Eclipse Records and the Earths Rotation

Modern physics allows us to calculate, with extraordinary accuracy, the orbits of the earth about the sun and the moon about the earth. This enables scientists to predict the characteristics of solar (and lunar) eclipses for many centuries into the future, and in particular, to determine "paths of totality." These are narrow tracks across the surface of the planet defining those places where that rare and spectacular event—a total eclipse of the sun— will occur.

It is also possible to operate backwards in time and "predict" the occurrence of total solar eclipses in the past. Unfortunately, there are innate errors in this process that increase the further back (or forward) one tries to go. A significant effect is tidal friction, which has the effect of gradually slowing the rate of rotation of the earth. To illustrate the point, suppose that we made the assumption that the earth has rotated at the exactly same rate for the past two millennia. In fact it has slowed down slightly, and during that time it has actually rotated by one eighth of a turn more than we thought. The result will be that our eclipse track "predictions" for an eclipse occurring in the year 0 would be correct in latitude but some 45 degrees off in longitude, since the moon's shadow would actually have fallen further round the earth's surface than we expected.

It was first realized in the 1970s that this argument could be turned around. Eclipses were recorded by the ancient Chinese, Babylonians, Greeks, and Romans, as well as by astronomers in medieval Europe. As a result, there exist many scores of records of total solar eclipses (and other sun-moon-earth related astronomical events) dating between about 700 b.c.e. and C.E. 1600 with sufficient information on the timing of their occurrence to enable us to identify them with "predicted" eclipses. By comparing, through time, the difference in longitude between the "predictions" and the actual observations, it is possible to construct a remarkably accurate estimate of the rate of deceleration of the earth's rotation. Even so the modeling is not straightforward, and the main progress was made in the 1980s by the British historian of astronomy Richard Stephenson working together with astronomer Leslie Morrison.

This type of study uses archaeology and history—in the form of ancient eclipse records—to inform modern physics and astronomy. It has been named applied historical astronomy to distinguish it from archaeoastron-omy, which does precisely the opposite. Archaeoastronomy uses modern astronomy to reconstruct the appearance of ancient skies with the ultimate aim of learning something about people's beliefs and practices in the past. See also:

Archaeoastronomy; Lunar Eclipses; Solar Eclipses.

Babylonian Astronomy and Astrology; Chinese Astronomy; Swedish Rock Art. Years B.C.E. and Years before 0.

References and further reading

Espenak, Fred. Historical Values of Delta T.

http://sunearth.gsfc.nasa.gov/eclipse/SEhelp/deltaT2.html. Morrison, Leslie, and Richard Stephenson. "The Sands of Time and the Earth's Rotation." Astronomy and Geophysics 39 (1998), 5:8-5:13. Stephenson, Richard. Historical Eclipses and Earth's Rotation. Cambridge: Cambridge University Press, 1997.

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