Krishna Chaturdashi

Krishna Chaturdashi is the twenty-ninth tithi overall, the fourteenth day of the dark fortnight, falling on the eve of the new moon when the Moon has all but disappeared. In the fivefold grouping it is a Rikta tithi. It carries the most powerful of the Shiva observances: Maha Shivaratri, the great night of Shiva, is kept on the dark-fortnight Chaturdashi of Phalguna, observed with night-long fasting and vigil. The monthly dark Chaturdashi, Masik Shivaratri, is likewise devoted to Shiva. The waning Moon is now invisible against the Sun's glare. The panchanga treats the tithi with caution for ordinary matters while marking its nocturnal Shaiva festivals as among the most sacred of the year. These belong to the devotional calendar.

Because Maha Shivaratri is calculated by the tithi prevailing during the nishita kala at the heart of the night rather than by the sunrise convention used for most calendar dates, the festival can fall on either of two consecutive Gregorian calendar dates depending on the year and one's time zone, which is why different regional panchangas occasionally publish different dates for the same observance in the same year, a genuine and openly acknowledged point of calendrical disagreement rather than an error on either side. The monthly Masik Shivaratri, observed on every dark-fortnight Chaturdashi rather than only the one in Phalguna or Magha, follows the same midnight-tithi calculation on a smaller, non-festival scale twelve times a year.

Behind the observance sits a piece of arithmetic. A tithi is the time the Moon takes to open a further twelve degrees of separation from the Sun. Krishna Chaturdashi occupies the arc from 336 to 348 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 Krishna Chaturdashi carries Vishti, the karana also called Bhadra, and the second Shakuni, first of the three fixed karanas that close the month. 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 waning through this arc, rising later each night and showing about 2 per cent of the disc lit, a thread visible only in twilight. The phase follows from the elongation and not the other way round, so the tithi and the shape of the disc are two readings of one measurement.