Eclipse of the moon

© Christopher Collins/Alamy

Eclipse of the moon

There are at least two lunar eclipses every year, one in spring and another in autumn. An eclipse requires syzygy—the alignment of the sun, Earth, and moon. With the sun behind the Earth, our planet’s shadow, the umbra, falls onto the moon’s surface, giving it a reddish appearance. That’s why a total lunar eclipse is sometimes called a blood moon.