the ship's position when in sight of land wasn't too difficult, except
when exploring uncharted waters. Pilots became familiar with land marks,
water colour, scend of the sea, tidal stream lore, even the smell of
a certain area. This led to the sensible policy of employing pilots
for a specific area, as with Trinity House pilots today. It is noteworthy
that the Trinity House for the Advancement of Navigation and Training
in Pilotage was formed by Henry VIII in 1514. Thomas Spert, Master of
the Mary Rose in 1513, was appointed master of Trinity House
at its inception.
The pilot was directly concerned with the science of astronomy, he needed to know the "age" and bearing of the moon for the purpose of tidal prediction. Tide tables were already in being, although the true cause of tides was not yet understood.
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.
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.