Why do solar eclipses happen on an 18 year cycle?
Eclipses are possible when the nodes of the lunar orbit are lined up with the Sun (“eclipse seasons”) which happens about twice a year. But in order for an eclipse to happen, the moon must be either new (for a solar eclipse) or full (for a lunar eclipse). A new moon happens only once every month. Combining these cycles gives a cycle of approximately 18 years.
