Gaocheng Observatory: Guo Shoujing's Yuan Star Tower (1276)

The Gaocheng Astronomical Observatory, near Dengfeng in central China, is a brick tower built around 1276 under the Yuan dynasty to measure the sun’s shadow with unprecedented accuracy. Designed under the direction of the astronomer Guo Shoujing, it supplied data for the Shoushi calendar of 1281, one of the most accurate calendars of the pre-modern world. It is generally regarded as the oldest surviving observatory building in China.

Background: why emperors needed a calendar

In imperial China the calendar was a political as well as a scientific matter. The emperor was believed to rule with the “Mandate of Heaven”, and an accurate calendar, which correctly predicted solstices, new moons and eclipses, was seen as a sign of legitimate rule. New dynasties often issued new calendars.

When Kublai Khan founded the Yuan dynasty and moved to conquer the Southern Song, the calendar in use had accumulated errors. In 1276 he ordered a full reform. The work was led by the official Wang Xun and the engineer and astronomer Guo Shoujing (1231–1316), with the scholar Xu Heng among the advisers.

What was built at Gaocheng

Guo Shoujing organised a network of observation stations across the empire, often said to number 27, from the far south to Mongolia. Gaocheng was chosen for its central station because Chinese tradition regarded the Dengfeng area as the centre of the earth. Nearby stands a Tang-dynasty stone marker of 723 commemorating the Duke of Zhou, who was said to have measured shadows there in antiquity.

The Gaocheng tower is a truncated pyramid of brick and stone, about 9.5 metres high, with two small rooms on top that bring the total height to about 12.6 metres. Two staircases wind up its sides. On the north side a groove in the tower faces a long, flat stone scale known as the “sky-measuring scale”, about 31 metres long, set exactly along the north–south line. Water channels in its surface allowed it to be checked for level.

How the shadow tower worked

The idea behind the tower is that of a gnomon, a vertical shadow stick, scaled up enormously. Traditional Chinese gnomons were 8 chi (about 2.5 metres) tall. A horizontal crossbar at the top of the Gaocheng tower stood roughly five times higher, so its noon shadow on the scale was five times longer and could be measured more finely.

A tall gnomon has one weakness: its shadow edge becomes blurred. Guo Shoujing solved this with a small device described in Yuan sources as a “shadow definer”, a plate with a pinhole that projected a sharp image of the sun and the crossbar onto the scale. By measuring the noon shadow day after day, especially around the winter solstice, astronomers could calculate the moment of the solstice and the length of the tropical year with great precision.

The Shoushi calendar

The results fed into the Shoushi calendar, whose name means “Season-Granting”. It was presented to Kublai and came into use in 1281. It fixed the length of the year at 365.2425 days, the same figure adopted in Europe with the Gregorian calendar three centuries later. It also used advanced methods of interpolation to model the uneven motions of the sun and moon.

The Shoushi system proved remarkably durable. The Ming dynasty adopted it with only small changes under the name Datong, and in this form it remained in use until the end of the Ming in 1644, about 364 years in all. Guo Shoujing himself went on to serve as an engineer, designing the canal that brought water and grain boats into the Yuan capital at Dadu, today’s Beijing.

Key people

• Guo Shoujing (1231–1316): astronomer, mathematician and hydraulic engineer who designed the instruments and observation network.
• Wang Xun (1235–1281): mathematician and official who led the calendar office.
• Kublai Khan (1215–1294): Yuan emperor who ordered the reform in 1276.

Legacy

The observatory survived the fall of the Yuan and was repaired several times in later centuries. In 2010 it became part of the UNESCO World Heritage Site “Historic Monuments of Dengfeng in The Centre of Heaven and Earth”, alongside the Shaolin Temple. Today it is often compared with other great medieval observatories, such as the Ulugh Beg Observatory built in Samarkand in the 1420s.

Guo Shoujing remains a celebrated figure in China. An asteroid is named after him, and a large Chinese telescope, LAMOST, has been given the name Guo Shoujing Telescope.

Timeline

• 723: A Tang astronomer sets up a stone shadow marker at Gaocheng honouring the Duke of Zhou.
• 1271: Kublai Khan proclaims the Yuan dynasty.
• 1276: Kublai orders a reform of the calendar; work begins on the Gaocheng tower around this time.
• 1279: The Yuan conquest of the Southern Song is completed.
• 1281: The Shoushi calendar comes into use.
• 1644: The Ming dynasty falls; the Shoushi-based Datong calendar is replaced.
• 2010: Gaocheng becomes part of a UNESCO World Heritage Site.

Why it matters

The Gaocheng Observatory is a rare surviving monument to medieval Chinese science. Its simple principle, a very large shadow stick read with great care, produced a calendar as accurate as anything Europe achieved before the late 16th century.