Why do shadows merge before objects touch?

[wasteofo2]

I've noticed that the shadows of objects seem to melt together before the objects casting the shadow touch. For instance, I hold my index fingers a foot apart and begin to move them together. The two shadows will meet and alter the appearance of themselves before my fingers have actually touched. Try it to see what I mean more specifically.

Why does this happen?

 

[Integral]

You have probably heard the terms umbra and penumbra associated with eclipse, these are the names given to the regions of a shadow. In the umbra region the source is completely masked while in the penumbra region a portion of the source is visible. All shadows produced by a extended source (in other words all shadows) have umbra and penumbra regions. What you are seeing is the meeting of the penumbra regions. By itself neither penumbra region is dark enough to be visible but when they meet they block enough of the source to become visibly dark regions.