At Dolphin Cay, our home on Orcas Island, Washington, we have standing just outside our front door an old ship's binnacle--a 3.5-foot tall wooden post fitted with a maritime compass and leveling weights.

But in the black of night, with the dim, blue-black porch light burning behind it, this benign binnacle takes on an insidious nature. Coming down our hallway anytime after midnight, it looks as though some rather short, stocky person is peeking in at us from just outside the front door. Even though we know its our dear old binnacle, the strange shadows it casts by porchlight and moonlight never fail to startle us and make us do a quick double-take.

The shadows transform the familiar into the threatening.

I suspect most of what we fear about the dark is not the fact that we can't see anything.…