The surface of the Earth is obscured by buildings, foliage and cloud cover, all of which mask the lie of the land. Our view of the Moon's surface is not obstructed by anything and so it is possible to view the geography and contours of the Moon easily from the Earth.
If we look a little closer at the shape of a crater we can try to use the shadows cast by the rim mountains to work out their height.
Diagram of the shape of a crater from the side.
If we know the angle of the light falling on the crater and the distance that a shadow falls across the surface of the Moon, then we can use the length of the shadow to estimate the depth/height of any geographical feature. You might have used shadows in the past to measure the height of tall buildings or trees by using the shadow that a metre ruler casts on the ground and then comparing this to the length of the shadow that is cast by the building or tree.
Using the shadows to estimate the height of features on the Moon is a technique that has been used to help map out many of the Moon's features, Galileo was known for using a similar method.
You can try it for yourself in the first of our extension exercises.