Historical accounts and fictional authors invoke the mystery of a total solar eclipse. Eclipse watchers have called it life-changing, surprisingly beautiful, even mystical. After seeing their first eclipse, many dedicate their free time to chasing eclipses around the globe.
Yet to an astronomer, an eclipse is the consequence of simple geometry. The moon lines up between the Earth and the sun, such that its shadow falls on the Earth; due to a coincidence of relative size and location, the apparent size of the moon (in truth about 400 times smaller and 400 times closer than the sun) precisely matches the apparent size of the sun in the sky, so it can block sunlight completely for those in the path of the shadow across Earth.
That brings us to the Eclipse of the Century on August 21, 2017. Along with hundreds of thousands of other Americans, I have traveled to see this eclipse at 100% totality, visible along a strip that is roughly 70 miles wide, from Oregon to South Carolina.
I'm in Sun Valley, Idaho, where hundreds of astronomers are attending the 16th meeting of the High Energy Astrophysics Division of the American Astronomical Society. Here, the period of totality will last more than a minute. (The entire eclipse, from when the moon starts crossing the solar disk to when it clears it, will last nearly three hours.) Those willing to drive 60 miles north in eclipse-heavy traffic will see more than two minutes of totality.
We are all eager to see the day become night, stars and planets briefly shine in a dark sky, the air cool and winds shift, and animals react to the fleeting end of daylight.
The sun supplies vital energy to heat the planet, grow crops and illuminate our lives. In ancient times, before Greek astronomers figured out that simple geometry caused the effect, the sudden blotting out of the life-enabling sun must have been terrifying.
It was also completely natural to link it to some earlier event: an argument, a political change, any kind of coloring outside the lines, anything that might make humans feel guilty. "Surely," our ancestors must have thought, "it is because we did [this thing] that the gods are now punishing us." It's human nature to interpolate cause and effect even when two events are unrelated.
Alas, no mysticism attaches to a solar eclipse. It's simple: The moon orbits the Earth, and every so often, for viewers somewhere on Earth, it lines up precisely with the sun, blotting out all but the outermost bits of solar light. Because the orbits are understood, future eclipses can be predicted with high precision.
The weirdness -- the coincidence that is a true puzzle at the moment -- is that the size of the moon and sun are, to an Earth observer, more or less the same. Specifically, the ratio between the radius of the moon (1,080 mi) and its distance from Earth (238,856 mi) -- what astronomers call its angular size -- is about one quarter of a degree (360 degrees constituting a full circle around the sky). The angular size of the sun -- i.e., its radius (432,687 mi) divided by its distance (92,955,559 mi) -- is more or less the same (0.26 deg for the moon, 0.27 deg for the sun).
These angular sizes depend on the size of the moon, the size of the sun, the moon-Earth distance and the sun-Earth distance. Since the Earth, moon and sun formed at different times in different ways, it seems strange that they ended up with such a similar geometric ratio. If you want to ponder a mystery, here it is.
In fact, everything seems mysterious before we understand it. Eclipses were once perceived as a deus ex machina; now we know they bear no relation to terrestrial activities.
Other natural phenomena are the same. Before Newton formed his theory of gravity, the orbits of planets around the sun were mysterious. (Indeed, many thought the sun and other planets somehow circled the Earth.) Now the law of gravity explains planets around other stars, as well as the collection of stars into galaxies and the motions of galaxies across the universe. What was once a foggy mystery is explained by science.
That means things that still appear to be mysteries -- like the coincident size of the moon and sun, or the precise masses of elementary particles, or the Big Bang origin of the universe -- may one day be explained very simply by scientists. (Indeed, there are already theories for the above.)
I'm planning to enjoy my first total solar eclipse. (My only other eclipse experience was in 1970, at about 90% of totality, as seen from my home near Boston.) I think I'll enjoy the weirdness of it, along with everyone else. But at the back of my mind, I'll be thinking about how wonderfully science explains nature.