Explaining Eclipses
Who first explained eclipses correctly?
Today we get excited about eclipses as marvelous lightshows in the heavens. It was not always so. Eclipses, and especially total solar eclipses, once were seen as harbingers of great catastrophes. Who first figured out what they were and how they came about?
We owe our present-day understanding to two early Greek philosophers: Parmenides and Anaxagoras.
Parmenides wrote a philosophical poem about 490 BC. In it he criticized the cosmological theories of his predecessors, and he offered a new theory of his own. He also offered two important insights into astronomy. First, he noted that the shiny part of the moon in all its phases is always facing the sun. Second, he inferred from this that the moon shines with borrowed light. In other words, the moon is a dark body that reflects sunlight (Parmenides fragments 14 and 15).
Anaxagoras, using Parmenides’ insights, figured out that the moon was an earthy body, since it is opaque, and spherical in shape, to account for its phases. Furthermore, when the moon is invisible (about three days each month the moon disappears from view at the time of the “new moon”), it is lurking in the vicinity of the sun. The moon disappears because the moon is, from the perspective of earth, totally in shadow. It must then be closer to the earth than the sun and at an angle near that of the sun, so that the backside of the moon (from our perspective) is illuminated, but the frontside is dark.
If the moon should happen to cross in front of the sun … it would block out the sun. Voilà, a solar eclipse.
When the moon is full, on the other hand, it is on the opposite side of the sky from the sun, so that the full moon is just rising as the sun is setting, and the full moon is just setting as the sun is rising. In that position, the earth lies between the sun and the moon. If, then, the earth should line up perfectly with the sun and moon, the earth would block the sun’s light to the moon and the moon would fall into the earth’s shadow. That would be a lunar eclipse.
Anaxagoras seems to have tried out his theory on a solar eclipse that was visible in Athens in 478 BC, when Anaxagoras was just 22 years old.
Before Anaxagoras, Greek philosophers had offered a number of ingenious theories about how eclipses occurred. After him, all the other theories disappeared and everyone accepted Anaxagoras’ theory.
A century and a half later, Aristotle cited the theories of lunar light and eclipses as paradigms of scientific explanation.
Is philosophy useless? Some people, including scientists, think so. But philosophers effectively put astronomy on the path of a science. Aristotle himself offered the first extant proofs that the earth is spherical around 350 BC. A century and a half later, Eratosthenes offered an accurate estimate of the earth’s circumference, using geometry and simple geography.
The Babylonians, the Egyptians, and the Chinese were all better observers and recorders of heavenly events than the Greeks. But it was the bold theorizing of the Greeks which (after a long string of explanatory failures) finally got eclipses, and the elements of astronomy, right. And with that insight they turned stargazing into a science.
For a recent discussion of the value, or lack thereof, of philosophy, see Paul Austin Murphy.
On the origins of scientific astronomy, see D. W. Graham.
