While the primary purpose of the lighthouse structure is to provide a platform for the light, they also frequently hold sound producing devices for fog, radiobeacons, possibly weather instruments and other equipment. The markings on a lighthouse allow them to be identified in the daylight hours by boaters. A lighthouse usually has a large rotating light inside with a light on top - red, green, or white. This light on top can be steady or flash in a variety of patterns. Charts give the characteristics of the light.
Some lights have sectors that can show a danger area.
From certain directions, a red light may be seen. If you can see the red sector on Annisquam light, in Gloucester, MA, you are traveling into danger. Lights may also have several white or red sections showing you safe and unsafe areas. When you see a red sector on the chart, the bearing given is that from the vessel to the lighthouse. That bearing is also sometimes called a danger bearing Back to List Visibility Visibility is determined by how high the light is and how bright it is.