The Thamil calendar, while based on the Hindu calendar, varies in terms of the timing of certain events and celebrations due to the manner in which such dates are calculated.
Calendars came into existence as early as the Bronze Age, as humans began to divide up time based on natural cycles, such as the length of a day or the route of traversal of the sun and moon across the sky. While used for practical purposes, such as determining planting and harvesting cycles, calendars also came into use for religious purposes, denoting the dates on which certain festivals or worship ceremonies should be held.
The Hindu calendar, also known as the Panchanga, is an ancient geocentric calendar developed through observation of the transit of sun and moon across the sky relative to certain constellations. Being based on both the sun and moon, it is known as a lunisolar calendar, but as such, there are sometimes conflicts in determining festival dates based on one cycle or the other. There are several offshoots of the Hindu calendar that lean towards one cycle or the other, including the Thamil calendar.
The Thamil calendar varies from the Hindu calendar in that it is based heavily on the solar cycle, and is known as a sidereal solar calendar. This means that it operates based on the position of the sun relative to certain constellations, rather than calculating time based on the equinoxes, resulting in a slight difference in the length of a year. While it uses the same months as the Hindu calendar, the Gregorian calendar date on which a festival or ceremony might be designated to take place can vary from the Hindu calendar to the Thamil calendar due to their relative emphasis on the influence of the solar or lunar cycle.
The Thamil calendar, having months and years approximately the same as the Hindu calendar, also operates on a sixty-year cycle. This cycle is based on the movement of the planet Jupiter through the various constellations, and begins and ends when the planet reaches a certain spot in the sky. This cycle can be considered similar to a “century” in western calendar systems, and marks the restart of the calendar cycle. Several other civilizations have utilized this idea of a calendar cycle based on astronomical observations, such as the Chinese and the Aztecs.
The Thamil almanac, or Panchangam, which details this calendar cycle, has two variants, one which is used for civil and religious calculations (Vakiya Panchangam) and one which is used for astrological calculations (Trikanitha Panchangam). While available in print, there are also versions available online, both on websites and in app form.