Why Do Blood Moons Occur?

Millions of people will witness this year’s first Blood Moon, a.k.a. total lunar eclipse.

Our ancestors had many prophecies surrounding such events. The Maya, for instance, believed that the Blood Moon was caused by a cosmic jaguar swallowing the moon. Others have seen it as a sign the end times are near.

Thankfully, science offers its own explanation for the celestial phenomenon. The excitement this time is that the four consecutive and complete lunar eclipses — or the tetrad — occur at approximately six-month intervals and will all be visible over the U.S. this year and next. TIME’s science editor Jeffrey Kluger explains.