Why does the Moon shine?

Credit Galileo Project, JPL and NASA

Perhaps rather bizarrely, the Moon is actually very dark, and it doesn’t glow for the reasons you might think. The ancients thought that the Moon produced its own light, but we now know definitively that this is not the case. Rather, our Moon reflects the light of the Sun in accordance with its orbit.

The entire Moon does not constantly reflect light – only the half in direct view of the Sun. As the Moon is tidally locked to the Earth (ie we only ever see one face), our view of the lit half changes constantly, ranging from a disc to a thin crescent. On a full Moon, the Sun is directly lined up with the Earth-Moon line; when we see a thin crescent, on the other hand, the Sun is illuminating just the side. However, the Moon does not reflect light quite like a mirror, although it is similar. All objects in space have an albedo, which is a measure of how well they reflect light. To give you an idea of how this works, material like ice has a high albedo, whereas soil has a low albedo.

However, the Moon’s albedo is actually very low – similar to that of coal. Its bright glow is instead the result of something called the opposition effect. You may have come across this when seeing a car’s headlights shine on a dark road: the road appears brighter than it would if light were not incident upon it. The Sun plays the part of the headlight in this case, directly shining on the Moon and leading to its bright glow. The large amount of debris on the surface of the Moon also contributes to its reflectivity.

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