Millennia ago, eclipses provided rare opportunities to measure the universe

Tyler Nordgren writes: As a kid visiting the Oregon coast I often wondered, “How wide is the ocean, and what is there beyond the horizon?” As I grew older and turned my sights to the night sky, I wondered something very similar: “How far away are the stars, and are there other planets there?” Even though very few of us have ever circumnavigated the globe, and no human being has ever ventured into space beyond the moon, we do know some of the answers to these questions. Immensity isn’t immeasurable. While these vast numbers may make little sense in our daily lives, we at least know they are known.

Consider what it must have been like to live in a world where this was not true: where the sense of immeasurability, the certainty of the unfathomable, was commonplace, and the thought that the world could be known was a novel idea. The philosopher Anaxagoras was born in about 500 B.C. in the eastern Mediterranean on what is now the coast of Turkey. It was a time when philosophy had only recently turned its attention to the natural world. Less than a hundred years before, Thales of Miletus supposedly predicted the solar eclipse that ended a war, thus implying that our world was predictable and events were not just the random whims of the gods.

In such a world of physical phenomena, Anaxagoras was the first, as far as we know, to understand that eclipses occur when one heavenly body blocks the light from another. This rejection of gods and dragons as the causes of eclipses was a revolutionary thought by itself, but Anaxagoras took it further: If solar eclipses happened only because the Earth had moved into the shadow of the moon, he reasoned, then the size of the shadow must tell us something about the size of the moon. Additionally, since the moon covered the sun, the sun must be farther away. Yet to appear nearly the same size, the sun must be larger than the moon. Herein lies the power of scientific thought: Measure the extent of the shadow sweeping across the Earth, and you know the moon must be at least as big as the shadow, and the sun larger still. Mysticism provided no such opportunity: If eclipses occur when a demon devours the sun, there is no reason to believe that any measurement we make here on Earth should reveal the demon’s size.[Continue reading…]

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