5c: Why don't solar eclipses turn up every month?
The moon's orbit is slightly tilted towards the earth's orbit. Thus, in most of the new moon events Moon's center moves above or below the ecliptic. Only at the so-called nodes, Moon's orbit meets Earth's orbit. Earth, Moon and Sun just stand in a line. If the new moon meets precisely one of the two nodes, then the rarely occurring eclipse can be observed (see: Fig. 40).

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5d: Why don't eclipses take place on an annual basis at least?

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