Why do people say at the crack of dawn?

Why does dawn have a crack? When someone says the word crack a lot of things come to mind like: 1 sidewalks
2 eggs 3 bones and of course butts! Okay, to get off of that subject. I don’t know about you, but i think that something that is supposed to be so important to our day (a.k.a. Dawn) should have a crack in it could be scary. I wonder if one of these mornings we will wake up and see some aliens coming through the crack in our dawn trying to take our cows for experiments. Hopefully for burgers sakes that may never happen.

So why do people say at the crack of dawn?