Route Finding
The art of Navigation is as much about knowing where you are and having moved knowing where you have arrived.
On the Land this knowledge was never difficult since there were usually named geographical and structural reference terminal and way points. Having noted these a return route is never a difficult task.
On the Oceans the beginning of a voyage has a reference harbour starting point. Once the craft has lost all land sight around the horizon reference points rely on an estimation of the heading direction and speed of travel.
The combined effects of wind, ocean currents and swells make the estimation of speed and direction a matter of experience and error correction. Frequently errors are cancelled but without accurate fixing of geographical position the errors accumulated can often end in disaster.
Although the Arabs and Europeans developed instruments to navigate in the Mediterranean Sea, Indian Ocean and lands south and north of the Straits of Gibralta the voyages were fairly modest in endeavour.
Many ships have foundered on rocky shores because of faulty navigation. This became a very important issue in the centuries of global exploration during the 15th to 18th Century when competing European nations sought to gain new lands and exploit the newly discovered riches. Up to the beginning of the 16th Century the Atlantic and Indian Oceans had been explored by European voyagers, mostly during the 15th Century.
The biggest prize was to establish a trade route to the S.E. Asia and it took a Portuguese sailor, Ferdinand Magellan, frustrated with a lack of support from his own ruling King, approached the King of Spain, Charles 1st, for support to find a route around Cape Horn to S.E. Asia. Magellan found a passage through the islands of the Horn and set sail North-East on a blissfully calm ocean which he named Pacific. He must have caught a rare calm week or two in a normally stormy location. He kept sailing missing all the islands of Polynesia until he sighted Guam, 14,400 km in a straight line from the Straits of Magellan. Unfortunately he got caught up in a local war in the Philippine Island of Mactan while assisting the Chief of the neighbouring Island of Cebu and was killed. His 2nd-in-Command Sebastian del Cano took over and completed the circumnavigation of the globe returning to Spain around the Cape of Good Hope.
The commercial activities of Portugal in the early 15th century was a leap in progress in practical navigation. Trade expeditions were sent out by Henry the Navigator and led first to the discovery of the islands Porto Santo NE Madeira in 1418, the rediscovery of the Azores in 1427, the discovery of the Cape Verde Islands in 1447 and Sierra Leone in 1462. Henry worked to systemize the practice of navigation. In order to develop more accurate tables on the sun's declination he established an observatory at Sagres. Combined with the empirical observations gathered in oceanic seafaring, mapping winds and currents, Portuguese explorers took the lead in the long distance oceanic navigation. Henry's successor, John II continued this research, forming a committee on navigation. This group computed tables of the sun's declination and improved the mariner's astrolabe, believing it a good replacement for the cross-staff. These resources improved the ability of a navigator at sea to judge his latitude. However the lack of a means to gauge longitude on long voyages lead to huge errors in positin fixing. On the voyage of Magellan when the ships eventually arrived in the Philippines one navigator Alvo was nearly 53 degrees in error in his determination of logitude.
http://en.wikipedia.org/wiki/History_of_navigation accessed 30 May 2012
Magellan navigational instruments consisted of 35 compasses, a wooden astrolabe, six metal astrolabes, 21 wooden quadrants, and 18 hour glasses.
The Astrolabe
An astrolabe
An 18th Century Persian astrolabe – maker unknown. The points of the curved spikes on the front rete plate, mark the positions of the brightest stars. The name of each star being labeled at the base of each spike. The back plate, or mater is engraved with projected coordinate lines. From the Whipple Museum of the History of Science in Cambridge.
A wooden quadrant
For marine navigation, the earliest examples of the quadrant were found around 1460. They were not graduated in degrees but rather had the latitudes of the most common destinations directly scribed on the limb. When in use, the navigator would sail north or south until the quadrant indicated he was at the destination's latitude, turn in the direction of the destination and sail to the destination maintaining a course of constant latitude. After 1480, more of the instruments were made with limbs graduated in degrees.
http://en.wikipedia.org/wiki/Quadrant_(instrument) accessed 12/6/2012
The Cross-staff
A Jacobs cross staff
The cross-staff was difficult to use. In order to get consistent results, the observer had to position the end of the pole precisely against his cheek. He had to observe the horizon and a star in two different directions while not moving the instrument when he shifted his gaze from one to the other. In addition, observations of the sun required the navigator to look directly at the sun. This could be a painful exercise and made it difficult to obtain an accurate altitude for the sun. Mariners took to mounting smoked-glass to the ends of the transoms to reduce the glare of the sun.
http://en.wikipedia.org/wiki/Cross-staff#Cross-staff_for_navigation accessed 12/6/2012
Backstaff is the name given to any instrument that measures the altitude of the sun by the projection of a shadow. It appears that the idea for measuring the sun's altitude using back observations originated with Thomas Harriot.[2] Many types of instruments evolved from the cross-staff that can be classified as backstaves. Only the Davis quadrant remains dominant in the history of navigation instruments. Indeed, the Davis quadrant is essentially synonymous with backstaff. However, Davis was neither the first nor the last to design such an instrument and others are considered here as well.
The Davis Quadrant as it evolved by the mid 17th century.
Captain John Davis invented a version of the backstaff in 1594. Davis was a navigator who was quite familiar with the instruments of the day such as the mariner's astrolabe, the quadrant and the cross-staff. He recognized the inherent drawbacks of each and endeavoured to create a new instrument that could reduce those problems and increase the ease and accuracy of obtaining solar elevations.
Thus Davis was able to optimize the construction of the quadrant to have both a small and a large arc, allowing the effective accuracy of a single arc quadrant of large radius without making the entire instrument so large. This form of the instrument became synonymous with the backstaff. It was one of the most widely used forms of the backstaff. Continental European navigators called it the English Quadrant.
http://en.wikipedia.org/wiki/Backstaff accessed 21/6/2012
