What is a lightship?

A lightship is like a floating lighthouse. It anchors in one spot to warn shipping of such nearby dangers as sand banks or dangerous currents. At night it projects a powerful beam of light. The lamp is fitted in such a way that the beam stays steady, even if the ship is being tossed about by waves. Lightships also have their name painted on each side, so that during daylight hours ships can use them to check their own position.