An eclipse happens when one astronomical body passes into the shadow of another, or when one body blocks your view of another so completely that the blocked object briefly goes dark. The two eclipses this book covers both involve the same three players — the Sun, the Earth, and the Moon — just arranged differently. In a solar eclipse, the Moon gets between the Sun and the Earth, and the Moon's shadow falls on the Earth's surface. In a lunar eclipse, the Earth gets between the Sun and the Moon, and the Earth's shadow falls on the Moon. Either way, you need all three bodies lined up in roughly a straight line. Astronomers call this alignment a syzygy (SIZ-uh-jee) — from a Greek word meaning "yoked together." Every solar and lunar eclipse is a syzygy, though not every syzygy produces an eclipse, for reasons you'll see in the next few subsections.

It's worth separating "eclipse" from two related words you'll see in astronomy writing, because students often blur them together. An occultation is when a larger body passes in front of a smaller, more distant one and hides it completely — the Moon occults a star nearly every night somewhere on Earth, simply by drifting in front of it. A transit is the reverse setup: a smaller body passes in front of a much larger one and only blocks a tiny piece of it, like a small dot crawling across a bright disk. Venus transiting the Sun is the classic example — Venus looks like a small black dot inching across the solar disk, blocking a tiny fraction of the Sun's light. An eclipse is really occultation's close cousin: the Moon eclipsing the Sun is essentially an occultation of the Sun, but by convention we reserve the word "eclipse" for cases involving the Sun, Earth, and Moon, and use "occultation" for everything else (the Moon covering a star or planet, one moon of Jupiter covering another, and so on).