The skies in early May have been a treat so far, as the Eta Aquarids meteor shower – the trail from the famous Halley’s comet – have burst through our atmosphere. But there’s another reason to look to the stars in the coming days – a mysterious phenomenon known as Earthshine.
Sometimes known as ‘the ashen glow’, ‘the old Moon in the new Moon’s arms’ or ‘the da Vinci Glow’, Earthshine has been mesmerising onlookers for millennia and now is the best time of year to catch a glimpse of the event.
When can I see Earthshine?
Weather permitting, you can see Earthshine this evening, Thursday 9 May, just after sunset (8:37pm BST in London, 8:01pm EDT in New York City, 7:44pm PDT in Los Angeles).
We’ve just had a new Moon, which is where the Moon is positioned in between the Earth and the Sun, and just after this event is the best time to catch a glimpse of the Earthshine effect.
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Now, the only bit of the Moon that’s illuminated by the Sun is a small crescent (or waxing crescent, to be precise), and the Earthshine effect becomes visible.
There are two reasons why Earthshine might seem brighter when the moon is a crescent.
First, with only a thin crescent of the Moon’s surface bathed in sunlight, the faint Earthshine has less competition. It's like a flashlight being easier to see in a dark room compared to a brightly lit one.
Second, during this crescent phase, the Earth is nearly fully illuminated from the Moon's perspective. Imagine yourself standing on the moon – you'd see a bright, almost full Earth casting Earthshine onto the lunar surface.
Here’s how the Moon is shaping up over the next few nights:
- 9 May: 3 per cent waxing crescent Moon
- 10 May: 7 per cent waxing crescent Moon
- 11 May: 14 per cent waxing crescent Moon
- 12 April: 22 per cent waxing crescent Moon
Weather permitting, Earthshine should be visible on all these evenings (although those closest to the New Moon on 8 May will be best). Look for it shortly after sunset in a dark location for the best chance of seeing it.
April and May are the best times in the year to witness the Earthshine effect, so this could be your last chance to observe it before next year.
What is Earthshine?
Earthshine is a faint glow on the unlit side of the Moon visible around a New Moon. Essentially, it’s light from the Earth, rather than the Sun, bouncing off the Moon like a giant mirror in space.
The process goes like this:
- Sunlight bathes Earth, bouncing off our oceans, land, and clouds.
- Some of this reflected light travels back out into space.
- A tiny portion reaches the Moon, illuminating the side facing us that's hidden from the Sun's direct rays.
This light has travelled a long distance, first to Earth and then to the Moon, making it much fainter than the sunlit part. This gentle, indirect glow is what we call Earthshine.
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Why is this the best time of year to view Earthshine?
During spring, particularly noticeable now in the Northern Hemisphere, Earthshine becomes more prominent by around 10 per cent due to the heightened reflectivity of the Earth’s surface.
"The reflectivity of the Earth – its albedo – changes throughout the year. Snow and ice reflect lots of sunlight, but not so much light is reflected by land, and the vegetation on it. Our oceans are even less reflective, as water is so transparent," Dr Darren Baskill tells BBC Science Focus.
As the Northern Hemisphere tilts towards the Sun following a winter of facing away from it, the remaining snow and ice in polar regions act as reflective surfaces, bouncing a significant amount of sunlight back into space.
This reflected light then reaches the Moon's surface, casting Earthshine onto its darkened portion.
By next month the snow and ice will have all but disappeared, making Earthshine that bit harder to spot, especially for those of us living in artificially lit urban areas.
"In addition, the “young” crescent Moon is higher in the sky as spring turns into summer, which also makes the phenomena easier to observe," Baskill adds.
What’s da Vinci got to do with it?
Among his numerous achievements, Leonardo da Vinci was one of the earliest to accurately describe Earthshine, leading to the phenomenon now often referred to as the 'da Vinci Glow' (though he didn't coin the term himself).
Da Vinci meticulously sketched the Moon and various celestial bodies, correctly deducing that Earthshine likely originated from sunlight reflecting off the Earth.
However, his theory wasn't without its flaws. He initially attributed the effect mostly to the oceans, overlooking the significant contribution of winter snow and ice.
Before da Vinci came along, some astronomers believed the Moon might be translucent, allowing sunlight to pass through. This theory suggested that sunlight hitting the Moon from behind would shine more readily through the thinner edges, creating a brighter crescent and dimmer Earthshine.
What affects Earthshine?
As previously noted, the brightness of Earthshine is influenced not only by the presence of snow and ice but also by various other factors.
One significant factor is cloud cover, as clouds can both reflect and absorb sunlight. Thick, white clouds are particularly effective at reflecting sunlight, thus enhancing Earthshine. Generally, the more cloud cover there is over the Earth, the brighter the Earthshine will appear, assuming the Moon is still visible, of course.
In 2017, scientists revealed a concerning trend: the Earth's albedo, or its overall reflectivity, has been decreasing over the past few decades. This decline suggests that Earthshine is becoming less visible.
Such a phenomenon could indicate global warming is intensifying, as more heat is absorbed by the Earth's oceans and atmosphere, leading to less sunlight being reflected into space.
Did you know?
- If the Sun were to suddenly disappear, the crescent Moon would vanish first, but Earthshine would faintly illuminate the Moon for a brief additional three seconds.
What equipment do I need to see Earthshine?
In short, not a lot. A pair of eyes should do the trick.
If you want to enhance the viewing experience, a pair of binoculars will help you pick out otherwise hidden details on the lunar surface.
About our expert
Dr Darren Baskill is an outreach officer and lecturer in the Department of Physics and Astronomy at the University of Sussex. He previously lectured at the Royal Observatory Greenwich, where he also initiated the annual Astronomy Photographer of the Year competition.
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