Amavasya

Amavasya is the thirtieth and final tithi, the new moon, when the Moon stands in conjunction with the Sun and its disc is dark and unseen. It closes the dark fortnight and the lunar month. Its presiding deities are traditionally the Pitris, the ancestral fathers, and the day is the chief occasion for shraddha and tarpana, the rites of offering to the ancestors, the fortnight of Pitru Paksha culminates on an Amavasya. The new moon of Kartik is Diwali, kept with lamps against the darkest night. In electional astrology the panchanga generally avoids Amavasya for auspicious beginnings, treating it as a day for ancestral and inward observance rather than outward ventures. These are the conventions of the calendar tradition; the astronomical basis is the exact conjunction of Moon and Sun that leaves the night moonless.

Pitru Paksha, the fortnight of ancestral rites that culminates on this Amavasya, is fixed to the dark fortnight of the lunar month of Bhadrapada or, in some regional reckonings, Ashvin, and its sixteen days each correspond to a different possible tithi of death, so a family observes shraddha on whichever day within the fortnight matches the deceased relative's actual date of death, with the Amavasya itself serving as a catch-all day for ancestors whose precise date of death is unknown or unobservable. Diwali's date, the Amavasya of Kartik, is calculated according to which tithi prevails at dusk, since the lighting of lamps against the moonless dark is the observance's defining act, making this one of several major festivals whose correct calendar date depends on identifying the tithi at a specific time of day rather than at sunrise.

The tithi is the oldest of the five limbs of the panchanga and the least intuitive, because it is defined by an angle in the sky and not by a sunrise. Amavasya occupies the arc from 348 to 360 degrees of that separation. Since lunar motion quickens and slackens across the month, the arc is crossed in as little as about nineteen hours or as much as about twenty six. The panchanga settles the mismatch by naming each civil day for the tithi in force at its sunrise. A tithi shorter than the interval between two sunrises can therefore be skipped entirely, which the tradition calls a kshaya tithi, and one longer than that interval can carry its name over two civil days, an adhika or vriddhi tithi. Both occur in the ordinary course of a year, and both are why an observance can appear on two dates in the same almanac.

Each tithi is halved again, and each half carries a karana, so the first half of Amavasya carries Chatushpada, the four-footed karana, and the second Naga, the serpent karana that ends the lunar month. Seven of the eleven karanas repeat, cycling eight times through the month, while four are fixed to particular half tithis at the month's opening and close.

At the close of this arc the Moon returns to conjunction with the Sun and the cycle begins again. Through Amavasya the disc shows under one per cent of the disc lit, which is to say effectively none, and the Moon is lost in the Sun's glare. In the amanta reckoning used across much of southern and western India the lunar month ends here, which is the root of the standing discrepancy between regional calendars over which month a given festival belongs to.