in India. China also had an idea of the Moon staying in a "house" every day. Twelve of these lunar nakshatras give their names to the purnimas and the months in the calendar. For example, the month Magha, beginning mid-February, and with Magha purnima on 27 February, is named after the lunar nakshatra (star) Magha. The idea was that on Magha purnima the Magha nakshatra in the sky would find the position of the Moon. It is worth checking this out if you know how to find the star Magha in the night sky.

The lunar calendar
Observers soon realized a curious fact. The Moon returns to the same position in the sky in around 27 days. So there are 27 nakshatras marking its path. So 28 March is Phalguna purnima, with the Moon's nakshatra (and the name of the month) being Phalguna.
Why do Full Moons change like this? To remember this, ancient people named the months after nakshatras. So the month name would tell people where the purnima would be. Thus the calendar was born. 
We have described this using Indian names, but something like this was recorded in all ancient civilizations. In the picture shown, each dot marks the start of a new zodiac sign, and are separated by 30 degrees. Here, the Sun is shown located at the start of Aquarius, that is, in Jan-Feb. The star signs are shown in the table. Notice that while the Sun takes a year to go through all these nakshatras, the moon takes around a month.
You can leap ahead into modern times. Purnima is when the Moon is opposite the Sun. But the Moon only takes 27 days to go around the Earth. Why is a lunar month nearly 30 days then? What has happened is that in the 27 days that the Moon takes to go around the Earth, the Earth itself has moved ahead in its orbit around the Sun. The stars on the opposite side of the Sun are now different. It takes two days longer for the Moon to again come opposite the Sun, when Sun-Earth-Moon are again in the same plane. So the month from Full Moon to Full Moon is 29.53 days rather than 27 days. 
Ancient calendar makers did not realize Earth goes around the Sun. But they were clever too. They realized that the nakshatras gave them a way of telling which stars were behind the Sun. Can you guess how they did it?

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BOX: Months, Full Moons and nakshatra names
Month name, Purnima date (2021), Tamil month name
Pausha, January 28, Thai
Magha, February 27, Masi
Phalguna, March 28, Panguni
Chaitra, April 27, Chittirai
Vaishakha, May 26, Vaigasi
Jyeshtha, June 24, Aani
Ashadha, July 24, Aadi
Shravana, August 22, Aavani
Bhadra, September 20, Purattasi
Ashwina, October 20, Aippasi
Kartika, November 19, Karthigai
Mrigashira, December 19, Margazhi

Pausha purnima (Thai) is on January 28, Magha (Masi) purnima is on February 27 and so on.
END OF BOX