Of all the nine planets (Navagrahas), moon (Chandra) plays an important role in Hindu calendar and astrology. Time calculations, in the Hindu calendar, depend on three factors: earth, moon and sun. This calculation has five dimensions; tithi, day, nakshatra, yoga and karana. A calendar showing these five items is called Panchanga. Out of these tithi and nakshatra are related to Chandra. Rashi in Hindu calendar and panchang is also based on the movement of moon.
The Hindu calendar, in general, is based on the lunar month, that is the time required for the moon to revolve around the Earth. One lunar month is about 29.5 days.
Twelve lunar months make one lunar year, which contains 365.2 days, approximately.
The lunar month is divided into two parts of approximately fifteen days each. In the first part (shukla fortnight), the moon’s shades are in the phase of increasing illumination, culminating with the full moon day. In the second part (Krishna paksha) the shades fade until the fifteenth day, which is new moon day.
When the angle between the moon and the sun become 360 degrees, thirty divisions are made of this angular distance, each division of 12 degrees being called a tithi. Each of the twenty-seven divisions of a lunar month is called a nakshatra (asterism).
Half of a tithi is called a karana, which is a difference of six degrees between the sun and the moon. The twenty-seven nakshatra are show in the panchanga. Tithi and nakshatra are important in observing death anniversaries and birthdays. They are also useful for telling the fate of a person.
All auspicious events are determined with reference to the tithi, the nakshatra and the moon’s position. Some Hindu communities avoid the new moon day for such events.