| You've been underway for days. It's 2am and you're | | | | 4.7 miles. You have increased your horizon distance |
| on watch in the blackest of nights, straining through | | | | from 2.9 miles to 4.7 miles! |
| binoculars to pick up the blink of the light ahead that | | | | How to Find Geographic Range of a Light for Landfall |
| marks the channel entrance. | | | | Combine your own HE with that of any other object |
| Your nautical GPS chart plotter says that it's visible | | | | of known height, and you can determine when you |
| from 18 miles away. Now you are 16 miles | | | | can expect to sight the object. Let's use the opening |
| away--according to the electronics. What went wrong | | | | scenario. |
| with your sailing navigation? Are you lost? | | | | Imagine that we are 6 feet tall, so our HE will be 2.9 |
| Know Your Horizon Distance | | | | miles. Next, look at the description of the light chart |
| Have you ever stood on the beach on a crystal clear | | | | symbol. It says: Fl 20s 86ft 18M. The light beam stands |
| day and looked out at the horizon. It looks like you can | | | | 86 feet above the water. The abbreviation indicates |
| see for a million miles. But did you know that the | | | | that the light can be seen from 18 miles away. But |
| average human can see only 2.9 miles to the horizon? | | | | that's a theoretical distance based on the candlepower |
| The curvature of the earth causes your "Horizon | | | | of the light--but does not take into account the |
| Distance" to be much shorter. You can increase your | | | | curvature of the earth! |
| horizon distance if you elevate yourself. This is why | | | | Instead, use the light's height to calculate the HE: |
| lifeguard towers are higher on beaches. | | | | square root of 86 = 9.27 X 1.17 = 10.9 miles. Combine |
| This same phenomenon happens with lighted aids to | | | | your HE and the light's HE. 2.9 miles + 10.9 miles = 13.8 |
| navigation like buoys, small light structures, or towering | | | | miles sighting distance. |
| lighthouses. The distance their lights can be seen | | | | Prepare to Make Landfall |
| depends on the height of the light beam above the | | | | Measure on your nautical chart or chart plotter 13.8 |
| water surface. | | | | nautical miles from the charted light symbol, back along |
| In chart navigation, your height or the height of a light is | | | | your sailing trackline. Use binoculars to scan the horizon |
| called "height of eye", or HE. To calculate the distance | | | | ahead when you arrive at this position. You should see |
| to the horizon from your HE, use this simple calculation: | | | | the friendly wink of your light ahead. Remember that |
| square root of the object X 1.17. So, if you are 6 feet | | | | this calculation does not take into account haze, fog, or |
| tall, you take the square root of 6 feet = 2.45 X 1.17 = | | | | rough weather. Any of these could reduce the |
| 2.87 or 2.9 miles. | | | | distance of your horizon. |
| If you want to see further, you could walk over to the | | | | Use these easy sailing navigation secrets to boost |
| lifeguard tower and climb up. Let's say the tower | | | | your chart navigation to give you confidence and |
| stands 10 feet off the surface. You climb to the top of | | | | peace-of-mind. You will become a more skilled and |
| the tower and stand up. Now you have elevated your | | | | knowledgeable sailing skipper--wherever in the world |
| HE to 16 feet (10 feet tower height + 6 feet of your | | | | you choose to cruise. |
| height). Take the square root of 16 = 4 X 1.17 = 4.68 or | | | | |