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The Background and History of Navagation
The need for celestial or astro-navigation is being questioned more and more as GPS sets become cheaper and in common use. GPS is here to stay, is very "user friendly", reliable and extremely accurate. Astro-nav is not user friendly, not very accurate and can only be used in ships when both celestial bodies and the horizon are simultaneously visible. Its use is promoted because it is the only system available to the mariner crossing oceans that does not rely on electronics. It requires a sextant, an accurate timepiece, a nautical almanac and a set of mathematical tables plus, of course, the knowledge of how to use them. Total electronics failures have been reported in yachts suffering lightening strikes, even spare portable GPS sets, switch-off and in the locker have been affected. Further, it is a fundamental of navigation never to rely absolutely on one system. Those students entering the course as the start of a professional career at sea require this knowledge for career advancement. Those, whose reasons may be mostly recreational, normally find the subject challenging and rewarding. Believe it or not most students enjoy it!!
Why do we sail when it is far more convenient and simple to motor?? Because we find it more aesthetically pleasing; we like it. Same with astro!
The Instruments
Mariners have been determining their position using celestial navigation with varying degrees of accuracy for many years. Over time a variety of different instruments have been employed.
The Marine Quadrant
Gunter's marine quadrant was in use from around 1450 to 1650. Originally the quadrant was made from a quarter circle of wood, brass or copper with a graduated scale marked on the 
circumference from 0º to 90. The celestial body is sighted across the two projections. Altitude is given directly by reading off where the plumb line cross the scale along the curved edge.
The Cross Staff
The cross-staff was used by ancient astronomers to measure angular separation between celestial objects. It also served a secondary function of aiding to determine distances, latitudes and heights of terrestrial objects such as hills, mountains and islands. The earliest recorded use of this object was by the German mathematician, cartographer and navigator Martin Behaim, and this was in the 1480s.
In simple terms, the cross-staff is a cross made from two pieces of wood, one long and the other short, with the shorter piece being movable. The longer piece comprises a long, squarish staff, made of hardwood, while the shorter length, also called the crosspiece or transom is made of the same material and it slides along the staff. The staff is then held up to the eye level and viewed. The crosspiece is adjusted so that the celestial object is just visible above the upper part of the crosspiece.
The cross-staff was simple to construct, light and portable, making it a handy and cheap navigational tool. It did frustrate some who were trying to measure the sun's altitude in the sky, as that required them to look into the sun. Of course, this problem was soon overcome by the use of smoked glass for the crosspiece. Navigators were also susceptible to parallax errors.
Marine Astrolabe
The history of the astrolabe begins more than two thousand years ago. The principles of the astrolabe projection were known before 150 B.C., and true astrolabes were made before A.D. 400. The astrolabe was highly developed in the Islamic world by 800 and was introduced to Europe from Islamic Spain (Andalusia) in the early 12th century. It was the most popular astronomical instrument until about 1650, when it was replaced by more specialized and accurate instruments.
The Mariner's Astrolabe, which was popular in the late 15th and early 16th centuries, was a simple brass ring, graduated in degrees with a rotating alidade for sighting the Sun or a star. The ring was cast brass, quite heavy and cut away to keep it from blowing around in the wind. It was not a very good instrument and errors of four or five degrees were common.
The Astrolabe was used by lining up the star through the two sighting holes on the movable adlidade, the zenith distance of the body could then be read directly from the graduated scale.
The Kamal
This extremely simple instrument appeared in the Middle Ages and was still in use in 1875 in the Indian Ocean. A Kamal is a small recantgualr piece of wood with a notch cut in the centre of one side. A fixed length of cord is stretched between the observers teeth and the middle of the board. The cord length is adjusted to match the altitude of of the Pole Star at the destination port, making it possible to navigate along a parallel of latitude.
The Sextant
All of the instruments described above were at best accurate to 1º (in other words 60nautical miles). The modern marine sextant is a precision optical instrument enabling observer to achieve high levels of accuracy.
