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The first thing to understand is a Synodic month. The term synodic month refers to the period it takes for the Moon to return to the same phase when observed from Earth - specifically, the time interval between successive new moons. A synodic month lasts about `29.53058867` days, which is approximately 29 days, 12 hours, 44 minutes, and 3 seconds.
In a synodic month, the Moon goes through its complete set of phases, from new moon ЁЯМС to full moon ЁЯМХ and back to new moon ЁЯМС again. This is the period that most people are familiar with when they think of the "monthly" cycle of the Moon. One such synodic month is called a Masa.
There usually are twelve Masa in a year. These are:
In Tamil, the masa names are as follows: Chithirai, Vaikasi, Aani, Aadi, Aavani, Purattasi, Aippasi, Karthikai, Margazhi, Thai, Maasi, Panguni.
Masa is a synodic month approximately 29.53 days long. So a year based on the Moon (i.e., a lunar year) is 354 (i.e., `29.53 * 12`) days long. A year based on the Gregorian calendar (i.e. a solar year) is 365.25 days long. So, a lunar year is 11 days short of a solar year (`365.25 - 354 = 11`days).
To bridge the gap, an extra lunar month, known as Adhika Masa, is inserted approximately every 32.5 months or roughly every 2.5 years. This resembles how one extra day is added in a leap year, once in four years. Adding an Adhika Masa makes the solar and lunar cycles in sync.
The Adhika Masa is considered an inauspicious time for major activities.