21. Roughly how long is the time interval between total solar eclipses occurring on Earth?
From Quiz Oh, When Darkness Comes
Answer: 18 months
If the Moon and the Earth travelled in circular orbits, and they moved in the same plane, then there would be a total solar eclipse every month, whenever there is a New Moon. (This is assuming that this hypothetical situation has the Moon at the right distance for totality.) However, their orbits are elliptical and inclined to each other, leading to the eclipse seasons that occur roughly each six months, with one or two eclipses in each season. However, many of these will be partial or annular eclipses. On average, there will be a total solar eclipse once every 18 months.
However, these eclipses are not all seen at the same spot. If you had to guess how long you need to wait to see another eclipse at the same spot where you just saw one, the answer would be somewhere between 350 and 420 years, depending on where you are.
The number 75 was thrown in as the familiar (to astronomy students) time for the return of Haley's Comet.