Earthshine tonight: How to see the otherworldly Moon event

Over the last few months, you may have been lucky enough to glimpse the Moon making an eerie appearance just before and after the new Moon phase. The unlit portion of the lunar surface has appeared to glow, in a phenomenon called Earthshine.

And July may be your last chance to see it this year.

In this article, we explain when you can see this lunar glow, what causes it, and what it has to do with Leonardo da Vinci, one of history's most revered polymaths.

You can also make the most of clear nights this year with ourfull Moon UK calendarandastronomy for beginners guide.

When can I see Earthshine?

Clouds permitting, you can see Earthshine on Wednesday 19 July in the evening, after the Sun has set at 9:07pm BST, and before the Moon sets at 10:30pm.

As the Moon will be low on the horizon at this point, try to find a spot where the western horizon is unobscured by vegetation or buildings.

Earthshine is visible in the mornings a few days before the new Moon, and in the evenings a few days after the new Moon, so if like for us, clouds scuppered your viewing a few days ago, there’s another chance.

And as a bonus, we’ll also be able to see a bright, magnitude 1.76 Mars a little further towards the west, before it too, sinks below the horizon.

Earthshine is most apparent during the Moons waxing or waning crescent phase, as the illuminated section appears narrower enabling a greater portion of the shadowed Moon to be lit up by the reflected Earthlight.

Why is Earthshine best seen in the summer?

The late spring and summer months are the best time of the year to see Earthshine, when the northern hemisphere is tilted towards the Sun. At higher latitudes, the last of the winter snow and ice is clinging on, giving the poles a higher albedo (they reflect more light). With more light reflected onto the Moon, the more apparent Earthshine is.

"While Earthshine will be best seen in April and May, the effect will still be visible in June and July," saysDr Darren Baskill,astronomy lecturer at the University of Sussex.

"It will just be a bit fainter due to the reduced amount of snow and ice in the northern hemisphere."

As Earthshine also depends on the amount of sunlight actually reaching the Arctic, it’s not such an event in the winter months, although not impossible.

What does Earthshine look like?

Earthshine appears as a dim, pale glow on the unlit portion of the Moon, resembling a soft, ethereal light that softly illuminates the lunar surface. It’s a lovely sight, and worth looking out for if you can, as you can often make out the whole of the lunar disk using nothing more than your eyes.

What causes Earthshine?

Earthshine is caused by the faint glow of sunlight reflecting off the Earth's surface and bouncing back onto the Moon. As it does so, it illuminates the unlit portion and gives the Moon a subtle, ghostly appearance.

It’s also known as the Da Vinci glow, ashen glow, or ‘the old Moon in the new Moon’s arms,’ and occurs during the part of the lunar cycle when only a thin crescent is illuminated by direct sunlight; the rest of the Moon being illuminated by indirect sunlight.

As the sunlight reflects off Earth’s land masses, clouds, oceans, and the atmosphere itself, this light is scattered and diffused, then is reflected back out into space. And it’s this reflected light that hits the Moon, landing on the lunar ‘night’ side – the unlit portion.

Why does Earthshine vary so much?

The composition of Earth's atmosphere, the extent of cloud cover, and the angle of sunlight play significant roles in determining the brightness – and even colour – of Earthshine, resulting in unique variations each time.

The Earth's atmosphere acts as a crucial factor in how Earthshine appears to us. As sunlight passes through our atmosphere, it undergoes scattering and absorption, which affects the different wavelengths to varying degrees. This atmospheric filtering directly impacts the colour and strength of Earthshine, and it’s this light that is subsequently reflected onto the Moon.

The type of ground cover on Earth also contributes to how much light is reflected. Land, for example, typically reflects approximately 10-25 per cent of light, while clouds can reflect much more; around 50 per cent.

Why is it called the Da Vinci glow?

Back in the late 15^th and early 16^th century, polymath Leonardo Da Vinci turned his mind to some of the greatest scientific mysteries of the age. Everything from anatomy to engineering, geology to astronomy, how water flows and why fossils are found on mountaintops.

And, hidden in the corner of one of the pages of his notebook, the Codex Leicester (bought by Bill Gates back in 1994 for over $30 million) is a tiny sketch of Earthshine on the Moon, alongside the speculation that the pale glow of the dark portion of the Moon is caused by reflected sunlight from the Earth.

This speculation turned out to be correct, but it would be another hundred years before German astronomer and mathematician Johannes Kepler would prove it.