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Why Isn’t There a Lunar Eclipse Every Full Moon?
Every lunar eclipse requires a Full Moon, but most Full Moons are not eclipsed. The missing ingredient is alignment with the plane of Earth’s orbit.
The Moon’s orbit is tilted by about five degrees. At most Full Moons it passes north or south of Earth’s shadow; an eclipse occurs only when Full Moon is also near an orbital node.
A tilted orbit
If the Moon orbited in exactly the same plane that Earth follows around the Sun, the Sun, Earth, and Moon would line up at every Full Moon. Instead, the lunar orbit crosses that plane at two points called nodes. Away from a node, the Moon misses Earth’s shadow.
The roughly five-degree tilt sounds small, but Earth’s dark umbral shadow is narrow at the Moon’s distance. That separation is enough for most Full Moons to pass cleanly above or below it.
Eclipse seasons
Eclipses become possible during periods when the Sun appears near one of the Moon’s orbital nodes. These eclipse seasons repeat roughly every six months and can contain solar eclipses at New Moon, lunar eclipses at Full Moon, or both, depending on the exact geometry.
Three kinds of lunar eclipse
A total lunar eclipse places the whole Moon inside Earth’s dark umbra. A partial eclipse covers only part of it. During a penumbral eclipse, the Moon crosses the lighter outer shadow, often producing a subtle dimming rather than a dramatic dark bite.
Lunar eclipses are safe to watch directly. Their visibility still depends on whether the Moon is above your local horizon while the event occurs.
Questions, answered
Frequently asked questions
Can a lunar eclipse happen at New Moon?
No. A lunar eclipse requires Earth to lie between the Sun and Moon, which is Full-Moon geometry.