The Hindu calendar: tithi, nakshatra and intercalary months
The Indian lunisolar calendar: the tithi or lunar day, the two fortnights, the twenty-seven nakshatras, the amanta and purnimanta systems and adhika masa.
Many regional calendars have coexisted for centuries in the Indian subcontinent, but almost all share a common basis: a lunisolar system in which the Moon marks the ritual days and the Sun regulates the year. The great Hindu festivals, from Holi to Diwali, are set by this calendar, and so their dates change every year relative to the Gregorian calendar. Its units of measurement, born of ancient Indian astronomy, are among the subtlest ever devised to describe the motion of the Moon.
The tithi, the lunar day
The basic unit is the tithi, the lunar day. The month is divided into thirty tithis, each corresponding to an increase of twelve degrees in the angular distance between the Moon and the Sun. Because the Moon does not move at constant speed, a tithi can last less or more than a solar day: a tithi may happen to be current at no sunrise at all, or at two consecutive sunrises. As a rule, the tithi current at sunrise gives its name to the day.
The thirty tithis are divided into two halves, called paksha: the bright fortnight, shukla paksha, from new Moon to full, and the dark fortnight, krishna paksha, from full to new. The full moon is called purnima, the new Moon amavasya. Many observances are identified simply by the number of the tithi and the fortnight, such as the eighth day of the dark half, the traditional date of Krishna's birth.
The lunar mansions
Alongside the tithis, Indian astronomy divides the zodiac into twenty-seven nakshatras, the lunar mansions, which the Moon crosses at a rate of about one a day during its circuit of a little over twenty-seven days. Some ancient sources count twenty-eight. The nakshatras already appear in Vedic texts and in the Vedanga Jyotisha, the oldest Indian treatise on calendrical astronomy, whose dating remains debated.
The names of the months derive from the nakshatras. A month takes the name of the mansion in which its full moon normally falls: Chaitra from Chitra, Vaishakha from Vishakha, Kartika from Krittika, that is, the Pleiades. The twelve months are Chaitra, Vaishakha, Jyeshtha, Ashadha, Shravana, Bhadrapada, Ashvina, Kartika, Margashirsha, Pausha, Magha and Phalguna, and each thus keeps in its name a reference to the night sky.
Two ways of counting the month
There are two main conventions. In the amanta system, common in southern and western India, the month ends with the new Moon; in the purnimanta system, prevalent in the north, it ends with the full moon. The same night can therefore belong to months with different names depending on the region, without changing the date of the festival. Diwali, for example, falls on the new Moon that closes the month of Ashvina in amanta reckoning and lies in the middle of Kartika in purnimanta reckoning.
To stay in step with the Sun an intercalary month, adhika masa, is added about every thirty-two or thirty-three months. The rule resembles the Chinese one: the lunar month during which the Sun does not enter a new zodiac sign is doubled. Traditional calculations rest on classical treatises such as the Surya Siddhanta, and years are counted in several eras, including the Vikram Samvat, which begins in 57 BC, and the Shaka era, which begins in AD 78.
In 1957 the Indian government adopted a reformed national calendar, based on the Shaka era and on solar months of fixed length, alongside the Gregorian calendar for civil use. The traditional lunisolar calendar is still printed every year in the panchangas, the almanacs that give tithis, nakshatras and moments held to be auspicious. In those pages dense with numbers survives an astronomical tradition more than two thousand years old.