An eclipse happens when three celestial bodies line up and one blocks the light of another. The most familiar are solar and lunar eclipses. In a solar eclipse, the Moon passes between Earth and the Sun, casting a shadow on Earth's surface. In a lunar eclipse, Earth passes between the Sun and the Moon, casting its shadow on the Moon.

Solar eclipses are spectacular but rare. A total solar eclipse occurs somewhere on Earth about every 18 months, but it is only visible along a narrow path. The Moon's shadow is small, and it moves fast. Totality lasts just a few minutes. That is why eclipse chasers travel the world to see them.

Lunar eclipses are more democratic. They are visible from the entire night side of Earth and last for hours. During totality, the Moon turns a deep red because sunlight is bent through Earth's atmosphere. That is why it is called a blood moon.

Eclipses are not just beautiful. They have been used to test physics. The 1919 solar eclipse confirmed general relativity by showing that the Sun's gravity bends starlight. Eclipses also help astronomers study the Sun's corona, which is normally hidden by the bright photosphere.