THE SCIENCE OF ECLIPSES
Every now and then, the Earth, Moon and Sun line up to produce an amazing spectacle in the sky
WORDS ANDREW MAY
DID YOU KNOW? Many ancient cultures explained solar eclipses in terms of a huge invisible creature devouring the Sun
Most locations see a ‘blood Moon’, or total lunar eclipse, every two or three years
© Getty / NASA
Solar eclipses, particularly of the total or near-total kind, are among
the most dramatic and memorable of all astronomical events. But
what causes these striking phenomena in the first place, and why do they occur at certain times and not others?
Eclipses occur due to the alignment of three different celestial bodies: the Earth, Moon and Sun. Without going into too much astronomical detail, you can picture the Moon as revolving around the Earth in a roughly circular orbit, while both bodies are illuminated by the much more distant Sun. The Moon takes about a month to complete a single orbit, during which time there are two occasions when the Sun, Moon and Earth lie along an approximately straight line. When it happens in that order, and the Moon looks very close to the Sun in Earth’s sky, we call it a ‘new Moon’; when it’s in the order Sun-Earth-Moon, we call it a ‘full Moon’, because the Moon’s disc is fully illuminated by sunlight when viewed from Earth.
Most months, this alignment isn’t perfect because the Moon orbits Earth in a slightly different plane from the orbit of the Earth-Moon system around the Sun. For example, a new Moon usually appears slightly offset from the Sun in the sky, rather than bang on top of it. But occasionally, the three bodies really do lie in a perfectly straight line, and that’s when we get an eclipse. The eclipse that occurs at new Moon is called a ‘solar eclipse’, and it results in all or part of the Sun’s light being blocked out by the Moon.
There’s an odd coincidence that makes solar eclipses even more striking on Earth than they would be on other planets. That’s
because even though the Sun is around 400 times larger than the Moon, it’s also 400 times further away, so the two objects look almost exactly the same size in the sky as seen from Earth. This creates the possibility of a ‘total solar eclipse’, in which the Moon fits neatly over the Sun, just leaving its outer atmosphere, or corona, visible.