On Wednesday 26 May, just after 9pm, if you looked into the heavens you would have seen the moon bathed in red. Dubbed the ‘blood super moon eclipse’, the first lunar eclipse of 2021 was more than just a visual spectacle.
The ‘super moon’ part of the title doesn’t hold much scientific significance – it simply means that the moon was at perigee, or its closest point to the earth, and appeared about five per cent larger than usual.
But ‘blood moon’ refers to a scientific principle that is helping scientists look for life on distant planets.
What is a lunar eclipse?
“Eclipses happen when one astronomical object passes between us and the sun, or when we pass between one astronomical object and the sun,” Lecturer in Astrophysics Dr Benjamin Pope said.
A solar eclipse occurs when the moon passes between Earth and the sun, while a lunar eclipse is when Earth passes between the sun and the moon.
“The Earth’s shadow is much bigger than the moon, so it blocks it out for some period of time,” Dr Pope said.
This means that during a lunar eclipse, totality (when the light is completely blocked) can last for more than an hour, whereas during a solar eclipse it only lasts for a few minutes.
Wednesday’s eclipse was visible at totality for 14 minutes from 9.11pm in Brisbane.
Why does the moon turn red?
At the peak of the lunar eclipse, the moon turns a deep shade of red. If you were standing on the moon, this would look even more impressive.
“You’d see the dark side of the earth, and you’d probably see cities lit up at night,” Dr Pope said.
“And it would be ringed in red – there would be this bright red ring of atmosphere around it.”
The reason for this is that light is scattered as it moves through the air.
“Blue light is scattered a lot, and red light is less,” Dr Pope said.
“This means that sunsets are red, while the midday sky is blue.
“What you’re seeing is the red light filtered through the Earth’s atmosphere. Some of it – not a lot, but enough that you can see it – is leaking onto the moon. And that causes the blood moon.”
How can lunar eclipses help us find life on other planets?
This principle of scattering, which explains why the moon turns red, is helping scientists analyse the atmospheres of far distant planets – and might one day even help them find signs of life.
“The same reason that we know why the moon turns red … is how we actually explore the atmospheres of distant extrasolar planets,” Dr Pope said.
When a distant planet passes in front of its star, the atmosphere of the planet will form a tiny arc of light around the planet. Using extremely precise measurements, scientists can analyse how light is scattered in that arc of light to determine what particles are in the atmosphere.
Current technology lets scientists detect not just water vapour clouds, like we have on Earth, but clouds of salts and minerals.
On 31 October 2021, NASA and the European Space agency are launching the James Webb Space Telescope. The largest space telescope ever built, one of its goals is to analyse the atmospheres of distant planets, and perhaps even detect signs of life.
“We’re developing techniques to detect the much fainter signal of not just water, but oxygen, phosphine, methane and other biogenic gases – that is gases exclusively formed by life, or at least in combination exclusively formed by life,” Dr Pope said.
“For example, it’s very hard to get methane and oxygen in the same atmosphere because they burn. So, the only reason we still have both on Earth is because we’re continuously producing both.
“We probably won’t be discovering life, but we might be discovering a lot of very exotic things.”
Shouting at the moon: how we interpreted lunar eclipses in the past
Humans have generally understood the science behind lunar eclipses for thousands of years, but there have still been some interesting superstitions arise over time.
Dr Beth Spacey, from UQ’s School of Historical and Philosophical Inquiry, specialises in medieval European religious history, and she gave us an insight into how lunar eclipses were perceived by Christians in western Europe during this time.
“People are aware of the astronomical reasoning, but they interpret that as the natural processes instilled by God at Creation,” Dr Spacey said.
She said it could be hard to tell what the average person thought about eclipses at the time, because most of the sources were written by members of the Church.
“We have accounts where a priest will get upset because his community have been shouting at a lunar eclipse or making loud noises, because they think that the moon is in danger and that they need to protect it.”
“There seems to have been a belief that the moon was in danger – either from magicians or malevolent sorcerers bewitching the moon, or a concern that it was being swallowed by some sort of monster.”
However, Dr Spacey wonders whether the reports of these beliefs might have been exaggerated.
“We see these reports from across the period … and they always look the same. So, I wonder if there’s a bit of a motif for taking the mickey out of peasants, or at least warning people against attributing the eclipse to false beliefs.”