Why eclipses do not happen every month
The Moon's orbit is tilted by about five degrees relative to Earth's orbital plane around the Sun. Most new and full Moons therefore pass above or below the alignment needed for an eclipse. Eclipses occur when the phase and the Moon's position near an orbital crossing point line up together.
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Phase alone is not enough
A solar eclipse requires the Moon to pass between Earth and the Sun along a suitable line of sight. A lunar eclipse requires the Moon to pass through Earth's shadow.
Those arrangements correspond to new Moon and full Moon, but the Moon's orbital plane is tilted relative to Earth's orbital plane. The two planes intersect along a line with crossing points called nodes.
An eclipse needs both the correct phase and a suitable position near a node. Otherwise, the Moon misses the necessary alignment.
- Most new Moons pass north or south of the Sun in the sky.
- Most full Moons pass clear of Earth's shadow.
- Certain periods favour the required alignments.
The orbit's geometry changes over time, creating eclipse seasons rather than a perfectly aligned event at every monthly phase.