# Old moon in the new moon's arm

I understand the phenomenon I want to explain is something known as Earth Shine. What is Earthshine and what causes Earthshine? Is it really normal? What exactly is the less illuminated part in the diagram?

Yes, earthshine is completely normal. It is simply illumination of that part of the moon that is facing away from the sun. This illumination is sunlight that is reflected towards the moon by the Earth. Strictly speaking, it is that reflected light from the Earth that is itself reflected back to Earth so that we can see it. The intensity of Earthshine is highly variable. If the moon was almost in line between Earth and sun then an observer on the dark side of the moon would see the the whole of Earth's disk brighly illuminated by the sun, and hence the earthshine would be about at its maximum. Conversely, when the Earth is nearly between Moon and Sun, an observer on the moon would only see a crescent Earth (apart from man-made city lights), so the Earthshine would be at a minimum.

• i am in no way related to this field. so my ideas would be real stupid. so you say we see the complete moon because of the light from the earth. then how is the full moon lit up? only one side of the moon should be visible.. this idea is coz of the fact that the sun can only lit up half of the earth.
– Vini
Dec 10, 2015 at 6:32
• The moon is far enough away that, wherever on Earth it's visible, you see basically the same thing. This means that everyone looking at the Moon sees the area that is lit by daylight areas of the Earth. You can't see the side of the moon that isn't lit by Earthshine, because that's an area which nobody on the Earth can see. Dec 14, 2015 at 1:48

You can think of it as the "reverse" of moonshine. When you walk outside at night and there's a full moon, you can actually see quite a lot around you.

Same thing - this happens on the moon when there's a "full earth" that lights up everything around. And since you're on earth, you see this from the outside, on the moon.

• i am in no way related to this field. so my ideas would be real stupid. so you say we see the complete moon because of the light from the earth. then how is the full moon lit up? only one side of the moon should be visible.. this idea is coz of the fact that the sun can only lit up half of the earth.
– Vini
Dec 10, 2015 at 6:33
• @Vini the full moon is lit by the sun, not the earth Dec 10, 2015 at 14:18