Here's a breakdown:

* The Moon's orbit: The Moon orbits the Earth in a tilted plane, not perfectly aligned with the Earth's orbital plane around the Sun.

* Solar and Lunar Eclipses: For an eclipse to occur, the Sun, Earth, and Moon need to be perfectly aligned.

* Alignment Issue: Because the orbits are tilted, the Moon usually passes above or below the Sun's path during its monthly cycle, preventing an eclipse.

* Lunar Nodes: The points where the Moon's orbit intersects the Earth's orbital plane are called "nodes." Eclipses only happen when the Sun, Earth, and Moon are aligned at or near these nodes.

Why they don't happen every month at the nodes:

* The nodes are shifting: The nodes of the Moon's orbit are not fixed but shift slowly due to gravitational influences.

* Timing: The Moon's orbit and the Earth's orbital position around the Sun have to align at the nodes for an eclipse to occur. This happens relatively infrequently.

In summary, eclipses are not a monthly event because the orbital planes of the Earth and Moon are not perfectly aligned, making alignment of the Sun, Earth, and Moon a rare event.