Navigation in the Renaissance proceeded without much attempt to take account of geographical longitude (that is, position east–west), which in practice could not be found, though it was of course marked on geographers' maps. Latitude, that is, position north–south, could be measured, so the commonest technique was to get to the correct latitude for one's destination and then sail to it directly along the line of latitude. Finding latitude is equivalent to finding the height of the celestial pole, but the Pole Star is not very bright so at sea other stars were used, their height being taken as they crossed the meridian. The method was to establish the meridian by finding north, with the compass if necessary, and then take the height of the star with a quadrant (in use from about 1450) or with a so-called mariner's astrolabe, a sighting instrument invented by Portuguese seafarers in the late fifteenth century, consisting of a heavy disc (usually with pieces cut away to leave a cross shape) that hung vertically from a suspension ring and carried an alidade with pinhole sights. The quadrant was fitted with a plumb line for checking that it was vertical, the mariner's astrolabe was simply heavy. After about 1470 navigators used the height of the sun, sighting being done by adjusting the alidade so that the shadow of the upper vane of the sights exactly covered the lower vane (a procedure called 'shooting the sun'). If the sun was used, north had to be found with the compass, if necessary making a correction for magnetic variation (tables were available), and astronomical tables were required to find the position of the sun relative to the pole on the day in question. Ongoing problems with the calendar were irrelevant because the measurements were not very exact. Contemporary literature is much concerned with improved instruments for navigation, such as the cross-staff, long used by astronomers but introduced onto ships in the early fifteenth century, or the backstaff; the uses of new instruments, usually ones obtainable from the author, are found in many treatises on navigation, for instance one by the well-known instrument-maker Michel Coignet (1549–1623), Onderwysinghe op de principaelste puncten der navigatien (1580). The most famous, and probably the most useful, treatise was that of Simon Stevin, De Havenvinding (1599; an English version by Edward Wright, The Haven-Finding Art, appeared in the same year).
Globes and charts helped track the course, and raised some interesting mathematical problems (see Mercator, Gerardus), but a great deal seems to have been done by very rudimentary methods, the vessel's position being found by 'dead reckoning', that is, by assessing its speed by throwing the log overboard and counting the number of knots of its line that ran through the hand in a given time (measured by a sandglass). Simple arithmetic then established how far the ship had travelled. Underwater archaeology has shown that most ships carried very little navigating equipment, even for, say, the long voyage from the Basque country to Labrador (Canada). However, we also have evidence from lists of equipment carried by expeditions; Martin Frobisher (c.1535–1594), setting out in search of the North-West passage in 1576, took such expensive items as a blank metal globe, presumably for calculation (cost £7 13s. 4d.), an astronomical ring-dial, for telling the time at known latitude (£1 10s.), and an astrolabe (£3 10s.), presumably a planispheric astrolabe rather than the simple mariner's astrolabe and useful for astronomical calculations. His eighteen hourglasses together cost only 17s. and his two standard texts on practical mathematics, Cunningham's The Cosmographical Glass (1559) and Recorde's The Castle of Knowledge (1556), together cost only 10d. This kind of list shows that there was indeed a market for elaborate equipment, which no doubt not only aided the navigators but also reassured the financial backers of expeditions.
E. G. R. Taylor, The Haven-Finding Art (1956);
D. W. Waters, The Art of Navigation in England in Elizabethan and Early Stuart Times (1958).
From The Oxford Dictionary of the Renaissance
