Marine Sextant facts I bet you did not know !




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    Marine Sextant facts I bet you did not know !


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    Whether you use a Marine Sextant or not, yet it still affects your maritime life on-board a ship. Its a pity that such precise and wonderful nautical instrument has become a facebook profile pic for navigational officers. No wonder people have not only forgotten how to use a marine sextant but how it affects their life indirectly till today. Here is a collection of such forgotten facts.

    1. Navigational watches were set as per Sextant
      Navigational watch schedule on most ships are
      0800-1200 / 2000-2400 : Third officer
      1200-1600 / 2400-0400 : Second Officer
      1600-2000 / 0400-0800 : Chief Officer
      While I am not explaining how to use Sextant this time, but trust all mariners will agree that "Intercept" and "Long by chron" method of sight calculation of sun may not require the same amount of precision as "Mer Alt" of sun or sight calculation of stars !

      A young Third officer with relative less experience, in older days, was kept at these watch timing as he cannot fix a ship positions at night due to lack of clear horizon and in the morning he will get only 1 position line with the sun.
      Relatively more experienced Second mate can take a mer-alt of the sun to get an accurate latitude of the ship.
      Like wise the most experienced navigating officer, Chief Officer, would get twilight both at morning and evening making it possible to fix ship's position with 3 or more stars within few minutes.

    2. Marine chronometer was invented because of Marine Sextant
      Precise calculation of time was essential for best position fixing using Celestial bodies. Most clocks in 18th century were gravity based, pendulum clocks which were worthless on rolling ships. Prize worth million pounds (at today date) was announced by British government (Longitude Prize) in 1714 depending upon the accuracy of marine clock invented. After almost 50 years of failed attempts, a British carpenter named John Harrison invented in 1761, the most expensive clock (almost 30% of ships cost then) known as a marine chronometer. The first true chronometer was the life work of one man, John Harrison, spanning 31 years of persistent experimentation and testing that revolutionized naval (and later aerial) navigation and enabling the Age of Discovery and Colonialism to accelerate.
      Source : Marine Chronometer
      Since its accuracy mattered so much in those non GPS age, a chronometer error log was started to check and keep the chronometer timing synchronized with coast radio station. The truth is, a marine chronometer later got modified/improved to pocket watch and wrist watch.

    3. Few miles of intercept is OK
      While "Practical Navigation" book did taught the young Deck Cadets about the marine sextant, it has also made them believe that answer has to be within 2 to 3 miles. My friend, sailors used to be 50 miles off the track how can they get 1 mile intercept ? Infact in 18th century when challenge to make the precise clock was laid down, the parameter was set to have max 30 miles error. Remember we are talking about DR Positions, no data about set and drift. Today 2nd mates set XTE of 0.1 miles and your ECDIS starts to cry when your ship touch it !

    4. Noon report position depended on Marine Sextant
      Noon reports are send to the concerned parties everyday. In those non-GPS days, the ships position in open sea depended only upon the sextant. While for the whole world all calculations are done from end of the day (read midnight), for ship its the noon time. For sure a major reason is that all departments are awake at noon, unlike night time, but a part of it is also depends on the accurate latitude observation from the Sextant at noon.

    5. Sextant is not only for celestial navigation
      Mariners have forgotten that a marine sextant can also be used for terrestrial navigation. Yes, I am talking of the horizontal Sextant angle and vertical sextant angle. In fact a marine sextant is the only device which can determine your distance from shore lights if your electronic position fixing devices like RADAR and GPS fails.
    On the technical side there are more questions to ponder :
    • Why is Long by Chron named such and what has chronometer to do with it ?
    • When to use Long by Chron over Intercept method for position fixing ?
    • When will your sextant sight not make an arc on horizon upon swinging ?
    • What assumptions do we make while doing sight calculation and when your Position line may not be perpendicular to Azimuth ?
    With 2 GPS on-board, assuming that GPS may fail is being impractical, even then, your Android phone GPS works without internet and can be used with moderate accuracy for finding your position. No doubt these hi-tech devices have there own importance and are far more accurate in position fixing but remember that a Marine Sextant and your Nautical Almanac is purely mechanical things with nil external dependency (read electricity/power).

    While I agree that expecting officers to use it during port approaches for position fixing is not practical but just because nobody uses it on-board, the awesomeness of a Marine Sextant to find your position on this vast earth doesn't diminish ! Feel free to add more information which I may have missed.  Marine Sextant facts I bet you did not know

    Rishi Kashyap | 5 years ago | EDIT | REPLY



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