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Marine Navigational Equipment: The Sextant

Is the sextant a carthorse in the 21st century? Abstract The purpose of this report is to decide whether the sextant is outdated and effectively useless in the modern age or whether it still has a use in modern navigation. To decide this I explored modern navigation techniques, which is mostly done by global positioning systems, I researched the way that these work and the methodology behind the use of a sextant. The conclusion that I came to was that modern day sextant use is rare unless practicing for an emergency.

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Since its release GPS has dominated and is now the main source of navigation on most vessels (big or mall). The reason for this dominance is the reliability and effectiveness of the GPS system. So unless there is a major problem it leaves the sextant as a practically useless device aboard a ship. This means that fewer people are learning how to use a sextant and are carrying spare GPS as a backup device rather than the sextant. However the sextant has been used for centuries as it is a reliable device but takes skill and knowledge to use which is dying out of modern sailing.

Introduction There are many different ways of navigating whilst at sea most of them involve taking bearing from a landmark that you can see and then plotting that bearing on a chart and triangulating your position using landmarks. However this becomes hard when you are out in open water and cannot see any noticeable landmarks. This then requires another method of positioning such as a sextant or GPS system. The sextant has been used for century’s and is one of the oldest and most reliable forms of navigation.

This essay is about the differences and modern day relevance that the sextant has on current navigation techniques. The sextant The sextant was developed in 1760 by a man called John Hadley (lfland 1998). He first developed a device that he called the Octant that is the basic design of the sextant. Until this point sailors had used several similar devices to determine the latitude of the vessel but determining longitude was much harder. This was due to it being necessary for the individual to know the exact time.

Until a method of precise timekeeping was developed it was very hard for navigators to work out the longitude of the vessel causing many shipwrecks. They developed a technique that allowed them to work out the time by measuring the angle between the moon and a celestial here to be many mistakes. In 1759 John Bird built the first working sextant by making the original octant bigger allowing it to take reading up to 120 degrees rather than the octants 80 (lfland 1998). This allowed for more accurate measurements to be taken of celestial and lunar observations.

Sextants work by measuring the angle between any two objects that are visible. By measuring the angle between the sun and the horizon you can determine the latitude based on the time of day. Brighthub Engineering. (2013) “The optical principle used in a sextant is this: given that a ray of light is reflected rom two mirrors in succession, then the angle between the first and last direction of the ray is twice the angle between the mirrors. (Manu 2013) ” The time and the angle of when the object was measured are used to gain a position line on a chart.

The sun is mainly used as a celestial object with the readings being taken at solar noon. There are several different terms for taking the measurement these include sighting/ shooting the object and taking a sight. The other main celestial object that is used is Polaris (the North Star) with the reading being taken at night. These readings along ith a series of calculations allow the user to work out the latitude of ones vessel. Global positioning system (GPS) GPS is a modern system that is used as a navigation guide this system is more commonly considered a help to navigating rather than a true positioning system.

This system works by having between 24-32 satellites orbiting the earth continuously. The system was designed in the 1960’s during the cold war with only military use until 1983 when the then president sanctioned that the public could use it too. The GPS device uses radio signals to contact 3 or more of the satellites to triangulate its osition relative to all 3 satellites. Trilateration then occurs where the device calculates the distance between your position and each satellite. This allows for even greater accuracy and the more satellites there are in range the more accurate the position will be.

The system has now been so perfected that wherever you are on the planet you should be within range of at least 4 satellites therefore allowing you to accurately determine your position anywhere in the world. (Physics. org) There are other positioning systems that run alongside the GPS system such as GLONASS this is Russian based satellite navigation system and is the only other system that offers worldwide coverage. It doesn’t however have access to as many satellites as the GPS system making it less accurate yet offers a separate system to GPS which is an American based system. Federal space agency 2012) The EIJ is currently developing a new system called the Galileo System. This will be a civilian based system using 30+ satellites. This aims to be completed this year and is attempting to be the most accurate of the global positioning systems (Althos 2009) Conclusion the instrument. Most modern day navigators are trained to use the sextant however hey tend not too while afloat on modern ships. This is due to the reliability of GPS and the speed at which it gives an accurate position.

A sextant can be used in almost any situation to predict a geographical position it does not rely on electricity ( not a certainty whilst at sea) its only reliance is the skill of the user. There are several ways that the measurement can be incorrectly taken given that there are no errors in manufacture of the sextant (a problem with older sextants. ) These include refraction, ( the light in the atmosphere refracts causing incorrect readings) dip, (the height of he eye of the observer) semi diameter (where the navigator cannot take the reading from the centre of the celestial object. and parallax ( the altitude reading is taken from the Earth’s surface not the centre which is the true altitude. ) All of the above problems however can be taken out of the calculation using data found in almanacs and then applied to the answer (Manu 2013). Most of the problems relating to the accuracy and the reliability of the given position are based around human error from the person taking the reading. Therefore it is hard to say how accurate sextant readings are as it could change from user to user.

This is why GPS is still only seen as an aid to navigation not as a navigating method as even though it is accurate quick and simple to use it still is not 100% reliable. However there are still some places at certain times that it will not be possible to determine a position from GPS due to the lack of a satellite or electrical problems. This is where the skills of the navigator will come into work and the sextant is then a reliable tool. The opinion of most people is that instead of using the sextant as a backup system there should be simply a backup GPS system that can be used in the case of an emergency.

However today ven US naval navigators are made to take sextant readings every day to make sure that they are in a relative position and that the navigator is able to effectively take the reading if required in an emergency. This is the main problem with modern day navigation the GPS system is so reliable that it rarely fails, meaning that sextant use is irrelevant until an event occurs to disable the GPS. 1 . Althos. (2009). Global Positioning System. Available: http:// www. wirelessdictionary. com/wireless_dictionary_GPS_definition. html. Last accessed 26/10/13. 2. Brighthub Engineering. (2013). The sextant. Available: http:// img. hs4. om/77/ff77fa97f24deb09ba2519f81 e68f11240fdc09eb8_large. Jpg. Last accessed 27/10/2013. 3. Federal space agency. (2012). Navigation space systems. Available: http://glonass-iac. ru/en/guide/. Last accessed 27/10/2013. 4. Manu. (2013). Marine Navigational Equipment: The Sextant. Available: http:// www. brighthubengineering. com/seafaring/31615-marine-navigational-equipment- the-sextant]. Last accessed 28/10/2013. 5. Peter lfland. (1998). The History of the Sextant. Available: http://www. mat. uc. pt/??”helios/Mestre/NovembOO/H61 iflan. htm. Last accessed 23/10/2013. 6. Physics. org. 0. How does GPS work?. Available: http://


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