During the reign of King Sejong in the early Joseon period, astronomy reached a world-class level. By compiling the Chiljeongsan, an independent astronomical calendar calculating celestial movements based on the latitude of Hanyang (Seoul), Joseon could measure its own time. Additionally, the sundial Angbuilgu was installed in the busy streets of Jongno so that commoners could easily tell the time, ensuring that knowledge was not monopolized by the elite.