The western calendar is based on the sun, in which a year is the time required for the earth to complete one orbit around the sun. This precisely measures 365 days 5 hours 48 minutes and 46 seconds.
The Indian calender is ingeniously based on both the sun and the moon; it uses a solar year but divides it into 12 lunar months. A lunar month is precisely 29 days 12 hours 44 minutes and 3 seconds long. Twelve such months constitute a lunar year of 354 days 8 hours 48 minutes and 36 seconds. To help the lunar months coincide with the solar year, the practice of inserting an intercalary (extra) month arose. So 60 solar months = 62 lunar months. Hence an extra month, called the Adhik Mas, is inserted every 30 months i.e. every 2 ½ years.
Lunar days in the Indian calendar are called tithis. They are calculated using the difference of the longitudinal angle between the position of the sun and moon. Because of this, tithis may vary in length. Consequently, the tithi may or may not have changed by the time the day has changed at sunrise. And that is why we find at certain times a tithi being omitted, and at certain times, two consecutive days sharing the same tithi.
In the Indian calendar, seasons follow the sun, months follow the moon, and days, both the sun and the moon. The era in the Indian calendar is called the Vikram Era, or the Vikram Samvat as it is called, which began in 57 BCE. To calculate the corresponding year of the Common Era, 57 years should be subtracted from the Indian year if the date falls between the beginning of the Indian year and the end of the Western year i.e. between Kartak sud 1 and 31 December. If the date falls between the beginning of the Western year and the end of the Indian year i.e. between 1 January and Aaso vad 30, then only 56 years should be subtracted.
Months of the Indian Civil Calendar and Correlation with Gregorian Calendar.
| Months |
Days |
Begins |
| 1. Chaitra |
30 / 31* |
March 22 / 21* |
| 2. Vaisakha |
31 |
April 21 |
| 3. Jyaistha |
31 |
May 22 |
| 4. Asadha |
31 |
June 22 |
| 5. Shravana |
31 |
July 23 |
| 6. Bhadra |
31 |
August 23 |
| 7. Asvina |
30 |
September 23 |
| 8. Kartika |
30 |
October 23 |
| 9. Agrahayana |
30 |
November 22 |
| 10. Pausa |
30 |
December 22 |
| 11. Magha |
30 |
January 21 |
| 12. Phalguna |
30 |
February 20 |
Day
The Hindu calendrical day starts with local sunrise. It is allotted five "properties", called anga -s. They are:
- the tithi active at sunrise
- the vaasara or weekday
- the nakshatra in which the moon resides at sunrise
- the yoga active at sunrise
- the karana active at sunrise.
Together these are called the panchanga -s where pancha means "five" in Sanskrit
TIthis
The (anticlockwise) angular distance between the sun and moon as measured from the earth along the ecliptic (circle on the sky in which the sun, moon and planets seem to move) can vary between 0° and 360°. This is divided into 30 parts. Each part ends at 12°, 24° etc. The time spent by the moon in each of these parts (i.e. the time taken for the angular distance to increase in steps of 12° starting from 0°) is called one tithi.
The month has two paksha -s or fortnights. The first 15 tithi -s constitute the bright fortnight or shukla paksha and the next 15 tithi -s constitute the dark fortnight or krishna paksha . tithi -s are indicated by their paksha and ordinal number within the paksha . The 15th tithi of the bright fortnight (full moon) is called purnima and the 15th tithi of the dark fortnight (new moon) is called amavasya .
The tithi in which the moon is at the time of sunrise of a day is taken to be the tithi for the day.