It's a remarkable coincidence, but the Sun, which has a diameter of 400 times greater than our Moon, is almost exactly 400 times farther away. That means, when the alignment is right and the Moon passes directly between the Earth and Sun, the Moon almost exactly covers the visible part of the Sun's surface for a few minutes and leaves the eerie glow of the Sun's outer corona visible to Earthly observers over a long but very narrow path across the Earth's surface. This is a total solar eclipse.

Total solar eclipses are a common event somewhere on Earth, and they occur twice a year, on average. However, a total solar eclipse is rare at any particular place on Earth. It takes many hundreds of years - on average - for such an event to revisit a particular location. Nor does the Moon's shadow necessarily cast itself in convenient locations during a total solar eclipse. These events can occur in populated areas, but are more likely to occur in oceans, arctic regions, or in the middle of a desert simply because these regions cover more of the Earth than densely populated urban regions.

During a total solar eclipse, as the Moon passes between the Sun and Earth, it casts two types of shadows. The umbra is the narrow dark shadow cast by the Moon, while the penumbra is a fainter outer shadow. During an eclipse, both shadows travel along the surface of the Earth at more than 1,000 miles per hour as the Moon moves along in its orbit, and these shadows move west-to-east across thousands of miles of the Earth's surface during the eclipse. The path of the umbra is only about 70 miles wide, roughly, and within its path an observer sees the full total solar eclipse during which the Moon blocks the bright face of the Sun. This is the path of totality. You MUST be in this path to see the total eclipse. Along the centerline of this path, you will see between two and seven minutes of totality, depending on the Earth-Moon-Sun alignment during the eclipse. Off the centerline, but still within the narrow path of the umbra, the duration of totality decreases.

The penumbra is much wider than the umbra and spans many thousands of miles on either side of the path of totality. Within this shadow, an observer sees a partial solar eclipse in which the Sun's face is only partially covered by the Moon. More of the Sun's face appears covered for observers closer to the path of the umbra.

Not all solar eclipses are total. If the Earth, Moon, and Sun are not perfectly aligned, but they are still aligned to within their angular diameters, the Moon does not completely cover the face of the Sun and does not cast an umbra onto the Earth but only a penumbra. Observers along the path of the penumbra will see a partial solar eclipse, but no one will see a total eclipse. And if the Moon passes directly between the Earth and Sun when it's at near its most distant point in its monthly orbit around Earth, where its apparent diameter is slightly too small to completely cover the Sun, a thin ring of the Sun's light is still visible around the Moon's dark disk. This is an annular solar eclipse.

A total solar eclipse is by far the most spectacular of the three types, and the eclipse of August 21, 2017 will be of this type. Figure 2 shows the outline of the narrow path of the umbra and much wider path of the penumbra of this event.