Why do eclipses happen?

Astronomy Now: AN graphic by Greg Smye-Rumsby. Solar eclipses are relatively rare; they happen when the Moon moves directly and precisely in front of the Sun. Why are they so infrequent, and why do solar eclipses always come in pairs with lunar eclipses? It is all down to a mix of orbital alignments and cosmic coincidences. The Moon is able to cover the Sun’s disc precisely because the Sun is about 400 times bigger than the Moon, and also about 400 times further away, meaning they have the same size in our sky. That’s a huge coincidence! With all that sky above our heads though, what causes the Sun and the Moon to be in the same position in the sky at the same time? It’s all a question of orbits. Earth orbits the Sun along a plane that we call the ecliptic – all of the planets orbit along this plane, more or less. The Moon, however, orbits Earth at an angle that is tilted to the ecliptic, by about five degrees. This means that on each orbit around Earth (a lunar orbit lasts 27 days), the Moon only crosses the ecliptic at two locations and these are the only two …

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