Broadly speaking a sextant is an instrument that measures the angle between two objects that are visible. Primarily, it is used to measure the angle between a celestial body and the horizon, a process normally known as sighting the object or taking a sight. The angle measured and the time at which it was measured is then used to identify the location of the user on the grid map of the world. Thus sextants are basically navigational tools and have been successfully used by seaman and even other travelers over the years. The most common process of this is to sight the sun at noon to find the latitude of one’s location.
The instrument is called a sextant because of the scale of the angles on it which runs up to 60o. The principle of the instrument was invented by Sir Isaac Newton during his life time but the actual tool was developed later on by two individuals separately. Englishman John Hadley and Thomas Godfrey of Philadelphia and around the same time too i.e. 1730. Slowly this tool replaced the then existing navigational instrument which was called Davis quadrant also known as the backstaff. Though there is one similarity between the sextant and the Davis quadrant and that is that both of them measure angles of the celestial bodies relative to the horizon, but that is just about it as far as similarities are concerned. Unlike the backstaff, this instrument allows the direct observation of stars making it possible for anyone to use it even at night. In addition to this as a sextant measures relative angle, it does not require a steady aim, thereby making it more comfortable to use.
Over the years the sextants have evolved into basically two types, the half-horizon mirror and the whole horizon mirror. The half horizon mirror is the traditional one wherein, the celestial body and the horizon are on either side of the field view. The index mirror has to be moved until the bottom point of the celestial object is reflected in line, with the horizon reflection on the fixed mirror. On the other hand the whole horizon view mirror makes it possible to find the exact point where the horizon line and the celestial body line touch. This instrument also has a filter to protect the eyes from the sun. To tackle the adverse weather conditions like fog there is a mirror that acts as an artificial horizon by reflecting a bubble in a liquid filled tube, strange method but quite effective and accurate.
This valuable navigational instrument has to be taken care of because it is very delicate and would be damaged with the slightest of mishandling. Often it is damaged beyond repair during their functioning period as well they need to be adjusted for maintain the accuracy of the measurements. They are stored in weatherproof cases because it can be warped by the weather and become as useless as when being dropped. Even in today’s world it is a trustworthy companion when other modern navigational systems like the GPS fail, which is why it is always kept as a backup.
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References: clipperlight, wisegeek