Ancient Eclipses and the Length of the

. . . time that takes survey of all the world . . .

William Shakespeare, Henry IV Part I

It may seem surprising, in the Space Age with cosmic phenomena being studied using hugely sophisticated instrumentation, that relatively crude ancient eclipse records are invaluable, even irreplaceable, to modern science.

Let me give an example of the value of such eclipse records. Imagine you suspect the day is getting longer, because the rate of spin of the Earth is very gradually slowing. You can measure that spin rate directly in the short term using a host of high technology equipment: vast arrays of radio telescopes following the motion of extra-galactic objects across the sky, laser beams reflected from orbiting satellites, phenomenally precise clocks employing beams of cesium atoms or hydrogen masers. Data collected using such techniques indicate that the duration of a day in 2001 was about 0.17 milliseconds longer than it was back in 1991. It's a small change, but a decade is only a short interval, historically speaking.

Alternatively, one can investigate how the day length has changed not just over the past decade, but also over 200 or 300

decades. How is this possible? In the first millennium B.C. the Egyptians, Babylonians, and Chinese did not have atomic clocks. In fact they had no artificial clocks at all, apart from simple devices measuring water flow, which were hardly very precise. But they did have natural clocks provided by the Sun and the Moon in the sky.

Suppose that a total solar eclipse was observed and recorded from Athens in 500 B.C., on the local calendar scheme in use in that era. Such eclipses are so infrequent that we can identify the event using our knowledge of the apparent orbits of Sun and Moon about the Earth. The bare observation of the total eclipse tells you that on that date the Sun, Moon, and Athens were aligned (to within a tolerance equal to the width of the eclipse track, which is equivalent to a few minutes of time). This then tells you the local solar time for Athens in that era: that is, when the Sun rose, when it crossed the meridian, when it set.

Since 500 B.C. the day has continually been getting longer. Over a single century the day increases by about 1.7 milliseconds (although there are reasons to believe that this deceleration is variable). This tiny amount summed over 2,500 years gives a total shift amounting to about four hours, equivalent to one-sixth of a rotation of the planet. So, if the day length had stayed the same, the eclipse track would have been out over the Atlantic and would have escaped detection by the Greeks.

The mere recording of an eclipse from Athens so long ago would provide rather accurate information about how our rotation rate has slowed, without the Greeks having made any sophisticated scientific measurements. Actually, no such eclipse occurred at that place and time (I made it up as a thought experiment). However, there are records of a similar nature written down by disparate civilizations over the last three millennia. Despite the fact that the ancients had no lasers, artificial satellites, radio telescopes, or cesium clocks, their accounts of eclipses have made it possible to build up a consistent picture of how the length of the day has changed.

Not only is the day getting longer (making necessary the insertion of leap seconds), but so, too, is the month, because the Moon is slowly receding from the Earth. In this chapter we consider the implications of these trends.

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