The umbra is the darker shadow.
To understand why, it's helpful to differentiate between the different types of shadows:
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Umbra: This is the darkest part of a shadow. It's the region where the light source is completely blocked by an object. Think of it as the core of the shadow. If you were in the umbra, you would experience a total eclipse.
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Penumbra: This is the lighter part of a shadow. It surrounds the umbra. In the penumbra, the light source is only partially blocked. This results in a shadow that is less dark and more diffuse than the umbra. If you were in the penumbra, you would experience a partial eclipse.
Essentially, the darkness of a shadow depends on the amount of light blocked. Since the umbra is the area where light is completely blocked, it is darker than the penumbra where light is only partially blocked.
