The traditional New Year in many South and Southeast Asian cultures is based on the sun's entry into the constellation Aries. In modern times, it is usually reckoned around 14 April.
As most countries and cultures of South and Southeast Asia lie within the Indian cultural sphere, the development of their traditional calendars have been strongly influenced by some form of the Hindu calendar. As in many other calendars, the New Year was based on the northern hemisphere vernal equinox (the beginning of spring). However, the Hindu calendar year was based on the sidereal year (i.e. the movement of the sun relative to the stars), while the Western Gregorian calendar is based on the tropical year (the cycle of seasons).
In ancient times, the sun's entry into Aries coincided with the equinox. However, due to the earth's axial precession, the sidereal year is slightly longer than the tropical year, causing the dates to gradually drift apart. Today, the sun's entry into Aries occurs around 18 April, according to astronomical definitions. Some traditional calendars are still marked by the sun's actual movements while others have since been fixed to the Gregorian calendar.
The sun's entry into Aries is known as meṣa saṅkrānti in Sanskrit, and is observed as Mesha Sankranti and Songkran in South and South-east Asian cultures.
The specific New Year observances include:
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