Danger Angle

A danger angle is a horizontal or vertical sextant angle set to a calculated limit, ensuring that as long as the observed angle remains below (horizontal) or above (vertical) the danger angle, the vessel is clear of a specific hazard. A horizontal danger angle uses the angle subtended between two charted objects to define a circle of position; the danger circle encloses the hazard, and the vessel must remain outside it. A vertical danger angle uses the observed elevation of a lighthouse or cliff above the horizon to determine the minimum safe distance from the coast. Bowditch (American Practical Navigator) provides worked examples and formulas for both types. Danger angles are plotted on the chart during passage planning for any coastal passage near charted shoals or rocks visible from seaward. The OOW monitors the relevant sextant angle during the approach, maintaining a continuous safety check that is independent of compass error and GPS accuracy. The technique is particularly useful when passing headlands with an offshore reef, where a prudent navigator sets a horizontal danger angle to avoid the shallow water without deviating unnecessarily from the track. Familiarity with danger angle calculations is a STCW-assessed navigational skill.