What can you learn from watching a lunar eclipse?

The Oct. 8 lunar eclipse has stargazers giddy with anticipation, because it promises a seemingly impossible view of both the eclipsed moon and the rising sun at the same time. But since the days of the ancient Greeks, astronomers have revered lunar eclipses of all kinds as keys to some of the mysteries of the solar system.

A lunar eclipse occurs when the sun, the Earth, and the moon fall into alignment so that the Earth’s shadow obscures the moon. The phenomenon is one of the solar system’s many tricks of light and shadow that have fascinated people for millenniums. Some ancients regarded the disappearance of the moon with fear and trepidation; others saw it as a cause for ritualistic celebration. Classical Greek philosophers, as was their custom, took a more analytical approach.

Proof that the Earth is round

Contrary to popular belief, Christopher Columbus was not the first to suggest that the Earth was round. One of the first recorded arguments for a spherical earth comes from Aristotle – nearly 2,000 years before Columbus – says University of Puget Sound science historian James Evans.

“He mentions in his book ‘On the Heavens,’ that during an eclipse of the moon, you see the shadow of the Earth falling on the moon and it always looks circular in shape,” Professor Evans explains. “He offered other arguments, but that’s a pretty good argument right there.”

Like many of the ancient Greeks’ models, Aristotle’s theory has held up only in part. Today, we know that that Earth is not a perfect sphere but is an oblate spheroid, with a heavier concentration of mass around the equator than at the poles, as proposed by Isaac Newton in the late 17th century.

A key to the day and night sky

Ancient Greek astronomers relied on the lunar eclipse to compare the nighttime and daytime sky.

“One of the ways that the ancients used to measure the longitudes of the stars would be to measure how far a star is from the middle of the moon in degrees during the middle of the eclipse of the moon,” Evans says.

While the position of the sun is not visible at night, it can be easily determined during an eclipse when the moon is diametrically opposite the sun, Evans explains.

It was likely through this method that Hipparchus of Nicea discovered a phenomenon known as precession, the top-like wobble of the Earth as it spins on its axis. By studying historic lunar eclipses, Hipparchus realized that the constellations shifted their position in the night sky over time in 26,000-year cycle.

Still operating under the assumption that the Earth lay at the center of the universe, Hipparchus was not fully able to explain why this must be the case, but his observation became a vital building block for Isaac Newton’s later explanation that the Earth’s oblate shape caused the planet to precess.

Clues for future generations

Today, scientists are still using ancient eclipse data to unlock scientific mysteries.

The Earth takes 24 hours to complete a single rotation on its axis. However, in the late 19th and early 20th centuries, astronomical evidence began to suggest that the rotation of the Earth is slowing very slightly. Beginning in 1972, universal clocks began to periodically insert a so-called leap-second to account for this slowing. Modern astronomers and geologists studying this phenomenon use ancient records of eclipses in efforts to determine the rate at which the Earth’s rotational speed is changing, says Evans.

Ancient eclipse data is also proving useful in dating the Antikythera Mechanism, a clocklike machine discovered among the remains of an ancient shipwreck off the coast of a Greek island in 1901.

Historians have been able to estimate that the wreck likely dates back to the late 3rd century BC based on the art and coins recovered from the ruins and through radiocarbon dating of the ship’s timbers. But it is possible that the mechanism could have already been quite old by the time it was placed on the ship so some historians are trying to use eclipses recorded on the mechanism as an alternate means of dating.