The total lunar eclipse on December 11, 1685, stands as a pivotal astronomical event that unfolded at this royal residence. King Narai the Great, in conjunction with French Jesuits, witnessed the spectacle, marking the inaugural use of a telescope in Siamese history. This observation served to calculate latitude and longitude, aiming for a precise scientific determination of Lopburi's geographical coordinates. Historical sketches from this event provided global confirmation of the tangible adoption of Western scientific knowledge. It signifies the definitive commencement of modern astronomy and geographical coordinate calculation within the Kingdom of Thailand.