Analyzing ancient Chinese accounts of solar eclipses up to nearly 4,000 years old, Jet Propulsion Laboratory researchers have determined to within few thousandths of second the rate at which the Earth's daily rotation has been gradually slowing down.
Their results -- from examining Chinese reports of eclipses in the years 532 A.D., 899 B.C. and 1876 B.C. -- closely match previous studies.
They showed that the span of each day was shorter by about 22/1,000ths of second in 532 A.D., 42/1,000th second in 899 B.C. and 70/1,000th second in 1876 B.C.
The study, due for publication in the British journal Vistas in Astronomy, was authored by JPL astronomer Kevin D. Pang, Kevin Yau of the University of Durham, England, Hung-hsiang Chou of UCLA and Robert Wolff, former JPL staff member now with Apple Computer Inc.
Previously, other astronomers had analyzed accounts of solar eclipses from Arabian and Babylonian sources for historical evidence of the gradual slowing of the Earth's rotation.
The oldest of those Middle Eastern solar eclipse reports analyzed dated to about 700 B.C. Key gaps in the Arabian and Babylonian records occurred during the Middle Ages.
Dr. Pang and his colleagues turned to formidable 1.2-million-word collection of ancient Chinese astronomical records currently being prepared for publication by the Beijing Observatory.
The Chinese text contains records of thousands of eclipses extending back almost unbroken to the 19th century B.C.
In order for an eclipse record to be useful, Pang said, the time of the eclipse must be accurately known.
Since ancient accounts generally do not specificy time of day, Pang explained that he and his colleagues limited themselves to eclipses which occurred at sunrise or sunset. The time of sunrise or sunset can be computed retroactively.
One of the eclipses they focused on took place on November 13 of the year 532 A.D., when China was divided into separate northern and southern kingdoms. From historical records it appears that sunrise took place during solar eclipse as observed from Loyang, the capital of the then northern Wei kingdom.
A second eclipse studied by the team occurred on April 21 of 899 B.C., when historical accounts relate that the day "dawned twice" at the since-vanished city of Zheng. very similar "double sunrise" will occur over Helsinki, Finland, on July 22, 1990.
Finally, the researchers were able to extend their analysis back in time another 1,000 years -- far more ancient than any other eclipse study -- by examining records of solar eclipse which occurred on October 16 of 1876 B.C., according to another report published recently by Pang in the Journal of Hydrology.
Continued analysis of such ancient eclipse records is expected to refine our understanding of the history of rotation of the Earth, Pang said.
The JPL research is funded by NASA's Office of Space Science and Applications.