Solar Eclipse 101
Humans have recorded solar eclipses for millennia, and references to them can be found in some of humanity’s earliest texts, such as ancient Chinese academic documents. Debate even swirls around a line from Homer's Odyssey—"The sun has been obliterated from the sky"—and whether it can be tied to a historic eclipse.
Through the ages, the sudden darkening of the sun has been seen as a signal of the displeasure of the gods or an omen of bad things to come. But once astronomers figured out how solar eclipses worked, they became events to be studied and celebrated.
Solar eclipses happen only during a new moon, when the lunar orb moves between Earth and the sun. However, because the moon orbits Earth at a slight angle, the three bodies will only periodically line up on the same plane to create a solar eclipse.
During a total solar eclipse, the moon is in just the right position to briefly cast a shadow on our planet. As the moon moves and Earth spins, this shadow races across the planet's surface at some 1,400 miles an hour, creating a relatively narrow line called the path of totality. Only sky watchers within this line—typically 10,000 miles long and just 100 miles wide—experience a total solar eclipse. People near it see only a partial solar eclipse, in which not all of the sun is blotted out, while those farther away see no change at all.
Seeing a solar eclipse can be an unforgettable experience, but experts urge caution. Looking directly at the sun with your naked eyes, or through unprotected telescopes or binoculars, can cause eye damage and even permanent blindness. Special eclipse glasses that filter out the sun's most damaging rays can allow you to view the phenomenon safely, as can instruments fitted with solar filters, or simple devices called pinhole cameras that let you see a solar eclipse indirectly.
Following the path of totality
A total solar eclipse begins as a barely perceptible nibble out of the sun. Over the next hour or so, the darkness spreads and eventually consumes the solar disk, turning day to night. This state, called totality, can last as long as seven and a half minutes, although it is usually less than that. During totality, some stars and planets become visible, the air temperature drops, and animals from insects to cows may change their behavior. This brief phase is the only time it's safe to look at an eclipse with your naked eyes.
The only visible part of the sun during totality is its corona, the faint and normally unseen outer atmosphere that shimmers in the darkness like a fiery ring. Astronomers can use solar eclipses to study the corona from the ground, gathering clues to its behavior that may one day help solve the mystery of why the sun's upper atmosphere is actually hotter than its surface.
In 1919 Sir Arthur Eddington's expedition to watch a total solar eclipse from the island of Príncipe, off the west coast of Africa, helped confirm Einstein's general theory of relativity. According to Einstein's work, gravity from massive objects should warp the fabric of spacetime and so bend light.
Eddington realized that a total solar eclipse would provide the perfect test bed for this prediction, since light coming from much more distant stars should get bent ever so slightly as it passes by the sun, and the eclipse would allow scientists to see stars close enough to the sun's edge to detect this minuscule change. The experiment was a success, and news of the result helped launch Einstein into scientific stardom.
A rare event
Total solar eclipses are only visible on Earth because of a lucky coincidence: The moon's diameter and distance from Earth make its relative size just big enough to cover the sun's disk. If the moon were any smaller or farther away, we would see only partial eclipses. In fact, measurements of the distance between Earth and the moon show that our cosmic companion is slowly spiraling away from us, and in a billion years or so, the moon will have drifted so far from Earth that total solar eclipses will no longer occur.
In addition, the moon's orbit around Earth is not a perfect circle, and its precise distance changes with each orbital cycle. Even today, sometimes the moon's apparent size is too small to fully cover the sun during an eclipse. When this happens, we see what's called an annular eclipse, also dubbed a ring of fire. In this case, the moon appears as a blackened circle surrounded by sunlight. Even during its total phase, it's not safe to view this kind of eclipse without proper eye protection.
Total solar eclipses occur every one or two years, on average, and partial and annular eclipses are only slightly more frequent. Because solar eclipses are visible from such a small area on Earth each time, the chance of observing one from any single spot is less than once in a lifetime.