Tithi, Simply: The Moon's Daily Rhythm

Every year someone asks me why Ekadashi is listed on Tuesday in one calendar and Wednesday in another, or why a festival seems to begin in the afternoon. The answer lies in the tithi, the lunar day, which is the oldest and most basic unit of the Hindu calendar. Once it is understood, a great deal of apparent confusion disappears.

A tithi is defined by the relationship between the Sun and the Moon. Each time the Moon moves twelve degrees further ahead of the Sun in the sky, one tithi ends and the next begins. A full cycle of three hundred and sixty degrees therefore contains thirty tithis, which make up one lunar month.

Why tithis are uneven

The Moon does not move at a constant speed; its orbit is elliptical, and it travels faster at some points than others. As a result, a tithi can be as short as about nineteen hours or as long as about twenty-six. It almost never starts at sunrise. On any given calendar day, you may see one tithi end and another begin at, say, ten past two in the afternoon.

This produces two curiosities. Occasionally a tithi begins after one sunrise and ends before the next, so it never touches a sunrise at all; it is called a kshaya, or lost, tithi. Occasionally a tithi spans two sunrises; it is called a vriddhi, or increased, tithi. These are why festivals are sometimes observed on different days by different communities, each following a slightly different rule for which sunrise counts.

The two fortnights

The thirty tithis divide into two halves. Shukla paksha, the bright fortnight, runs from the day after the new moon to the full moon, Purnima. Krishna paksha, the dark fortnight, runs from the day after the full moon to the new moon, Amavasya. Tithis are named by number within each fortnight, so Pratipada is the first, Dwitiya the second, Tritiya the third and so on, up to Chaturdashi, the fourteenth. Ekadashi, the eleventh, is observed as a fasting day in both fortnights.

The five groups

For muhurta, the tradition sorts tithis into five groups that repeat through each fortnight. Nanda (the first, sixth and eleventh) is associated with joy; Bhadra (second, seventh, twelfth) with wellbeing; Jaya (third, eighth, thirteenth) with victory; Rikta (fourth, ninth, fourteenth) with emptiness; and Purna (fifth, tenth, fifteenth) with fullness. Rikta tithis are generally avoided for auspicious beginnings, though they are considered suitable for some other kinds of work, such as ending a dispute.

The tithi is not a lucky or unlucky day; it is a way of noticing where we are in the Moon's monthly rhythm.

How tithi is used in practice

For most religious observances, the tithi prevailing at sunrise, the udaya tithi, governs the day. For some, such as certain festivals and shraddha, the tithi at another time of day matters more, which is why calendars occasionally disagree. For muhurta, the practitioner checks not only which tithi applies but exactly when it changes, so that the chosen window sits comfortably within a suitable one.

None of this tells you how your day will go. It is a framework for timing beginnings and observances, inherited from people who watched the sky closely. If you are curious, follow the tithi in a local panchang for one lunar month. Watching it shift by an hour or two each day is the quickest way to understand it.

Vedic counsel and ritual are traditional and complementary practices — they sit alongside professional medical, legal and financial advice, never as a substitute for it.