Sextant angles for coastal navigation
February 19th 2007 03:23
Sextant angles for coastal navigation:
The sextant is a great tool for the navigator. It is wrongly thought that it is just for the ocean navigator who uses stars, planets the sun and moon to navigate by. The navigator hugging the coast is just as grateful for the sextant. It has no need for magnetism so it is not affected by the magnetic pull of the earth, by steel hulls or tools laying carelessly as they may be around compasses. A simple pair of mirrors reflecting an object to an eye piece that is set to align with the horizon or edge of the object having its angle measured. No great need for a brass masterpiece with a midget telescope in a fancy timber box either. These days there are good plastic versions that are accurate enough for the average punter.
The simplest calculation is “distance off” How far off from a lighthouse, cliff is our vessel? First identify the object you are going to “shoot” with your sextant on your chart. All light houses, lights or significant cliff tops are given as a height above sea level. Aim the sextant at the top of the object and by bringing down the reflection with the top mirror to the bottom which is aligned with sea level, using the arm on the side of the sextant, you will get an angle. This angle is shown on a scale next to the arm. Read off the degrees and minutes of the angle. (To read minutes there is a vernier scale that is a brief lesson in itself.)
Say the lighthouse is seventy-six metres above sea level. The scale on your sextant shows 4deg 36min. There are two methods to use here. Exact and rule of thumb. Lets be a bit cavalier and use the rule of thumb. Convert the angle to all minutes. So you have 4 degrees multiply by sixty to convert then add the odd thirty-six minutes. Total two hundred and seventy six minutes in total. Now it is necessary to multiply the height by 1.852 so we end up with a figure that relates to nautical miles but that is a bit hard to do in your head so why not just multiply by two as we are sort of feeling our way. That makes seventy six times two which equals one hundred and fifty two. This is divided by the angle of minutes so one hundred and fifty two divided by two hundred and seventy six (you will need a pen and paper or calculator probably) is .55 nautical miles. Quite close to the lighthouse! Now if you want to be exact we use old Pythagoras. The tan of the angle opposite (that is the one running between the hypotenuse and the adjacent side is equal to the length of the opposite side over the adjacent side. The Adjacent side is the side we are trying to determine the length of. With a bit of algebra we end up with seventy six divided by the tan of four degrees thirty six minutes which needs to be divided by 1852 to convert from metres to nautical miles the answer is .567 nautical miles, only half a cat’s whisker different.
It all sounds a bit daunting but when you use a cheap ($96) plastic sextant and end up with a reliable answer just using rule of thumb it makes you quite confident.
Now this has only told you how far off the lighthouse you are. You will need to draw a circle .55 nautical miles around the lighthouse, (obviously only on the water part of your chart) and your vessel is on that arc somewhere. Taking a relative bearing from you vessel to the light house will give you a back bearing you can use to cross the arc. Where those lines cross is where you are.
The sextant can also be used horizontally but the system is beyond my explaining in a blog like this so you would understand. It is however simple and easy to learn.
Happy safe navigating.
The sextant is a great tool for the navigator. It is wrongly thought that it is just for the ocean navigator who uses stars, planets the sun and moon to navigate by. The navigator hugging the coast is just as grateful for the sextant. It has no need for magnetism so it is not affected by the magnetic pull of the earth, by steel hulls or tools laying carelessly as they may be around compasses. A simple pair of mirrors reflecting an object to an eye piece that is set to align with the horizon or edge of the object having its angle measured. No great need for a brass masterpiece with a midget telescope in a fancy timber box either. These days there are good plastic versions that are accurate enough for the average punter.
The simplest calculation is “distance off” How far off from a lighthouse, cliff is our vessel? First identify the object you are going to “shoot” with your sextant on your chart. All light houses, lights or significant cliff tops are given as a height above sea level. Aim the sextant at the top of the object and by bringing down the reflection with the top mirror to the bottom which is aligned with sea level, using the arm on the side of the sextant, you will get an angle. This angle is shown on a scale next to the arm. Read off the degrees and minutes of the angle. (To read minutes there is a vernier scale that is a brief lesson in itself.)
Say the lighthouse is seventy-six metres above sea level. The scale on your sextant shows 4deg 36min. There are two methods to use here. Exact and rule of thumb. Lets be a bit cavalier and use the rule of thumb. Convert the angle to all minutes. So you have 4 degrees multiply by sixty to convert then add the odd thirty-six minutes. Total two hundred and seventy six minutes in total. Now it is necessary to multiply the height by 1.852 so we end up with a figure that relates to nautical miles but that is a bit hard to do in your head so why not just multiply by two as we are sort of feeling our way. That makes seventy six times two which equals one hundred and fifty two. This is divided by the angle of minutes so one hundred and fifty two divided by two hundred and seventy six (you will need a pen and paper or calculator probably) is .55 nautical miles. Quite close to the lighthouse! Now if you want to be exact we use old Pythagoras. The tan of the angle opposite (that is the one running between the hypotenuse and the adjacent side is equal to the length of the opposite side over the adjacent side. The Adjacent side is the side we are trying to determine the length of. With a bit of algebra we end up with seventy six divided by the tan of four degrees thirty six minutes which needs to be divided by 1852 to convert from metres to nautical miles the answer is .567 nautical miles, only half a cat’s whisker different.
It all sounds a bit daunting but when you use a cheap ($96) plastic sextant and end up with a reliable answer just using rule of thumb it makes you quite confident.
Now this has only told you how far off the lighthouse you are. You will need to draw a circle .55 nautical miles around the lighthouse, (obviously only on the water part of your chart) and your vessel is on that arc somewhere. Taking a relative bearing from you vessel to the light house will give you a back bearing you can use to cross the arc. Where those lines cross is where you are.
The sextant can also be used horizontally but the system is beyond my explaining in a blog like this so you would understand. It is however simple and easy to learn.
Happy safe navigating.
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