Purnima

Purnima is the fifteenth tithi, the full moon, when the Moon stands opposite the Sun and the disc is completely lit. It closes the bright fortnight and is a Purna tithi, the complete day, read in the panchanga as broadly auspicious. Its presiding deity is the Moon itself, Chandra or Soma. Many of the most important festivals of the year fall on Purnima: Guru Purnima honours the teacher, Buddha Purnima marks the birth of the Buddha, Sharad Purnima the autumn full moon, and the full moons of Kartik and Magha are kept for bathing and charity. Fasting and pilgrimage are common on the day. These are the conventions of the calendar and its festivals; the astronomical basis is the exact opposition of Moon and Sun that produces the fully illuminated disc.

Because Purnima is defined by exact opposition of Sun and Moon, it is also the necessary condition for a lunar eclipse, which can occur only when the Moon is full and additionally crosses close to one of the two nodal points where its orbital path intersects the ecliptic; most Purnimas pass without any eclipse since the Moon usually passes above or below the Earth's shadow, but when the alignment is close enough, the astronomical basis of the tithi and the astronomical basis of the eclipse are, quite literally, the same alignment observed at two different levels of precision. The four-month period of Chaturmasya, during which certain classes of activity are traditionally deferred, is bounded at its start and close by specific Purnimas, tying this fifteenth tithi to the wider structure of the ritual year beyond its individual monthly festivals.

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. Purnima occupies the arc from 168 to 180 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 Purnima carries Vishti, the karana also called Bhadra, and the second Bava, first of the seven movable karanas. Vishti is the one karana the muhurta literature marks as unfit for beginnings, and its presence in the first half is why this tithi is read with more care than its neighbours. 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 stands opposite the Sun and the month turns. Through Purnima itself the disc shows over ninety-nine per cent of the disc lit, a disc the eye reads as full. In the purnimanta reckoning used across much of northern India the lunar month ends here rather than at the new Moon, which is why the same tithi can sit in differently named months in two almanacs printed the same year.