- A lighthouse ‘dips’ when you can first see the ‘blink’ above the horizon (or when it just disappears).
- Before that you can see the ‘loom’ when it is still below the horizon (like the sun before dawn).
- Height of eye above sea level in a yacht is about 2.5 – 3 metres
- If the tide is below MHWS it increases the height of the light above sea level while the height of eye stays the same, so you can see the light from a greater distance.
- A dipping distance range and a bearing on the light is a fairly accurate position fix.
- The table in the back of the Almanac or in Reeds tabulates dipping distances for various heights of eye and light.
Dipping distance M miles = 2.08 × (√ (height of eye h metres + √ height of light H metres )
Points to pounder regarding rising and dipping of light
Because of the curve of the Earth’s surface, the light of major navigation marks (light houses and ships) will be out of sight even though the light may be powerful enough to reach as far a vessel. The distance you can see a major light is influenced by the height of the light and the height of eye of the observer as well as the power of the light itself.
When your height of eye is too low, but the light is powerful enough to reach as far as your position you will see a loom of light, which is the beam of the lighthouse shining in an arc as it crosses the sky. If it is a really dark moonless night, lights can be seen at a considerable range, it is even possible to take a bearing on the light in these conditions. On one cross channel passage I was able to take a fix from lights that were 50M away on the French and English coasts.
This effect will continue, until suddenly the light appears over the horizon. At this point, it is very simple to calculate the distance from the light, using a table in the almanac. As the curve of the Earth is reasonably constant, nautical almanacs contain a Rising and Dipping light table which easily gives the distance off from a major light.
We can take the bearing, so to plot a position on the chart all we do is plot the bearing, then measure off the distance from the light, that is the vessel’s position. Look inside the back cover of the Training Almanac (copy below). The table is one we use to calculate our distance off from a light when it just appears (or disappears) over the horizon.