May 21, 2014

The Maddison Project: Take 3

IN THE LAST EPISODE of the Maddison project we learned that maritime progress in the 15th century had shifted to Portugal, where caravels were now exploring the Atlantic. But there were also major improvements in navigation. New instruments were developed, and much better charts were being drawn.

“In the northern hemisphere, the pole star provided a more-or-less constant bearing and altitude,” Maddison points out. “On a north-south passage, a navigator could observe the pole star each day at dawn and dusk, when he could see both the star and the horizon. By noting changes in altitude, he could get some idea of changes in his position. In sailing east-west, he could keep a steady course by maintaining a constant polar altitude.”

All this had been done before, of course, but very crudely, using finger spreads or other rough means of measuring altitude. But in the 15th century the Portuguese developed the quadrant, which made it possible to judge latitudes and distances sailed.

“The sun’s altitude could not be measured with a quadrant, as its light was too bright for the naked eye, so a variant of the astronomer’s astrolabe was developed for mariners. because of the earth’s movement, the altitude of the sun was different every day, so altitude readings had to be adjusted for daily changes in the sun’s declination. These tables were constructed by the astronomer Zacuto in the 14702. After practical tests of the instruments and tables on trial voyages, a naval almanac, Regimento do Astrolabia et do Quadrante, was compiled, which was available to Vasco da Gama when he sailed to India in 1497.”

In 1569 the Flemish mapmaker, Gerard Mercator, developed a projection technique to represent the world’s sphericity on a flat surface. On his charts, parallels of latitude and meridians of longitude cut each other at right angles. Meridians were spread apart as they approached the poles, and to compensate the spacing of latitude degrees was increased progressively toward the poles. The result of this was on a Mercator chart, the line of a constant compass bearing was straight. This made it much easier to plot and maintain a course at sea, but strangely enough, Mercator charts were not generally used until the 17th century.

In 1594 the English navigator, John Davis, invented a simple back staff that could be used to measure the altitude of the sun without sighting the sun directly, and by the end of the 17th century it had replaced and seaman’s quadrant and astrolabe.

The backstaff, in turn, was superseded by a much more precise instrument, the reflecting octant, in 1731. It was the work of the English mathematician, John Hadley. It was further improved in 1757 by a sextant developed by the British navy. The sextant permitted a quick and accurate reading of any celestial object against the horizon.  

Today’s Thought

One ship drives east and another drives west

With the self-same winds that blow,

‘Tis the set of the sails and not the gales

Which tells us the way to go.

— Emma Wheeler Wilcox, Winds of Fate

Seattle police arrested two kids yesterday.  One was drinking battery acid, and the other was swallowing fireworks. They charged one and let the other one off.

(Drop by every Monday, Wednesday, Friday for a new Mainly about Boats column.)

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