Why do eclipses happen?

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 opportunities for an eclipse, be it a solar or a lunar eclipse.

A picture of the partial eclipse of 23 October 2014 taken using an HTC mobile phone at the eyepiece of a filtered 125mm refractor. Image: Nick James.

Now, an eclipse of either variety can only happen when the Sun, Moon and Earth are in a line. Usually, the Moon crosses the ecliptic when they are not in a line, so we do not see an eclipse. However, on odd occasions – roughly once every 18 months on average – the Moon crosses the ecliptic at a time when it is aligned with Earth and the Sun. If this crossing takes place when the Moon is between Earth and the Sun, we see a solar eclipse. Two weeks later the Moon moves to the other side of Earth, with the alignment still the same, causing the Moon to slip into Earth’s shadow. Here on Earth, we see a lunar eclipse.

So solar and lunar eclipses always come in pairs. This year, the eclipse of 10 June was proceeded by a total lunar eclipse on the night of 26/27 May which was visible from Australia, New Zealand and the Pacific region.