What is a Total Eclipse?

On Monday, April 8, a total solar eclipse will cross North America, passing over Mexico, the United States, and Canada.

A total solar eclipse happens when the moon passes between the sun and earth, completely blocking the face of the Sun.

According to NASA, “The sky will darken as if it were dawn or dusk.”

For this celestial event, the longest period of darkness will occur for about two minutes and 40 seconds. The last time the U.S. saw a total eclipse was in 1979.

Exactly how the sun, moon, and earth align determines what kind of eclipse we see.

With a Total Eclipse, the moon moves directly in front of the sun and barely covers the solar disk. This allows observers in the center of the moon’s shadow to briefly see the sun’s outer atmosphere, the corona, which is too dim to see when the bright solar disk is not covered.

The shadow that the moon casts has two main parts, which are the darker inner shadow called the umbra and a fainter outer shadow called the penumbra. Within the umbra, the sun’s light is completely blocked. In the penumbra, the sun’s light is only partially blocked.

To see a total solar eclipse, an observer must be within the umbra. Observers in the penumbra will witness a partial eclipse, with only part of the sun covered by the moon. Those outside the moon’s shadow will see no eclipse at all.

As the shadow extends away from the moon, the umbra narrows or gets smaller. By the time it reaches earth, the umbra covers a relatively small area on our planet compared to the penumbra. That means the area that experiences a total eclipse is much smaller than the area that experiences a partial eclipse. A typical umbral path may be less than 50 miles wide, while the penumbral zone can span a thousand miles or more.

Because earth is continuously rotating, and the moon is constantly moving in its orbit, the moon’s shadow will travel across earth’s surface, tracing a path. This is called the eclipse path. Within this area is a smaller path traced by the umbra, called the path of totality. The rapid movement of the moon in its orbit causes the shadow of the moon to sweep across the face of earth in just four to five hours. At any location along the path of totality, the total phase of a solar eclipse may last only a few minutes, or even less.

This will be the last total solar eclipse visible from the United States until August of 2044.

For further information, visit science.nasa.gov.