A solar eclipse occurs when the moon passes between Earth and the sun and all three bodies are aligned in the same plane. When the moon completely blocks Earth’s view of the sun and the umbra, or dark part of the moon’s shadow, reaches Earth, a total eclipse occurs. A total eclipse happens only along a narrow path 100 to 200 miles (160 to 320 kilometers) wide called the track of totality. Just before totality, the only parts of the sun that are visible are a few points of light called Baily’s beads shining through valleys on the moon’s surface. Sometimes, a last bright flash of sunlight is seen—the diamond ring effect. During totality, which averages 2.5 minutes but may last up to 7.5 minutes, the sky is dark and stars and other planets are easily seen. The corona, the sun’s outer atmosphere, is also visible.

If the moon does not appear large enough in the sky to completely cover the sun, it appears silhouetted against the sun with a ring of sunlight showing around it. This is an annular eclipse. Because the sun is not completely covered, its corona cannot be seen, and although the sky may darken it will not be dark enough to see the stars.

During a partial eclipse of the sun, the penumbra of the moon’s shadow strikes Earth. A partial eclipse can also be seen on either side of the track of totality of an annular or total eclipse. The moon will cover part of the sun and the sky will not darken noticeably during a partial eclipse.