The sextant is a very important item for the navigator. However, like everything else on this planet, it had to evolve. The very beginnings of the sextant were a bit more unrefined and couldn’t quit hit the nail on the head. While modern day brass sextants are outclassed by the global positioning system, which many navigators criticize for it’s many faults.
The need for a navigational tool in the nautical realm arose as a way for exploration to take place on the treacherous uncharted seas. In order to use a sextant, a few things had to be done first. For instance, an almanac had to exist that included the location of celestial objects and bodies in relation to our planet at every single hour of every single day for many years. Furthermore, a device capable of measuring time to a precise point must be utilized. This is called a chronometer. Cartographers were necessary to plotting and charting maps so that longitude and latitude could be found and marked by the observer. A simple mathematical formula to transform the relation of the celestial body and the horizon with the navigators position would also be needed. With these things in place the sextant would be the final key in locating an accurate position of one’s self on the globe.
However, long before the invention of the sextant, navigators had to rely on Polaris to find their way back to their home port. The Arabs were very good at doing this, and used a device known as a Kamal to their advantage. The Kamal relied on a short rope and an object that sighted Polaris at the top and the horizon at the bottom. A knot was tied at the exact location of which he could align the two. When returning from a voyage back home the navigator would adjust his sailing in order to bring Polaris into the same position he had when he left port.
In the 10th century Arabs gave Europe two very important astronomical devices that would lead to the sextant – the astrolabe and the quadrant. The quadrant was especially useful to the Portuguese explorers. Explorers such as Columbus would mark off the points of altitude witnessed of Polaris similar to the Arab way of tying knots in the Kamal. This would be done in ports that the sailors wished to return to, and would eventually the alturas would become published so other sailors could find their way around the coasts of Europe and Africa.
The astrolabe was a remarkable device for use on sea as it could retain its position amongst the ever changing harsh conditions at sea. It was used for more than 200 years because of this. The astrolabe used a circular scale, and rotatable alidade with sighting pinnules. When held at eye level a celestial object could be viewed through the pinnules and the altitude read from the point of crossing by the alidade on the scale.
The sextant is a fairly complicated device. It relies on the use of a telescope for which one uses to spot the horizon and superimpose a celestial body onto it to determine one’s location. This is done by way of two opposing mirrors, one of which is attached to a moving scale that allows the light from a celestial object to be reflected onto the image of the horizon. However, if the sextant is not properly calibrated then the location the navigator perceives will be incorrect. This can be a very deadly fault, especially when at sea and without any other means of calculating location. This is why it is important to have the sextant that is intended for repeated use calibrated as much as needed.
Calibration of the sextant should be done at a facility that specializes in such. For instance, a United States Air Force aircraft bubble sextant (very different from a nautical sextant) should only be calibrated in a proper military-maintenance facility. This is done by propping the sextant on a calibration device, timing the mechanism’s average, and setting the elevation wheel to angles of 0, 30, 45, and 60 to check the HS reading. This will then have to be examined against preset marks to determine the correct calibration of the sextant.
Furthermore, you can calibrate a sextant by setting it on a table a certain distance from a wall and checking its elevation as opposed to pre-measured marks on the opposing wall. For example, if the sextant is about 5 feet away from the wall, then marking five feet higher than the height of the sextant’s eyepiece should show an elevation of 45 degrees. An improper reading will be considered the index error, and should be taken into account accordingly.
When using the marine sextant, calibration can be done by positioning the alidade to the 0 degree mark. Next, you will locate the horizon through the eyepiece. The image in the mirror should be aligned with the horizon. If not, then you will need to use the adjustment screws to position the mirror so that the image is calibrated correctly. You will know proper calibration when the two images are perfectly aligned with one another.
It is important that the sextant is calibrated before each use in order to make sure that the location you are getting is as accurate as possible. You must also take into account errors that may occur due to certain conditions. Calibrating the sextant will mean the difference between accurately knowing where you stand, and being totally lost. Luckily with the advent of a global positioning satellite, those who have a hard time calibrating a sextant, will be able to utilize GPS to their advantage. Though, GPS is extremely unreliable for its limited power source and constant loss of satellite reception.
Upon first glance at the sextant one may feel intimidated by its many accessories. And the truth is, you really shouldn’t be. The basic use of the sextant is fairly easy; however, it is the charting, locating, and plotting that is the hardest part of using a sextant properly. The sextant works to find your position relative to a celestial object, such as the sun, moon, or stars. The celestial object is positioned over the horizon by way of two mirrors on the sextant that allow the navigator to determine the latitude they are currently in. The sextant can even work properly while on a moving ship. This is because the sextant sees the horizon directly and as unmoving, while the celestial object is viewed through two opposing mirrors that take into account the motion of the sextant from the reflection.
The sextant’s scale is one sixth that of a complete circle, which gives it the name “sextant”. The sextant’s two mirrors work in conjunction with the telescope, scale, and filters to determine an extremely accurate location. The mirror through which the navigator sees the horizon is half silvered to allow for light to come through. The second mirror opposing the first is attached to a movable arm that glides along the scale. The arm is moved so that the second mirror positions a celestial object’s light into the first mirror to give the appearance that the object is directly on the horizon. The angle between the two superimposed objects can be told by the points on the scale. This is the basic way of how to use a sextant.
Navigating the seas with a sextant is a very tricky affair that involves a lot of recalculating and astronomical references. The way to find latitude is by measuring the angle between sun and horizon while the sun sits at it’s highest point. The sun will be in the proper position at noon. From there you will have to reference a table compiled by astronomers that reveals where the sun should be at that particular time on that particular day. Due to the Earth’s steady motion, navigators can deduce that for every hour the sun will move 15 degrees. To measure this accurately and effectively navigators make use of a chronometer. A chronometer is basically an extremely accurate clock.
Using the sextant for celestial navigation as it was intended is a bit tougher. Celestial navigation measures angles in the sky in relation to the horizon in order to find the navigators global positioning. The position on Earth where the celestial object is aligned is known as the sub point, the location of which is found by referring to tables.
The measured angle of the celestial object and the horizon is directly correlated to the celestial object’s sub point and the navigators position. The measurement of this correlation defines a circle known as the celestial line of position (LOP). Of which the location and size is discovered by way of mathematical equations.