A position circle can be obtained from the rising or dipping of navigation lights over the horizon. The South African List of Lights and Radio Signals has a table of geographical ranges of lights. By entering the table with the height of the observer’s eye above water level and the height of the light above water level, the rising/dipping range of the light can be obtained. Using this range a position circle centred on the position of the light can be drawn on the chart. The formula for calculating the range is also given, ie

Ranges of lights.

There are three classifications of ranges of navigation lights:

1. Geographic range: The range of a light from the observer as it rises or dips over the horizon. It depends on the height of the light, the height of the observer’s eye and the atmospheric refraction.
2. Luminous range: The luminous range of a light is the expected horizontal visible range of a light depending on its luminous intensity and the clarity of the air and to a lesser extent on the character, length of flashes and the interval between them. The luminous range diagram in the SA List of Lights and Radio Signals provides the luminous range of lights in miles.
3. Nominal range: This is the luminous range in miles in a homogeneous atmosphere with a standard meteorological visibility of 10 miles.