In August, the United States will experience its first coast-to-coast total solar eclipse in nearly a century. Over the course of an hour and a half, the moon’s narrow shadow will slice across 12 states, from Oregon to South Carolina (SN: 8/20/16, p. 14). As many as 200 million people are expected to travel to spots where they can view the spectacle, in what could become one of the most watched eclipses in history. Excitement is building, hence the flurry of new books about the science, history and cultural significance of what is arguably one of Earth’s most awesome celestial phenomena.
Total solar eclipses happen when the moon passes in front of the sun as seen from Earth, and the moon blocks the entire face of the sun. This event also blocks sunlight that would otherwise scatter off the molecules in our atmosphere, reducing a source of glare and so allowing an unfettered view of the sun’s outer atmosphere, or corona. Total solar eclipses arise from a fluke of geometry that occurs nowhere else in the solar system, astronomer Anthony Aveni explains in In the Shadow of the Moon. Only Earth has a moon that appears, from the planet’s viewpoint, to fit so neatly over the sun — a consequence of the fact that the sun is a whopping 400 times as large as the moon but also 400 times farther away. Moons orbiting other planets are either too small to fully cover the sun’s face or are so large that they fully block any view of the corona.
In fact, the fluke of geometry is also a fluke of history: Because the moon’s orbit drifts about four centimeters farther from Earth each year, there will come a time when the moon will no longer appear to cover the sun, notes planetary scientist John Dvorak in Mask of the Sun. We already get a preview of that distant day: When the moon…
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