Extraordinary behaviour of the Moon

Eclipses may simply be the result of one world passing in front of another, but for those of us looking up at them from our planet they reveal the profound bond between the Sun, Moon and Earth.

Moon fact: On average, the same spot on Earth only sees a solar eclipse for a few minutes roughly every 375 years.

What are eclipses?

In the past, eclipses sparked celebration or fear in people who saw them as signs from the gods. But knowing the real reason why eclipses happen makes them no less dramatic to watch.

Though the Moon cycles through its phases like clockwork most of the time, now and then something extraordinary happens that transforms our sky: an eclipse. An eclipse happens when one astronomical object blocks you from seeing another.

Arguably the most spectacular of these are solar eclipses. During a solar eclipse, the Moon gets in the way of the Sun’s light and casts its shadow on Earth. That means that during the day, the Moon moves over the Sun and it gets dark.

Watching the Moon devour the Sun is a spectacle anyone can enjoy (with proper safety equipment – never look directly at the Sun). You just need to be lucky enough to be in the right place at the right time. A solar eclipse is not seen across the whole daylit side of Earth, only from places where the Moon’s shadow falls. It therefore briefly traces a thin line of darkness across our planet’s surface. And the effect can be different depending on your location.

If, from where you are looking, the Moon only blocks a chunk of the Sun, you are witnessing the least thrilling version: a partial eclipse where the light only slightly dims. A more exciting situation is when the Moon completely envelops the Sun. Briefly transforming day into night, this is a total solar eclipse.

There is also a special time when the Moon appears smaller than the Sun to create an annular eclipse. This happens when the Moon is near its farthest point from Earth, making it appear a little smaller. If an eclipse happens at this time, the Moon covers the Sun, but the Sun can be seen around the edges of the Moon like a ring of fire.

Cosmic questions

The fact that solar eclipses are as dramatic as they are raises an important question. Why does the Moon almost perfectly cover the Sun during an eclipse? For the Sun to be fully blocked by the Moon, it needs to look like it is roughly the same size as the Moon when viewed from Earth. As it happens, even though the Moon is 400 times smaller than the Sun, it's also about 400 times closer to Earth than the Sun is. This means that from Earth, the Moon and Sun appear to be roughly the same size in the sky. It is a complete coincidence.

Solar eclipses happen around new moon, when the Sun and Earth align on opposite sides of the Moon. But as there is a new moon every 29.5 days, another important question is: why are there only about two solar eclipses per year?

The reason is that the Earth’s orbit of the Sun and the Moon’s of the Earth are not perfectly aligned. They are in fact off by 5 degrees. As a result, a solar eclipse is only possible when a new moon happens to coincide with when the Earth’s and Moon’s orbits sync up – just 69 days of the year.

Painting the Moon red

Another spectacular event to watch is a lunar eclipse. During a lunar eclipse, the Earth gets in the way of the Sun’s light and casts its shadow on a bright full moon. Unlike a solar eclipse, when an eclipse of the Moon takes place, everyone on the night side of Earth can see it. And the results are often stunning. The Moon’s colour transforms from bright white to yellow, orange or even a deep crimson; which is the reason a total lunar eclipse is sometimes called a blood moon.

Why the Moon doesn’t simply vanish into darkness and instead undergoes a dramatic colour transformation is because some of the light from the Sun gets bent round the Earth. As the light does this it passes through the Earth's atmosphere, which filters out most of the blue light so that only longer, redder wavelengths make it through to illuminate the Moon. The exact colour we see depends on how much dust is in Earth's atmosphere. The more dust, the redder the colour.

Sometimes, a lunar eclipse (or blood moon) is unusual for other reasons. If it happens when the Moon is at its closest approach to Earth in its elliptic orbit, it is called a super blood moon. Supermoons make the Moon appear slightly brighter and 14% bigger than regular full moons. And if the eclipse coincides with the second full moon of the month, it is confusingly called a blue blood moon.

Other exotic names tagged on include wolf, beaver, harvest and frosty, and they all refer to the season in which the full moon occurs. It can lead to unwieldy monikers like super blue blood beaver moon. But these attention-grabbing names mask the true wonder of a lunar eclipse – how it transforms the Moon’s appearance to literally reflect the relationship between the Sun, Moon and Earth.

Scientific boon

For scientists, solar eclipses have played a crucial part in our understanding of the Universe. By shielding telescopes from the Sun’s glare, they have allowed us to explore and learn about the solar corona, a glowing ring of super-hot gas that extends millions of kilometres into outer space and causes solar winds.

The most scientifically important solar eclipse occurred in 1919. Then, a team led by Arthur Eddington captured pictures of stars behind the Sun as darkness fell. They compared the stars’ positions to where they would be on a normal night, and made a startling discovery ­– they had moved.

This meant the Sun had bent the starlight. The only theory that could account for the amount the starlight bent came from Albert Einstein. Eddington’s results catapulted Einstein into the limelight and started to turn scientific opinion in favour of his general theory of relativity. 100 years later, it remains a pillar of modern science.