The Solar Year

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 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.

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.

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.

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.