สำนักงานราชบัณฑิตยสภา

The Journal of the Royal Society of Thailand Vol. 41 No. 3 July-September 2016 172 Abstract Suwarnabhumi Calendar Loy Chunpongtong Adviser to the Royal Thai Calendar Calculation, Bureau of the Royal Household The common lunar year is shorter than the solar year by about 11 days. So the lunar calendar is kept adjusted to the solar calendar and therefore to the sidereal year by the addition of an intercalary month at suitable intervals. The Thai calendric system comprises both solar and lunar calendars. Such a calendar is called a lunisolar calendar The return of the sun to the same reference point in its path in the sky is the measure of the solar year and the basis of all solar calendars. However, this reference point may be chosen in two different ways. A fixed point on the ecliptic with reference to a background star - sidereal system and it has been used in the current calendar. Either of the two equinoxes or equinoctial points, which for calendrical and astronomical purposes is normally taken to be the vernal or March equinoctial points - tropical system. Apparently, every 70 years the current sidereal system causes one day slip from the tropical system. This has been make the Buddhist holidays gradually seasonal offset. This paper describes a method of calculation which the author invented using a unique mathematical technique and incorporating modern astronomical parameters to turn the current official sidereal system to tropical system. Keywords: Thai lunisolar calendar, Suwarnabhumi calendar, Sidereal year, Tropical year, Adhikamas determining calculation, intercalary year ปฏิทินสุวรรณภูมิ