Eclipse Made ‘Day Dawn Twice’ in China : NASA Scientist Solves Ancient Riddle

A NASA astronomer solved a Chinese riddle about the time “the day dawned twice” because of a solar eclipse, placing a date on a king’s reign and learning how much faster the Earth turned in 899 BC.

The discovery that a day on Earth was 43-thousandths of a second shorter in 899 BC than it is today “doesn’t change the price of tea in China, but it’s something of scientific interest,” said Kevin D. Pang, who presented his study at the annual meeting of the American Astronomical Society.

The finding is based on a study of ancient Chinese chronicles by Pang of the National Aeronautics and Space Administration’s Jet Propulsion Laboratory; physicist Kevin C. Yau of the University of Durham, England, and Hung-hsiang Chou of the East Asian languages and cultures department at UCLA.

The chronicles, called the Bamboo Annals because they were written on bamboo strips, recorded “all important events from the earliest in China down to about 299 BC,” Pang said. “It was probably passed down through the ages and was constantly updated by the (royal) court chroniclers.”

One passage in the annals says, “In the spring of the first year of the reign of King I of the Western Zhou dynasty, the day dawned twice at a place called Zheng,” which is near the present-day Hua District in Shaanxi Province.

Pang and his colleagues performed a computer simulation of the history of the rotation of the Earth around the sun and of the moon around the Earth, determining that the annals must have referred to an eclipse of the sun by the moon shortly after dawn on April 21, 899 BC.

“We solved the riddle of an ancient text, which is of historical interest because it puts an exact date (year) on the beginning of the king’s reign, which wasn’t known until now,” Pang said.

By calculating the positions of the sun, Earth and moon during the eclipse, Pang also was able to determine that the Earth revolved 43 milliseconds faster in 899 BC than it does now, meaning that days were that much shorter then.

“It’s generally known that the Earth has been spinning ever slower from at least 4 billion years ago,” Pang said, adding that before his study, the exact rotation rate was known only as far back as 700 BC.

The day dawned twice on April 21, 899 BC, because the real dawn was interrupted by the solar eclipse just before the sun rose.

The Bamboo Annals ended in 299 BC, when the bamboo strips were buried with King Hsiang of the Wei Kingdom. Pang said that saved them from destruction by a later ruler, Chin Shi Huang Ti, who helped build China’s Great Wall and army of terra cotta figures, and burned all the books he could find.

Grave robbers opened Hsiang’s tomb in AD 281, stealing jewels and other valuables but leaving the annals behind for study by “the most eminent scholars of the Pu Dynasty” and subsequent researchers, Pang said.