Navigation - Page 2 of 2

The measurement of North - South movement (latitude) and East - West movement (longitude) presented another set of problems. Latitude was measured by taking a "sight" of the altitude of a heavenly body, usually the altitude of the midday sun, or a star, primarily the Pole Star in Northern latitudes.

The instrument used for this purpose in the period of the Mary Rose would have been either an astrolabe or more likely, a cross staff. In conjunction with astronomical tables published in Lisbon in 1509, the latitude of a ship could be reasonably accurately determined.

Longitude was another matter entirely. At the time of the Mary Rose this was achieved by "dead reckoning", a form of guesstimate of distance and direction over a given time. The log and sand glass combination gave the speed of the ship, with the course being obtained from the compass. This information was plotted on the "traverse board", a form of peg board on a compass rose. Pegs were inserted every half hour of a four hour watch to show course and estimated distance covered. This information would be interpreted by the pilot and transferred to his "portolan chart" using his dividers for the plotting. The pilot could now estimate his east - west distance travelled to give him longitude. His cross staff would give him a latitude check. So far no traverse board or cross staff have been identified on the Mary Rose.

There were of course considerable errors, the roll of the ship could impede the flow of sand in the glass and bad weather could make sun or star sights impossible for days on end. The measurement of time was assisted by the use of pocket sundials. These gave the local time by the sun at the particular latitude they were set for.

Nine pocket sundials were found aboard the Mary Rose. The sun dials' built in compass needle enabled them to be orientated to magnetic north. Really accurate measurement of longitude had to wait for the development of a reliable ship born clock, the Harrison Chronometer of the 1770s finally allowed navigators to do so.

The chart had existed since the middle of the thirteenth century. These were called "portolans" and probably originated in Venice or Genoa. This sheep or goatskin chart gave coastal outlines with the positions of ports marked on them. Compass roses featured all over these charts to enable courses to be plotted. By the 16th century, these charts were in common use by the Portuguese.

The existence of dividers on the Mary Rose may indicate that the ship carried charts. The pilots of the period also had a "portolano", a form of sailing directions that went with such charts. This described coasts and ports, anchorages, rocks, etc. The successor to the portolano, the routier or "rutter," was becoming popular in the 16th century. These followed the format of present day sailing directions, giving tidal information and views of the coast as seen from seaward. In 1541, "The New Rutter of the Seas for North Partes" was published for circumnavigation of the British Isles.

By the 16th century the compass was becoming an essential aid for the navigator. The compasses on the Mary Rose were mounted on gimbals in a bowl to compensate for the movement of the ship. The compass bowls had glass tops and were set in wooden boxes for protection.

The pilot also needed a lodestone, a piece of magnetic oxide of iron. (The Pole Star was often referred to as "The Lodestar").

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