Timelines of Events

• 100 BCE

  • Emperor Wu of the Han Dynasty sets a Chinese calendar based on a solar year.

• 46 BCE

  • Julius Caesar changes the Roman calendar to include a leap day every four years and a year length of 365 days and 6 hours.

• 1437

  • Ulugh Beg, a Timurid astronomer, used a 164-foot gnomon to determine that the solar year is 365 days, 5 hours, 49 minutes, and 15 seconds long.

• 1582

  • Pope Gregory introduces the Gregorian calendar, a revision of the antiquated Julian calendar, using the same 365.25-day year as Guo's Shoushi calendar.

The Solar Year

• The old and traditional Chinese calendars were a blend of complicated lunar and solar cycles.

  • It’s 13 lunar months correspond to the seasons derived from the sun.

• A 365.25-day solar year was used when it was first formalized in the first century BCE during the Han Dynasty.

• China’s calculations were different. They were 50 years ahead of the West’s calendar.

• When Kublai Khan conquered China in 1276, the Daming calendar was used.

  • He then decided to impose his authority on a new calendar — the Shoushi calendar.

  • Guo Shoujing, the khan's brilliant Chinese chief astronomer, was given the job of creating the calendar.

• An armillary sphere is a tool used to simulate the positions of celestial bodies; Guo Shoujing created a water-powered version of this instrument.

Measuring the Year

• Guo's task was to establish an observatory in Khanbaliq in order to measure the length of the solar year.

• Guo started a series of observations to track the motion of the sun throughout the year in collaboration with mathematician Wang Chun.

• In 1279, they declared that a month had 29.530593 days and that the length of the actual solar year was 365.2524 days.

  • This is just 26 seconds longer than the currently accepted measurement.

Enduring Calendar

• Guo invented a giant gnomon with a height of 44ft. This allowed him to measure the angle of the sun with far greater accuracy.

• The Shoushi calendar was widely regarded as the most accurate calendar in the world at their time and it continued to be used for 363 years.

• In 1912, China adopted the Gregorian calendar.

Facts about Guo Shoujing

• He was born in the year 1231 in the province of Hebei's Xingtai. His family was poor.

• Guo Yong, his grandfather, was well-known for applying the Five Classics to the fields of astronomy, mathematics, and hydraulics.

• Guo Shoujing was a gifted youngster. He discovered how to construct a water clock as a teenager. Then he enhanced it.

• His new creation was known as the lotus clepsydra (a bowl in the form of a lotus flower into which water trickled).

• He started studying math at age 16, followed by hydraulics and astronomy.

• Guo created a number of astronomical instruments, including the armilla, the square table, the gnomon, and the Ling Long Yi, a water-powered armillary sphere.

  • The gnomon, which functions as both a clock and a sundial, determines the time by determining the position of the sun. The seasons can also be determined by it. Guo greatly improved the accuracy of this device.

  • The square table functions as a protractor and measures the angle and placement of the stars in the side.

  • The armilla calculates any celestial body's position as well as the angle of the sun.

  • A fancier and more precise armilla is the Ling Long Yi.