Moon Phase Calculator (Tithi)

What is a tithi?

A tithi is a lunar day, and it is not the same thing as the ordinary day you read off a clock. In the Vedic reckoning a tithi is defined by the angle between the Moon and the Sun. Each time the Moon pulls twelve degrees ahead of the Sun, one tithi has passed. Since the full circle is 360 degrees, the gap from one new Moon to the next holds exactly thirty tithis, and that span of thirty tithis is one lunar month.

Because the Moon and the Sun move at their own changing speeds, a tithi is not a fixed number of hours. It can run shorter or longer than a solar day, sometimes by a fair margin. This is why a tithi can occasionally skip a calendar date or repeat across two of them. The traditional panchang names the tithi that is current at sunrise as the tithi of that day, but the precise tithi at the moment of a birth is read from the exact Moon and Sun positions at that instant.

The tithi is the first of the five limbs of the panchang, the five part almanac that Vedic timekeeping rests on. The other four limbs are the nakshatra, the yoga, the karana and the weekday, but the tithi sits at the head of the list because so much of ritual life turns on it. Festivals, fasts and the right day for a ceremony are all fixed by tithi rather than by the solar date, which is why two people born a year apart can share a birth tithi while their calendar birthdays fall on different dates.

The two pakshas: waxing and waning

The thirty tithis of a lunar month split into two halves of fifteen, and each half is called a paksha. The bright half is the Shukla Paksha, the waxing fortnight, when the Moon grows from new to full. The dark half is the Krishna Paksha, the waning fortnight, when the Moon shrinks from full back to new. Shukla means bright and Krishna means dark, naming the light on the Moon rather than anything ominous.

The waxing half carries a sense of increase and building. The Moon is gaining light night after night, and tradition reads this as a supportive current for starting ventures, for growth and for anything you want to see expand. The waning half carries the opposite grain, a sense of release and completion. As the Moon loses light it is read as a time for finishing, for letting go and for inward, reflective work rather than outward push. Astrologers weigh the paksha when they time an undertaking, and many auspicious acts are set in the bright fortnight by preference.

Two tithis mark the turning points. Purnima, the full Moon, is the fifteenth tithi of the bright half, when the Moon stands fully opposite the Sun and shines at its brightest. Amavasya, the new Moon, is the fifteenth tithi of the dark half, when the Moon sits with the Sun and shows no face at all. These two are the hinges of the lunar month, and both carry their own weight in ritual, the full Moon toward fullness and worship, the new Moon toward the ancestors and quiet renewal.

The thirty tithis of the lunar month

Each paksha runs through the same fifteen named tithis in order, from Pratipada through to the fifteenth. In the bright half the fifteenth is Purnima, the full Moon. In the dark half the fifteenth is Amavasya, the new Moon. So a complete count gives Shukla Pratipada through Shukla Purnima, then Krishna Pratipada through Krishna Amavasya, thirty tithis in all. Each tithi has a presiding deity and a character that the older texts describe, and the table below lists the fifteen names with a short note on the flavour each one carries.

Tradition also groups the tithis into five families that repeat three times across each paksha, and astrologers lean on this grouping when they judge whether a day suits a given act. Nanda covers the first, sixth and eleventh tithis and is read as a day of joy. Bhadra covers the second, seventh and twelfth and is read as steady and supportive. Jaya covers the third, eighth and thirteenth and leans toward victory and effort. Rikta covers the fourth, ninth and fourteenth and is treated with care, since it is held to be empty for auspicious beginnings. Purna covers the fifth, tenth and fifteenth and is read as full and complete. Knowing your birth tithi places you within one of these families, which is part of what an astrologer notes when they first look at the lunar side of a chart.

What does your birth tithi mean?

The tithi that was running when you were born is your Janma Tithi, and a Vedic astrologer reads it for the emotional and instinctive grain of a person. The Moon governs the mind in this system, and the tithi measures the Moon against the Sun, so the birth tithi is taken as a marker of how the inner, feeling self sits against the outer, willing self. A person born near the new Moon, with the two lights close together, reads differently from one born near the full Moon, with the lights pulling apart at their widest.

The paksha of birth is part of this reading. Those born in the bright half, as the Moon gathers light, are often described as outgoing and growing in confidence as life goes on. Those born in the dark half, as the Moon lets go of light, are often read as more reflective and inward, drawn to depth rather than display. These are tendencies and not verdicts, and a full reading always weighs the tithi against the rest of the chart before it draws any firm line. The presiding deity and the family of the tithi add further texture, so a Chaturthi native tied to Ganesha or an Ekadashi native tied to fasting and devotion each carry a recognisable flavour.

The birth tithi also feeds practical timekeeping. Many families observe a tithi based birthday, marking the day each year when the same tithi returns rather than the fixed solar date, since that is the day the Moon repeats the relationship it held at the birth. Tradition counts certain tithis as gentler and others as sharper, and an astrologer will note where your birth tithi sits, but a single tithi is never read on its own. It joins the nakshatra, the Moon sign and the running dasha to build the lunar side of a chart.

How the lunar cycle is measured

The lunar month this calculator works with is the synodic month, the time from one new Moon to the next, which averages a little over twenty nine and a half days. Across that span the elongation, the angle by which the Moon leads the Sun, climbs steadily from zero at the new Moon to 180 degrees at the full Moon and on around to 360, which is the next new Moon. Dividing that climb into thirty equal steps of twelve degrees gives the thirty tithis, so finding a tithi is really a matter of measuring that one angle with care.

This calculator computes the Moon and Sun positions with the Swiss Ephemeris and reads the elongation between them, then converts to the sidereal frame using the Lahiri ayanamsa, the value adopted as the Indian national standard. The ayanamsa matters less for the tithi than it does for a nakshatra, since the tithi rests on the gap between two bodies rather than on the position of one against the fixed stars, but the engine works in the same sidereal frame the rest of the panchang uses so the result lines up with a traditional almanac. The tithi reported is the one current at sunrise on your birth date at your birth place, which is the convention a printed panchang follows.

Because a tithi can be shorter or longer than a solar day, the exact place and the sunrise that goes with it both feed the answer. A birth near a tithi boundary can land on either side depending on the sunrise at the birth place, which is why the calculator asks for the place and not the date alone. For most births well inside a tithi the result is steady, but near a junction the detail of place and time decides which lunar day the chart belongs to.