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Dipping Distance of a Lighthouse
Dipping distance M miles = 2.08 × (√ (height of
eye h metres + √ height of light H metres )
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
The table in the back of the RYA Training Almanac or in Reeds tabulates dipping distances for various heights of eye and light.
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.
You can plot the range on the chart before arrival so you just need to take a bearing.
See also LIGHTHOUSES for more on Nominal Range, etc.
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Height of Eye metres |
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|
Height of Light |
1 |
2 |
3 |
4 |
5 |
6 |
7 |
8 |
9 |
10 |
|
10 |
8.7 |
9.5 |
10.2 |
10.8 |
11.3 |
11.7 |
12.1 |
12.5 |
12.8 |
13.2 |
|
12 |
9.3 |
10.1 |
10.8 |
11.4 |
11.9 |
12.3 |
12.7 |
13.1 |
13.4 |
13.8 |
|
14 |
9.9 |
10.7 |
11.4 |
12.0 |
12.5 |
12.9 |
13.3 |
13.7 |
14.0 |
14.4 |
|
16 |
10.4 |
11.2 |
11.9 |
12.5 |
13.0 |
13.4 |
13.8 |
14.2 |
14.5 |
14.9 |
|
18 |
10.9 |
11.7 |
12.4 |
13.0 |
13.5 |
13.9 |
14.3 |
14.7 |
15.0 |
15.4 |
|
20 |
11.4 |
12.2 |
12.9 |
13.5 |
14.0 |
14.4 |
14.8 |
15.2 |
15.5 |
15.9 |
|
22 |
11.9 |
12.7 |
13.4 |
14.0 |
14.5 |
14.9 |
15.3 |
15.7 |
16.0 |
16.4 |
|
24 |
12.3 |
13.1 |
13.8 |
14.4 |
14.9 |
15.3 |
15.7 |
16.1 |
16.4 |
17.0 |
|
26 |
12.7 |
13.5 |
14.2 |
14.8 |
15.3 |
15.7 |
16.1 |
16.5 |
16.8 |
17.2 |
|
28 |
13.1 |
13.9 |
14.6 |
15.2 |
15.7 |
16.1 |
16.5 |
16.9 |
17.2 |
17.6 |
|
30 |
13.5 |
14.3 |
15.0 |
15.6 |
16.1 |
16.5 |
16.9 |
17.2 |
17.6 |
18.0 |
|
32 |
13.9 |
14.7 |
15.4 |
16.0 |
16.5 |
16.9 |
17.3 |
17.7 |
18.0 |
18.4 |
|
34 |
14.2 |
15.0 |
15.7 |
16.3 |
16.8 |
17.2 |
17.6 |
18.0 |
18.3 |
18.7 |
|
36 |
14.6 |
15.4 |
16.1 |
16.7 |
17.2 |
17.6 |
18.0 |
18.4 |
18.7 |
19.1 |
|
38 |
14.9 |
15.7 |
16.4 |
17.0 |
17.5 |
17.9 |
18.3 |
18.7 |
19.0 |
19.4 |
|
40 |
15.3 |
16.1 |
16.8 |
17.4 |
17.9 |
18.2 |
18.7 |
19.1 |
19.4 |
19.8 |
|
42 |
15.6 |
16.4 |
17.1 |
17.7 |
18.2 |
18.6 |
19.0 |
19.4 |
19.7 |
20.1 |
|
44 |
15.9 |
16.7 |
17.4 |
18.0 |
19.5 |
18.9 |
19.3 |
19.7 |
20.0 |
20.4 |
|
46 |
16.2 |
17.0 |
17.7 |
18.3 |
18.8 |
19.2 |
19.6 |
20.0 |
20.3 |
20.7 |
|
48 |
16.5 |
17.3 |
18.0 |
18.6 |
19.1 |
19.5 |
19.9 |
20.3 |
20.6 |
21.0 |
|
50 |
16.8 |
17.6 |
18.3 |
18.9 |
19.4 |
19.8 |
20.2 |
20.6 |
20.9 |
21.3 |
|
55 |
17.5 |
18.3 |
19.0 |
19.6 |
20.1 |
20.5 |
20.9 |
21.3 |
21.6 |
22.0 |
|
60 |
18.2 |
19.0 |
19.7 |
20.3 |
20.8 |
21.2 |
21.6 |
22.0 |
22.3 |
22.7 |
|
65 |
18.9 |
19.7 |
20.4 |
21.0 |
21.5 |
21.9 |
22.3 |
22.7 |
23.0 |
23.4 |
|
70 |
19.5 |
20.3 |
21.0 |
21.6 |
22.1 |
22.5 |
22.9 |
23.2 |
23.6 |
24.0 |
|
75 |
20.1 |
20.9 |
21.6 |
22.2 |
22.7 |
23.1 |
23.5 |
23.9 |
24.2 |
24.6 |
|
80 |
20.7 |
21.5 |
22.2 |
22.8 |
23.3 |
23.7 |
24.1 |
24.5 |
24.8 |
25.2 |
|
85 |
21.3 |
22.1 |
22.8 |
23.4 |
23.9 |
24.3 |
24.7 |
15.1 |
25.4 |
25.8 |
|
90 |
21.8 |
22.6 |
23.3 |
23.9 |
24.4 |
24.8 |
25.2 |
15.6 |
25.9 |
26.3 |
|
95 |
22.4 |
23.2 |
23.9 |
24.5 |
25.0 |
25.4 |
25.8 |
16.2 |
26.5 |
26.9 |
Posted 5th December 2012