Structural Changes of the Sun-and-Stars Time-Determining Instrument

We will show the structure and usage of the Sun-and-Stars Time-Determining Instrument (日星定時儀) that was first made under King Sejong (fifteenth century) during the Joseon Dynasty. It can measure solar time in the daytime and sidereal time at night. At the same time, a Simplified Sun-and-Stars Time-Determining Instrument (小日星定時儀) which evolved from the Sun-and-Stars Time-Determining Instrument, simplified the graduated scale to ‘du’, not ‘fen’ in the celestial-circumference degree (that is, 365.25 du). It also got rid of the pole-fixing ring (正極環) by which its instrument can be aligned to the meridian line. Lastly, the Simplified Sundial (小日影) manufactured at the time of King Sejong can still be found in Korea today. This eliminated the need to measure sidereal time by removing the celestial-circumference ring and the star-dial hundred-interval ring of the Sun-and-Stars Time-Determining Instrument. We suggest that the Sun-and-Stars Time-Determining Instrument evolved into the Simplified Sun-and-Stars Time-Determining Instrument, and then the Simplified Sundial. This family of Sun-and-Stars Time-Determining Instruments was used both for timekeeping and for military purposes and in ritual ceremonies.