1) tithi is a lunar day. A tithi is shorter than a solar day. There are
30 tithis in a lunar month; fifteen during the bright half known as shukla
paksha and fifteen during the dark half known as krishna paksha.

 

2) Different kinds of Tithis:

Nanda        the 1st, 6th and 11 th lunar days

Bhadra       the 2nd, 7th and 1 2th lunar days

Jaya            the 3rd, 8th and 1 3th lunar days

Riktha         the 4th, 9th and 14th lunar days
Poorna       the 5th, 10th and 15th lunar days

3) Lunations are divided into 30 tithis, or lunar days.

Each tithi is defined by the time required for the longitude of the Moon to increase by 12o over the longitude of the Sun.

Thus the length of a tithi may vary from about 20 hours to nearly 27 hours.

During the waxing phases, tithis are counted from 1 to 15 with the designation Sukla. Tithis for the waning phases are designated Krsna and are again counted from 1 to 15. Each day is assigned the number of the tithi in effect at sunrise.

Occasionally a short tithi will begin after sunrise and be completed before the next sunrise.

Similarly a long tithi may span two sunrises.

In the former case, a number is omitted from the day count.

In the latter, a day number is carried over to a second day.