仏滅紀元
During his monkhood, Mongkut studied both indigenous astrology and English texts on Western astronomy and mathematics, hence developing his skills in astronomical measurement. One way that he honed his mastery of astronomy, aside from the accurate prediction of the solar eclipse of August 18, 1868 (Wakor solar eclipse), was changing the official Buddhist calendar, "which was seriously miscalculated and the times for auspicious moments were incorrect."