Cheomseongdae is the oldest existing astronomical observatory in Asia. Constructed during the reign of Queen Seon-deok (632-647), it was used for observing the stars in order to forecast the weather. This stone structure is a beautiful combination of straight lines and curves, and was designated as National Treasure No.31 on December 20th, 1962. Cheomseongdae was built in a cylinder shape with stones 30cm in diameter. 362 stones were piled up to make 27 levels. Roughly 4.16m up from the bottom there is a 1㎡ square entrance and a space to hang a ladder under it. The inside is filled with soil up to the 12th level, and the 19th, 20th, 25th, and 26th levels all have long rocks hanging on two areas, shaped as the Chinese letter '井' (jeong). It stands 9.17m high and the base stone on each side measures 5.35m. The Vernal Equinox, Autumnal Equinox, Winter Solstice, Summer Solstice and the 24 solar terms (also known as the astronomical solar year) were determined by the observation of stars. The pavilion stone is believed to have been used as a standard of deciding directions, north, south, east and west. The 362 stones used to build Cheomseongdae represented the 362 days in a lunar year.