So I noticed something on the enterprise, and I have a question.

I was watching TNG season Seven, and at the end clip of one of the episodes, I noticed the enterprise has two 'beacon' lights at its rear, a green and a red one. I am assuming they represent navigational lights like the ones found on air places and boats but...

Since space has a third axis and no up-down east-west (you know what I mean), doesn't that make them redundant? Or at the very least, shouldn't there be four lights to represent both axis?