Ashtami

Ashtami is the eighth tithi of the bright fortnight, falling close to the first-quarter Moon when the disc is about half lit. In the fivefold grouping it is a Jaya tithi, associated with victory. Its presiding deity is traditionally Rudra, a fierce form of Shiva, and the goddess Durga is strongly linked to the eighth tithi as well. Durga Ashtami, in the bright fortnight of Ashvin, is one of the most important days of the Navaratri festival. In some reckonings Krishna is held to have been born on the eighth tithi of the waning fortnight, the festival of Janmashtami. The panchanga treats Ashtami with some caution for ordinary undertakings while honouring its many festivals. These are the calendar tradition's associations, anchored to the half-Moon of the waxing quarter.

Durga Ashtami's position within Navaratri places it at the traditional close of the nine nights' worship of the Goddess in her nine forms, immediately before Navami, and in several regional traditions the two tithis are observed together as Ashtami-Navami, blurring into a single extended culmination rather than being treated as two fully separate days. Krishna's traditional birth on the waning-side eighth tithi, at midnight in the month of Bhadrapada, is calculated by most panchangas according to the tithi prevailing at midnight itself rather than at sunrise, which occasionally produces disagreement between regional almanacs about which calendar date Janmashtami should be observed on, since the tithi in force can differ between the two reference moments in a given year.

Reckoning a tithi means measuring elongation, the angular distance of the Moon from the Sun, and cutting that circle into thirty equal twelve degree arcs. Ashtami occupies the arc from 84 to 96 degrees of that separation. The Moon does not move at a fixed rate, and the same twelve degrees may take a little over nineteen hours near perigee or close to twenty six near apogee. A calendar has to name whole days, so the rule is that whichever tithi is current at sunrise gives the day its name. 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 Ashtami 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.

The Moon is waxing through this arc, setting later each evening and carrying about 50 per cent of the disc lit. That figure is a consequence of the definition rather than an observation added to it: elongation fixes the phase, so naming the tithi already names how much of the disc is lit.