The celestial dance between the sun, the earth and the moon is predictable enough that astronomers know when the three will be aligned.
A solar eclipse happens when the moon casts a full or partial shadow on earth. The animation below shows how the shadow happens.
Although the sun is about 400 times larger than the moon, the distance in between them affects perspective and the sun nearly disappears when its alignment with the moon is viewed from earth.
During the Aug. 21 solar eclipse, the moon's shadow will glide over the earth.
For the first time in 99 years total solar eclipse observers in the U.S. from coast to coast will also get to see the sun's corona, the aura of plasma that surrounds the sun.
There will be changes on the solar radiation that reaches earth. The graphic below shows the U.S. exposure to sun's rays during the eclipse. The cooler the shade, the less the amount the solar radiation that will reach the area.
Viewers at the center of the moon's shadow from Oregon to South Carolina will see the day turn into night for about 2 minutes and 40 seconds and the ring of the sun will shine around the moon.
Observers in South Florida will see the moon cover only about 77 percent of the sun. The shadow of the moon will move from the coast of South Carolina and into the Atlantic Ocean.
The irregular topography of both the earth and the moon cause the shadow to change shape. The shadow is known as the lunar umbra during a total solar eclipse, and as the lunar penumbra during a partial solar eclipse.
Experts from The American Astronomical Society believe that the cosmic coincidence is temporary, as the moon continues to move away from earth about 1.5 inches per year.