Sector lights are a man-made pilotage

Pilotage

Pilotage is the use of fixed visual references on the ground or sea by means of sight or radar to guide oneself to a destination, sometimes with the help of a map or nautical chart. People use pilotage for activities such as guiding vessels and aircraft, hiking and Scuba diving...

 and position fixing

Position fixing

Position fixing is the branch of navigation concerned with the use of a variety of visual and electronic methods to determine the position of a ship, aircraft or person on the surface of the Earth.These techniques include:...

 aid that consist of strictly de-limited horizontal angle light beams to guide water-borne traffic through a safe channel at night in reasonable visibility. They are most often used for safe passage through shallow or dangerous waters. This may be when leaving or entering harbour. Nautical charts give all the required information and it is important that an up to date chart be used to validate navigation using sector lights.

In Europe, generally, there are three lights of different colours, each identifying a navigational sector:

• white - this sector is in the middle of the safe channel
• red - indicates the port (nautical) edge of the channel for vessels approaching the light source.
• green - indicates the starboard edge of the channel for vessels approaching the light source.

The world has different navigation stereotypes managed by IALA (International Association of Lighthouse Authorities). For example, the United States uses a signalling stereotype which is the opposite of Europe. In USA, the red light indicates the starboard side of the channel for harbour bound vessels, while the green light indicates the port (nautical) side of the same channel. An expression to remind of this is "red right returning".