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Why do solar and lunar eclipses not happen every month?
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Moon
(Left) Near side of Earth's Moon, photographed by the Galileo spacecraft on its way to Jupiter. (Right) Far side of the Moon with some of the near side visible (upper right), photographed by the Apollo 16 spacecraft.
Solar and lunar eclipses do not occur every month because the Moon’s orbital plane is inclined by about 5° relative to the ecliptic, which is the plane of Earth’s orbit around the Sun.12
Because of this tilt, the Moon typically passes above or below the Sun during a new moon and above or below the Earth's shadow during a full moon. For an eclipse to occur, the Moon must be near a node—the specific points where its orbital path crosses the ecliptic.123
These nodes are where the Sun, Earth, and Moon can align in a straight line, allowing for the casting of shadows. Because the Moon’s nodes move slowly along the ecliptic, these alignments only happen during specific periods known as eclipse seasons, which occur roughly every six months.23