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478 ªØ‘ ∑‘ π·≈–»— °√“™∑’Ë „™â „πª√–‡∑»‰∑¬ Vol. 29 No. 2 Apr.-Jun. 2004 The Journal of the Royal Institute of Thailand Abstract Calendar and Era in use in Thailand Visudh Busyakul Associate Fellow of the Academy of Arts, The Royal Institute, Thailand The ancient civilizations in the Middle East founded the lunar, solar, luni-solar and sidereal calen- dars. Egypt at one time employed three calendar systems simultaneously, namely, the solar or official system (with 365 days per year), the religious or lunar system (354 days per year), and the siderial Sothis Star year (for control purpose) of 365.25 days. The solar (or Julian) calendar had been in use in Europe for sixteen centuries when it was replaced by the Gregorian calendar in 1582 A.D., reducing the year length from 365.25 to 365.2425 days, a very close approximation of the astronomical year length. The true lunar Hejira calendar (354.367 days per year) is still in use by the Thai Muslims for religious purpose. Thailand employs several eras in its history, namely, Buddhist era, Kali Yuga era, Mahasakaraj era, Cullasakaraj era, and Ratanakosin Era. The modified Gregorian calender was officially sanctioned by King Rama V of the Bangkok period, and the Buddhist Era was then permanently adopted as the official choice of this country from the time of King Rama VI. The solar calendar is presently used by the general public in Thailand for everyday purpose, but the luni-solar calendar, based on Suriyayatra text, with the year-length of 365.25875 days, is still employed for religious functions and for astrological computation, in spite of the fact that it increases one more day of error of the vernal equinox every sixty years. This article also gives a handy formlar for conversion of eras in use in Thailand. Key words : calendar, era