Tamil New Year Vilambi - An astronomical note

சென்ற ஹேவிளம்பி ஆண்டு புத்தாண்டு வான்நிலையும் வரைபடத்தையும் நான் விளக்கிய தமிழ் கட்டுரை இங்கே

The period from one sunrise to another, is called a dina दिनः or aha अहः in Sanskrit. Since the Sun causes the day, he is called Dina kara (दिनकरः the maker of the Day). The English astronomical term for this is Solar Day.

The period from one moonrise to another, is called a thithi तिथि in Sanskrit. The English astronomical term is Lunar Day.

There are thirty thithi-s, fifteen from new moon to full moon (called waxing fortnight in English and shukla paksha in Sanskrit), and fifteen from full moon to new moon (called waning fortnight inEnglish and krishna paksha in Sanskrit). This thirty thithi period is called a month in English and maasa in Sanskrit. Note that each thithi or lunar day is slightly shorter than each solar day. The period between two full moons is only 29 1/2 days, not thirty, though, so twelve full moons will come upto 354 rather than 365 days.

Each night the moon changes its position in the sky against the background stars (because it is orbiting the earth). In the Vedic period, the name of the brightest star next to the moon on each night was observed to fall in a certain cylce…. Ashwini, Bharani, Kriththika, Rohini….with series ending in Revathy on the twenty seventh night. So each night or day was called by that star… or नक्षत्रम् nakshatram; the English astronomoical term for this is Stellar Day. They also noted that moon became  a full moon, only when it was next to some stars of this list of twenty seven; hence the months were named after those stars. 

So the month where the full moon was next to Chitra was called Chaitra; when next to Visaka was called Vaisaaki, when next to Kritthikaa was called Kaarthika, when next to Mrigasirsha the month was called Maargasirsha….and so on. You can see local adaptations of these names in various languages. So Vaisakhaa is called Baisaaki in some languages, Sravishtam is called Avittam in Tamil, etc.

In Sumeria, China, Egypt, and later in Rome, the local people saw patterns made by stars in they sky, reminding them of things like a bull or fish or crab and they named these collection of stars (con=collection; stellation=of star). Hence we have constellations like Taurus(bull), Cancer (crab), Pisces(fish). This idea of constellations was not used by early Indian astronomers, but adopted later, though it is not clear when this happened. Perhaps between the fifth century BC and fifth century AD, before Aryabhata, Varahamihira and others heralded a new epoch of Indian astronomy and mathematics.

This is a star chart of picture of tonight’s night sky, around 8 pm, towards the east. It wont change significantly for the next few days. On the full moon night, the moon will be seen near Chitra (Spica), hence the name for this month. The moon can be seen near corresponding stars (nakshatras) on those nights. None of the planets can be seen in the eastern sky now, until around 11pm when Jupiter will quite visible and bright. Venus, bright in the western sky, can be seen for about an hour or so after sunset.

Eastern Sky Vilambi Year
April 14, 2018 

The red line, called Ecliptic in the chart, is the path of the Sun and the Moon across the sky. You can see that is at an angle to the Equator, because of the earth's axial tilt.

Compare the above with the night sky last year on the same day, April 14 2017 or Hevilambi New Year’s night. The moon was near Vishaka, and Jupiter was visible.

Eastern night sky Hevilambi
April 14, 2017

And I also present the night sky below on April 14, 2014, or Jaya New Year’s night. Saturn and Mars were visible in the same region. Also by coincidence, the NewYear was also night of the Full moon, Chitra Pournami, so the moon was next to Spica (Chithra)

Eastern night sky Jaya
April 14, 2014

Here is a list of star names in Sanskrit, Tamil, the corresponding month names, and English/Latin names.

If you enjoyed this piece of astronomy, perhaps these other essays may also interest you.

Nilakantha Somayyaji’s Sanskrit mathematical pun

Aryabhata’s sloka for pi

Varahamihira’s salutation to Agastya

My essay on Aryabhata in The Week magazine

சூர்ய தினம், திதி, நட்சத்திரம், சிறிய குறிப்பு - தமிழில்