It is well evidenced by "Chiljongsan", an astronomical book on the theory of the calendar.

The book in six volumes was completed in 1442 and printed in 1444.

Chiljong means the sun, the moon, Mercury, Venus, Mars, Jupiter and Saturn. The book deals with the theory of positions and movements of heavenly bodies in a synthetic and systematic way. It is divided into interior and exterior parts, and each part is subdivided into small items.

The interior part tells about the method of setting the number of days for every month and the 24 divisions of the year, movements of the sun and the moon, method of setting the time by stars, method of deciding the lengths of the day and the night and other contents of the calendar theory which had been actually used in Korea. The calendar based on this theory had been used till the closing days of the feudal Joson dynasty (1392-1910).

The exterior part introduces the theory of the Arabic calendar. The theory was included in the book as a reference datum in making the calendar.

The book "Chiljongsan" showing Korea's astronomical development in the first half of the 15th century is a precious historical material in the study of the nation's astronomical development.