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A lesson about Earthshine.

Have you ever looked at the Moon after Sunset when the Moon is a thin crescent and seen the whole disk? What you are looking at is Earthshine and it can be incredibly beautiful under the right conditions.


When you look at the crescent Moon you see the light of the Sun reflecting off the surface of the Moon. Because of the position of the Moon in it's orbit you see only part of the lit surface, the part that is facing the Earth. A new Moon occurs just after the Moon has passed between the Earth and the Sun. While there is always half the Moon that is lit by the Sun, there is only a small part of this that we can see from Earth. This is why we see a crescent. 


As always, half the Moon is facing us, but as it is not all lit by the Sun, it is invisible to us. Except for Earthshine.


When the Moon is a crescent, the unlit part is facing toward a nearly fully-lit Earth. This means that a lot of light is reflected from Earth back to the surface of the Moon. As the Sun sets, and the unlit part of the Moon is against a background of the darkening sky, we can see it lit by sunlight reflected off the Earth.  


So when you look at Earthshine you are seeing the light of the Sun reflected off the Earth to the Moon and reflected back again to your eyes.


That's a very cool astronomy thing, don't you think?

Moon with Earthshine

Moon with Earthshine. Dylan O'Donnell. Public Domain

Earthshine is a very difficult thing to photograph. This is because the difference in light between the crescent part of the Moon and the dark part with Earthshine. The contrast is so great that your camera can't take a good image of one part without over or under-exposing the other part. 


In this image you can see the features on the unlit part of the Moon, but they are very dim and barely visible, but if you look hard you can see them.


Here is what Dylan O'Donnell, the photographer, said about the image:


It’s a layering of 2 separate stacks, both of 20 exposures each for a total of 40 exposures. This was done to reveal the faint and delicate crater details as well as the dull blue glow of the earth itself, reflecting back from the dark side of the moon.


I really like how the 20% moon’s character as a 3 dimensional object hanging in space, illuminated both by the earth and the sun, is revealed in this image, unlike any of my other lunar photos.


Taken from my backyard in Byron Bay NSW, Australia with a Celestron 9.25″ SCT w/Focal Reducer (6.3) and Canon 70D. Backyard EOS, AutoStakkert & PS6 used for capture and post processing.

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