Tithi

A tithi (lunar day) is the Moon's own way of counting days. It does not follow sunrises. It follows the angle between the Moon and the Sun: each time the Moon gains another 12° on the Sun, one tithi ends and the next begins. The tithi at a birth is one of the five limbs of the panchaanga, hora's almanac of the moment.

Twelve degrees at a time

What counts is the Moon's lead over the Sun: the arc from the Sun forward along the nirayana (sidereal) zodiac to the Moon. At the new moon it is 0°, at the full moon 180°, and just before the next new moon it is almost 360°. The first tithi runs from 0° to 12°, the second from 12° to 24°, and so on, so 360° holds thirty of them. That is one lunar month: it starts when the Moon catches up with the Sun, at the new moon, and ends when it catches up again.

Since the Moon's speed changes over the month, and so does the Sun's over the year, a tithi is not a fixed length of clock time. It lasts about a day, sometimes a few hours more, sometimes a few hours less, and so it can begin and end at any hour.

Two pakshas

The thirty tithis fall into two pakshas (fortnights) of fifteen. In Sukla paksha, the bright half, the Moon is 0° to 180° ahead of the Sun and waxing, from new moon to full moon. In Krishna paksha, the dark half, it is 180° to 360° ahead and waning, from full moon back to new moon.

Names

The two pakshas use the same names for their first fourteen tithis, mostly Sanskrit ordinals: Pratipat is the first, Dwitiya the second, and so on to Chaturdasi, the fourteenth. The fifteenth tithi of each paksha has a name of its own: Sukla paksha ends with Paurnami, the full moon, and Krishna paksha with Amavasya, the new moon.

The paksha's name in front tells the two apart: Krishna Dasami is the dark half's tenth tithi, the 25th of the month, while Sukla Dasami is the 10th. The chart page names a tithi this way, with its number from 1 to 30 in brackets after it.

Lords

Each tithi has a lord, one of the grahas (planets). Read down the table's rows, the lords follow the weekday order of the grahas, Sun, Moon, Mars, Mercury, Jupiter, Venus and Saturn, with Rahu as the eighth, then start again with the Sun; Ketu rules none. A tithi and its namesake in the other paksha share a lord, so Krishna paksha starts again with the Sun, and its last tithi, Amavasya, has Rahu where Sukla paksha's Paurnami has Saturn.

The table

The table gives the same facts as the book's Table 3: each tithi's number in Sukla and in Krishna paksha, its name and its lord. The book also lists other spellings of some names; hora uses the first.

A worked example

On 1 January 2000 at 12:00 UT, at Greenwich, hora places the Sun at 16°31′17″ in Sagittarius, a longitude of 256°31′17″, and the Moon at 19°28′14″ in Libra, a longitude of 199°28′14″ (see the chart; the rasi page shows how a position in a rasi becomes a longitude).

1. The Moon minus the Sun is 199°28′14″ − 256°31′17″ = −57°03′03″. It is negative, so add 360°: the Moon is 302°56′57″ ahead of the Sun.
2. 302°56′57″ divided by 12° is 25, with 2°56′57″ left over: twenty-five tithis are complete, so the twenty-sixth is running, and the Moon has 9°03′03″ still to gain before it ends.
3. In the table, 26 is in the Krishna column, on the row of Ekadasi: the tithi is Krishna Ekadasi, and its lord is Mars.

In the chart

The panchaanga on the chart page gives the tithi at the birth, by paksha, name and number, with its lord; its value there links to this page. The table under it tells when that tithi started and when it ends.

Next comes the lunar month: how the months these tithis make up are named.

Book reference: P.V.R. Narasimha Rao, Vedic Astrology: An Integrated Approach, chapter 1, § 1.3.8.1. Where to get the book