What do red and green harbor lights mean?

When returning to the harbor, you will see a green light on the left side of the ship channel and a red light on the right side of the channel. Go outside those lights and you may run aground onto the bottom in shallow water. Just remember this old adage: "Red on Right when Returning". The opposite is true when departing the harbor to go to sea, i.e., red light will be on the left and green light will be on the right.