Bringing the Universe to America's Classrooms
Why Isn’t There an Eclipse Every Month?

The Parts of the Moon's Shadow

Whether you see a partial or total solar eclipse depends on which part of the Moon’s shadow you are in.

An umbra is the inner, darker part of a shadow. The Moon's umbra is cone shaped and becomes smaller as it gets farther from the Moon. If you were within the Moon’s umbra, you would not see any part of the Sun—the Sun would be completely blocked by the Moon, and you would experience a total solar eclipse.
An illustration shows the relative locations of the Sun, the Moon, and Earth along with the cones of the Moon's umbral and penumbral shadows.

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This illustration is titled Total Solar Eclipse, Monday, August 21, 2017. This was the first total solar eclipse visible in the continental United States in 38 years. The Sun is located on the left edge of the image. Earth is near the right edge of the image, and the Moon is just to the left of Earth. The Moon’s umbra is shown as a dark-gray, cone-shaped shadow that gets smaller as it extends from the Moon toward Earth, with its point falling on a small part of the United States. It is labeled as “total eclipse”. The Moon’s penumbra is shown as a light-gray shadow that gets larger as it extends from the Moon toward Earth. At Earth’s surface, the lighter shadow spans North America. The penumbra is labeled as “partial eclipse.” A caption notes that the image is not drawn to scale. If drawn to scale, the Moon would be 30 Earth diameters away from Earth. The Sun would be 400 times that distance.

A penumbra is the outer, lighter part of a shadow. The Moon’s penumbra becomes larger as it gets farther from the Moon. If you were within the penumbra, you would see a partial eclipse of the Sun—the Moon would only block a portion of the Sun.

Take Notes

Why did people who wanted to see the total solar eclipse of August 21, 2017, have to travel to particular areas of the United States? Use information from the illustration to support your answer.

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