What is earthshine and when to see the next one?

Gabriel Funes captured the waning crescent moon with earthshine on the early morning of July 8, 2021, from Teide National Park in the Canary Islands, Spain
Gabriel Funes captured the waning crescent moon with earthshine on the early morning of July 8, 2021, from Teide National Park in the Canary Islands, Spain. (Image credit: Gabriel Funes/APOD)

Earthshine

Earthshine is a beautiful dim glow visible on the unlit part of the moon.

The illuminated part of the moon is visible when sunlight reflects from the moon directly. However, the unlit part of the moon is visible when sunlight reflects from Earth and back onto the moon. So earthshine is earthlight shining on the night side of the moon.

Earthshine is visible a few days before and after the new moon, when the moon is in its crescent phase.

Look in the eastern sky preceding sunrise (when the sky remains dark) on the unlit part of the waning crescent moon or in the western sky following sunset (when the sky becomes dark) on the unlit part of the waxing crescent moon to see earthshine.

You need no instruments, like a pair of binoculars or a telescope, to see earthshine. It is easily visible to the naked eye if your night sky is dark and clear.

Da Vinci glow

Earthshine is also known as Da Vinci glow because Leonardo da Vinci illustrated the phenomenon and explained its cause for the first time over 500 years ago. Visit here to see the sketch of Leonardo da Vinci.

Leonardo thought the unlit part of the moon shines when sunlight reflects from Earth’s oceans. However, nowadays we know that this is not entirely correct. In reality, 50% of the reflection comes from Earth’s clouds and only 10% from Earth’s oceans.

When to see the next earthshine in 2025?

The next earthshine will be visible on the unlit part of the waning crescent moon from February 23 to 25, 2025. Look for them in the eastern sky around an hour before sunrise.

On February 23, the waning crescent moon will be 24% illuminated, and the earthshine will be visible on the rest of the 76% unlit part of the moon.

On February 24, the waning crescent moon will be 15% illuminated, and the earthshine will be visible on the rest of the 85% unlit part of the moon.

On February 25, the waning crescent moon will be 8% illuminated, and the earthshine will be visible on the rest of the 92% unlit part of the moon.

Best time of the year to see earthshine

The strength of earthshine varies throughout the year as the amount of light reflected from Earth depends on part of Earth and the cloud cover.

However, it is more intense in the summer (April to June) due to global warming.

Why do we see earthshine when the moon is in crescent phase?

We know that a full moon can illuminate (light up) the landscape of Earth at night. Similarly, a full Earth can illuminate the landscape of the moon at night.

Now, similar to the phases of the moon, our Earth also goes through phases, as seen from the moon, and the phases of the moon and Earth are complementary.

During Earthshine, the earth looks nearly full from the moon when the moon looks slim crescent from the Earth
During earthshine, Earth looks nearly full from the moon, while the moon looks crescent from Earth. (Image credit: NASA/JPL-Caltech)

So a full Earth is seen from the lunar night sky when the moon looks crescent in our sky, and this full Earth is bright enough to faintly illuminate the darkened surface of the moon. It is very similar to a full moon, which illuminates the darkened surface of Earth.

So we see earthshine when Earth looks full from the moon’s perspective and the moon looks crescent from Earth’s perspective.

And we see bright moonlight when the moon looks full from Earth’s perspective and Earth looks slim crescent from the moon’s perspective.

References

Apparent magnitude of earthshine: a simple calculation

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Ashim

Ashim Chandra Sarkar founded Space & Telescope in 2022. He holds a M.Sc. in physics and has five years of research experience in optical astronomy. His passion for astronomy inspired him to open this website. He is responsible for the editorial vision of spaceandtelescope.com.

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