How big would a mountain be on the moon to be able to see it with the naked eye from the earth?

Probably smaller than you think.

Let us relate to geography (Ehm.. ‘ Selenography ‘) from the mountain of a very boring mountain going out:

The mountain in question has the shape of a rounded hill and consists of the same uniformly gray, powdery material as the surrounding lunar soil.This means that in completely diffuse light, we could hardly distinguish him from his environment. Not even if we were amply close enough to see him in detail.

The only thing that can distinguish this mountain from its surroundings is the fact that the light source that illuminates the moon (the Sun) is not diffuse, but clearly shines from a certain direction.

With regard to the exposure situation, we are therefore going from the most favourable conditions:

The mountain is seen from the Earth in the middle of the moon, and at the time of our perception from the Earth, the mountain is almost exactly on the Terminator (the boundary between the sun-lit and unlit part of the moon).This happens for our mountain (and virtually every point on the lunar surface) twice every 28 days; If the first quarter (sunrise) or last quarter (sunset) is. The Terminator is nothing but the line where the sun at that moment just comes up or just goes down. This is relevant to our thought experiment since then -successively -the following situations occur in respect of our mountain:

  • The mountain casts a very long shadow -many times longer than its height -on the lunar surface.
  • When at the foot of the mountain The sun has been below or has not yet occurred (and it is pitch black because there is no atmosphere on the moon that creates a twilight -a gradual transition of light and darkness -as on Earth) captures the top of the mountain Still/For some time sunlight on.

Both situations ensure that the presence of the mountain temporarily creates strong contrasts in the landscape.In addition, the first effect (the shadow) ensures that the mountain is magnified many times (a mountain of Pak’m bite a kilometer high, shadows of many kilometers can throw long if the sun is very low in the sky).

I do not now have the opportunity to calculate it more precisely (I am on the road and type this from my mobile) but I estimate that these effects ensure that a mountain of a kilometer or 10 in diameter with a height of about 1.5 kilometers already with the naked eye -as a Dot -can be visible at the optimum moment and under the optimal conditions that occur along the Terminator.You will of course not see a full mountain as an observer from the Earth. You see by contrast with the surroundings purely that there is ‘ something ‘.

This picture shows how the relief on the moon is magnified by the low position of the sun along the Terminator.Both of these effects (extra long shadows and illuminated mountains in the shadows) are reflected in the picture. (Photo-Credit: Jules Stoop)

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