A navigation light is a colored source of illumination on an aircraft, spacecraft, or waterborne vessel, used to signal a craft’s position, heading, and status. Commonly, their placement is mandated by international conventions or civil authorities. Navigation lights are used to prevent collisions at night or in times of reduced visibility, and are an essential tool in keeping you and your vessel safe. Nav lights allow you to see other nearby vessels, and allow other vessels to see you. Nav lights also provide information about the size, activity, and direction of travel. By understanding the characteristics of Nav lights, you can determine an appropriate course of action as you approach another vessel. On any vessel, navigation lights have a specific color, (white, red, green, yellow, blue), arc of illumination, range of visibility, and location, as required by law and regulations.

A red light will be mounted on the left or port side of the craft and a green on the right or starboard side. In general sailing vessels are required to carry a green light that shines from dead ahead to 2 points (22½°) abaft the beam on the starboard side, a red light from dead ahead to two points abaft the beam on the port side and a white light that shines from astern to two points abaft the beam on both sides. Power driven vessels, in addition to these lights, must carry either one or two (depending on length) white masthead lights that shine from ahead to two points abaft the beam on both sides. If two masthead lights are carried then the after one must be higher than the forward one.[2] Some boats operating in crowded areas may also carry a yellow flashing beacon for added visibility during day or night.

This article has been compiled from:
http://www.bosunsmate.org/seamanship/lights.php