Introduction of Navigational Instruments
With the development of shipbuilding and the increase in knowledge of astronomy, there was increased use of instruments. The cross-staff was used to find latitude early in the 15th cent. It consisted of two pieces of wood, the cross at right angles to, and sliding on, the staff. At each end of the 26-in. (66-cm) cross a small hole was bored, and at the end of the staff a sight was fixed. To measure the altitude of a heavenly body, the instrument was sighted in that direction, and the cross was moved forward or back until the heavenly body appeared through the upper hole and the horizon through the lower. The altitude could then be read on a scale marked on the staff. Another device used for finding latitude was the astrolabe. Both were far from accurate.
The navigating equipment carried by Columbus probably was simply a compass, a cross-staff, and a table of the sun's declination. Vasco da Gama on his first voyage around the Cape of Good Hope in 1497 used an astrolabe. The Flemish geographer G. K. Mercator's work in improving charts at the end of the 16th cent., the works of the Spanish scientist Martín Cortés during the same period, the determining of the earth's circumference, and the introduction of logarithms at the beginning of the 17th cent. by the Scottish mathematician John Napier all helped advance navigation.
By the middle of the 18th cent. a quadrant could be used to find latitude and a log line and half-minute glass could help keep track of distance traveled; but the problem of finding the longitude remained unsolved until the invention of the chronometer. The appearance of the Nautical Almanac (see ephemeris) in 1767 was a great step forward in navigation, and the 19th cent. saw the development of books on navigation that far surpassed any earlier instructions, such as the standard book by Nathaniel Bowditch, an American mathematician. The system of dead reckoning, which was much refined, is the art of finding a position by calculating the point of departure (i.e., the last known point of latitude and longitude), the course (as shown by the compass), the speed and the distance traveled according to the log, and the time elapsed. The use of buoys and the making of careful charts made navigation easier, while the fixing of positions by sextant and astronomical charts was greatly improved.
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