Why we do not get a solar eclipse at every new moon
If the Moon circled Earth in the same plane that Earth circles the Sun, every full moon would bring a lunar eclipse and every new moon a solar one. The two orbits are tilted against each other, so eclipses can only happen in two windows a year, called eclipse seasons.
An eclipse happens whenever one body slips into another's shadow, or something passes between it and the watcher. The three bodies lined up are said to be in syzygy. If the object is hidden completely, that is an occultation; if only partly, a transit. The word comes from the ancient Greek ekleipsis, meaning abandonment, downfall or the darkening of a heavenly body, built from roots meaning out and to be absent.
Everyday talk means solar eclipses, when the Moon's shadow crosses Earth, and lunar eclipses, when the Moon moves through Earth's shadow. But the idea stretches much further: planets fall into the shadows of their moons, moons into their planets' shadows or each other's, and a pair of orbiting stars will eclipse one another if their orbit is edge-on to us. Anyone on the night side of Earth can watch a lunar eclipse, whereas a total solar eclipse may return to a given spot only after many decades.
Shadows have three zones. Inside the umbra, Latin for shadow, the blocking body hides the light source entirely; for the Sun, that means covering the photosphere. The antumbra lies past the tip of the umbra, where the body sits wholly in front of the source but is too small to cover it. In the penumbra, from the Latin for almost, it covers only part. Stand in each and you see, in turn, a total, an annular or a partial eclipse. Lunar eclipses have no annular kind, because the Sun and Earth's antumbra falls far beyond the Moon, and seen from the Moon, Earth appears nearly four times wider than the Sun.
Astronomers time an eclipse by four contacts: when the dark disc first touches the bright one, when it lies fully inside, when it begins to leave, and when it clears the edge completely. Whether a given season brings a total, annular, hybrid or partial solar eclipse depends on how big the Sun and Moon happen to look from Earth at the time.
Source: Eclipse
