Two major developments in the 18th century vastly improved navigation: the solution to the problem of finding longitude and the improved availability of printed guides and charts. But navigation tools for dead reckoning and piloting were not to improve until the 1900s.
Guides for the Navigator
The eighteenth century saw major improvements in publications for navigators.
France and Britain established governmental hydrographic or surveying offices whose data, late in the century, began to be published as both charts and sailing directions.
Using data collected in North America and the Orient, the British English Pilot continued to be corrected and republished, but was superseded by government publications.
In 1774, J.F.W. Des Barres began publishing, in England, a marine atlas of the North American coast. The Atlantic Neptune showed detailed shorelines, islands, and depths based on his and other surveys between 1764 and 1773. During the American Revolution, it likely provided valuable information for the British during the Penobscot Expedition in 1779, and for general operations. The atlas continued in use for more than fifty years after its last publication in 1781.
No less momentous was the 1767 publication of the first Nautical Almanac. Published by the Astronomer Royal, Neville Maskelyne, to aid in solution of lunar distances for finding longitude, its tables were the product of German astronomer Tobias Mayer, who made efforts to correct previous works. The method required accurate measurement of the angle between the moon and known stars. The Nautical Almanac is still published, and gives the locations of the sun, moon, visible planets, and stars for every hour and day of the year, data essential for celestial navigation.
In 1770, Benjamin Franklin published the first map of the Gulf Stream, something well known to sailors, but the map was not published for mariners until the 19th century.
Tools for the Celestial Navigator
In addition to the method of lunar distances, the late 18th century saw the creation of another practical solution to finding longitude at sea: the marine chronometer. Both methods became widely used in the 19th century, when mass production provided chronometers and accurate altitude measuring instruments like octants and sextangs, and publishers printed lunar distance tables.
After more than a century of using the backstaff or Davis Quadrant for measuring the altitude of the sun, John Hadley invented a reflecting octant in 1731. This was the direct ancestor to the modern sextant. Instead of working with sun shadows, the Hadley octant, also known as a quadrant or Hadley’s quadrant, used mirrors to line up a reflection of the sun or other celestial object with the horizon. It could also be used for star sights, a major advantage. In 1751, the length of the arc was extended to allow measurements of up to 120°, something needed for lunar distance observations. This new instrument was the sextant.
Accurate determination of longitude was the major challenge to navigation. As an incentive, in 1714 the British government’s Board of Longitude offered £20,000 to the person who solved the problem. It was finally awarded to John Harrison in 1773, reflecting his lifelong effort to create a chronometer that was accurate at sea. The chronometer was set to the time at the Prime Meridian, and local noon was established by the sun. The difference in times translates to longitude.
It would take the work of many others to make chronometers practical and affordable. During the first part of the 19th century only naval vessels, the largest merchant ships, and exploration vessels could afford to carry them.
Lunar distances competed with the expensive chronometer. A navigator could find his longitude without a chronometer by using the moon as a giant clock, working its way past sun and stars.
By finding local time with daytime observations of the sun, the navigator needed to know the time at the Prime Meridian in Greenwich, the difference in time providing the longitude. The lunar sights measured the distance between the moon and nearby stars or the sun, and when compared to nautical almanac data for Greenwich of distances between the moon and stars, one could find the time at the Prime Meridian. The complex calculations were simplified by American Nathaniel Bowditch in his 1802 American Practical Navigator. Working lunars continued to be part of navigators’ training until the 1920s, when chronometers became affordable for any offshore captain.
Courtesy of Penobscot Bay History Online.
Image: Solving For Lunar Distances. This diagram illustrates how the moon appears to move through the stars. It shows that there is a difference between the measured altitude of the moon and its actual altitude, due to atmospheric refraction. Locations of the moon are shown at 3 and 6 hours, to show its actual positions as given in the Nautical Almanac, and that its observed position must be interpolated between the given positions.