Elements of the Indian calender

Sidereal Period and Nakshatras

The sidereal period of an object is the time it takes to complete one revolution against the background of stars. For instance, the Sun's sidereal period is the time it takes to traverse the entire ecliptic and return to the same position. The Moon's sidereal period is about 27.32 days, constituting one lunar cycle.

The Indian system divides the zodiac into 27 equal parts, each called a Nakshatra. Each Nakshatra spans 13 degrees and 20 minutes or 800 minutes of arc. These divisions help track the Moon's trajectory against the stars. Each Nakshatra is named after a prominent star within that division.

Rashis and Luminaries

The 27 Nakshatras are further mapped onto 12 Rashis, with each Rashi associated with two and a quarter Nakshatras. This division forms the basis for tracking the movement of celestial bodies and making astronomical calculations.

The Indian calendar is primarily based on the two luminaries: the Sun and the Moon. The elapsed time for the Sun to return to the same star, completing one revolution of the Earth around the Sun, is defined as the solar year. The lunar month is the period from one Full Moon to the next or one New Moon to the next. Twelve such lunar months constitute a lunar year.

Luni-Solar Calendar

While some Indian states follow a solar calendar and others a lunar calendar, the lunar calendar is universally used for fixing religious festivals and auspicious times. This makes the Indian system essentially a Luni-solar calendar.