What Is a Solar Eclipse?
Credit: Jay Pasachoff, Ron Dantowitz, Christian Lockwood, and the Williams College Eclipse Expedition/NSF/National Geographic
When the moon passes in front of the Sun and blocks it completely, it casts a shadow onto our planet turning daytime into night. What causes this extraordinary phenomenon?
A solar eclipse occurs when the moon passes between the Sun and Earth, casting the moon's shadow on Earth. This only happens occasionally, because the moon doesn't orbit in the exact same plane as the Sun and Earth do. Two things need to occur at once for you to see a solar eclipse: the Sun, moon, and Earth need to be in a straight line, with the moon between the Sun and Earth; and you need to be standing in the correct spot on Earth.
Lucky for us, our moon is the perfect size and is located at the perfect distance. When it passes in front of the Sun, it covers the Sun’s disk. During an annular eclipse, the moon is a bit closer to the Sun, leaving the edge of the Sun's disk uncovered. The result looks like a bright ring of fire in the sky. During a total eclipse, the Sun’s disk is completely covered, allowing the outermost layer of its atmosphere, the corona, to shine out around the moon in a spectacular display.
The Sun and moon aren’t really the same size, of course: the Sun is 400 times the diameter of the moon. But it’s also 400 times farther away from us, and this relationship between size and distance makes the Sun and the moon appear the same size in the sky. It’s a wonderful coincidence, and should not be taken for granted—Earth is the only planet in our solar system with a moon the proper size and distance to cause striking solar eclipses.
The motions of Sun, moon, and Earth bring the three bodies into alignment two to four times a year. But a perfect alignment, resulting in a total solar eclipse, only occurs about once every 18 months. During the day, as the moon passes in front of the Sun, it begins to cast a partial shadow (called the penumbra) onto Earth. At the height of the eclipse, the Sun’s light is entirely blocked, and the moon casts a full shadow called the umbra.
This diagram (which is wildly out of scale) shows a side view of the Sun-moon-Earth alignment. Only from within the tiny area where the dark umbra touches Earth will you see the Sun completely covered and witness an annular or total eclipse. From anywhere in the larger, lighter penumbra, you will see a partial eclipse.
The moon’s darkest shadow hits only a small part of our planet, however. The perfect alignment of the Sun and Moon is visible from a limited region on Earth. The area from where you can see an annular or total eclipse is never more than 167 miles wide, and is usually less. Outside that area you’ll see only a partial eclipse, or none at all.
The area on Earth where a solar eclipse is visible is not static, because the moon and Earth are not fixed objects: the moon orbits Earth and Earth orbits the Sun, in addition to spinning on its axis. These motions continue during an eclipse, so the spot on Earth where the moon’s shadow falls traces a curved path across the planet. This is the path of totality, the only vantage from which you can witness a full eclipse.
Although total solar eclipses happen regularly, the path of totality is extremely limited. If you stood in one place and waited to see a total eclipse, you could wait for 300 years or more.