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Structure of Hindu Calendar
Definition of Tithi (Day) in Hindu Calendar
In the Hindu calendar, the day starts with the local sunrise and the position of the Moon (Moon phase) at that time decides the Tithi (known as Udaya Tithi). This Tithi is applied for the entire day even if the Moon phase may change to the next Tithi a few minutes after Sunrise. This concept of Tithi in the Hindu calendar is different from the Jain Ägamic calendar. The Tithi is not connected with the Sunrise (Udaya Tithi) in the Jain Ägamic calendar.
Kshaya Tithi (Lost Tithi ) and Vriddhi Tithi (Extra Tithi)
Since in the Hindu calendar the Tithi is defined at the time of Sunrise, several times in a year a condition occurs that the same Tithi may exist between two consecutive Sunrises because during 24 hours the Moon phase did not change more than 12 degrees. This is called Vriddhi (extra) Tithi.
Similarly several times in a year a situation occurs that one Tithi (Moon phase) does not touch any Sunrise time at all. This is known as Kshaya (loss) Tithi.
This happens because the duration of each Tithi varies between 22 hours and 26 hours and the duration between the two consecutive Sunrises is about 24 hours.
Please note that only Kshaya Tithi occurred in the Ägamic Jain calendar but the Hindu calendar has both Kshaya (loss) and Vriddhi (extra) Tithi several times in a year. However the total number of days in a year remained the same in both Jain and Hindu Calendars.
Leap Year (13 months in a Year)
As explained before, both the Jain and Hindu calendar has an extra month added once every three years which brings seasons in phase with the month duration. Even though both the Hindu and Ägamic Jain calendars are Lunisolar calendars, they used different leap year calculation methods. In the Hindu calendar, any of the 12 months can be a leap month while the Ägamic Jain calendar defines a five year time cycle (known as Yuga) in which the month of Posha (midyear month of an ancient Jain Calendar) and Ashadha (the last month of an ancient Jain calendar) are only two leap months.
Changes made to the Hindu Calendar:
Jain Parva Tithis (Pious Days)
Jain Ächäryas have defined Tithis; 2, 5, 8, 11, 14, and 15 as pious or Parva Tithis or Parva days in both the Sud and Vad part of the Moon cycles of every month. People take special vows and perform various penances (Ärädhanä) on these days. To fulfill their Ärädhanä without any interruptions of Kshaya (lost) or Vriddhi (extra or double) Tithi, the ancient Jain literature has indicated some special rules to apply to the Hindu calendar defined in the following Gätha or Sutra:
"Kshaye Purva Tithi Kärya, Vridhhau Karya Tathottara; Shri Veer Jina Nirvanam, Karyma Lokanugairih:"
If a Kshaya or Vriddhi Tithi occurs on a Parva Tithi (pious day) in the Hindu calendar, follow the following rules for the conversion to Jain calendar:
In case of Kshaya (loss) Tithi, previous Tithi should be considered as Jain Parva Tithi In case of Vriddhi (double) Tithi, the 2nd Tithi should be considered as Jain Parva Tithi Do not change Mahavir Nirvana Day (Deepavali day) as defined in the Hindu calendar. Also this rule indirectly told us that we cannot change the leap month of the Hindu calendar.
This is how the Jain calendar is created using above rules applying to Hindu Calendar. Practically the following changes were made in the Jodhapur Hindu calendar.
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