History of Navigation

The Visual Bearing

After sailing along the route, a compass bearing of 295 degrees magnetic to the red bell buoy off Belfast Harbor is taken. Plotting it shows that the distance back to the green gong is about .65 nautical miles. If time of departure is recorded as well as the time of the bearing, speed can be calculated. This single line of position produces a running fix. If another bearing were taken at the same time as the 295 bearing, an accurate position could be established.

The Plotted Course

The course line is drawn on the chart. Above it is written both the magnetic and true courses while below it is the distance of the route.

Measuring the Distance

Take a pair of dividers and set one tip on the green gong buoy and the other on the red and white bell buoy. Then bring the dividers to the side of the chart near the course and measure the distance of that leg of the route. One minute of arc of latitude equals one nautical mile.

Reading the Compass Course

After "walking" the parallel rules across the chart to the nearest compass rose, the magnetic and true compass bearing from one buoy to the next is read on the compass rose.

Using Parallel Rules

The first step is to find the green gong buoy off Sears Island and the red and white bell off the northwest corner of Islesboro Island, the course to be plotted. Parallel rules are set along that line.

Loran Lines on Penobscot Bay Chart

Many small scale charts provide Loran lines, which indicate the time difference expected between receipt of  "master and slave" radio signals in the Loran electronic navigation system. This chart, Penobscot Bay and Approaches, #13302, has four different sets of lines, improving a navigator's chance of getting an accurate electronic fix. Loran receivers indicate by number which signals they are getting, and the navigator finds the numbers and uses them to plot a fix.

Bridge on Trawler Huntress 1979

View of the bridge aboard the 87-foot stern trawler Huntress, built at the Washburn & Doughty yard in Woolwich, Maine, in 1979. Note the radar monitor near the wheel. A modern bridge would have GPS, a fish finder, an auto pilot, and several radios. In this set up it is likely that the radio is out of view and there should be a LORAN available. This vessel has a compass with large compensators at each side, common on steel ships.

Octant or Hadley Quadrant

The Hadley quadrant was developed by John Hadley in England and by Thomas Godfrey in Philadelphia, both in 1732. The Hadley design took precedence and became the stock celestial navigation tool well into the 19th century, due to its simplicity and lower price than the more modern sextant. As most celestial navigation in the age of the Down Easter centered around sun sights and occasional lunar distances, there was not great need for owning a more expensive sextant.

Pocket Chronometer or Chronometer Watch

Because the ship’s chronometer was normally stowed in the cabin in a safe and well-supported environment, times for celestial sights were taken from the chronometer watch. To ensure accuracy, the time on the chronometer watch was compared with the time on the ship’s chronometer before and after the sights were taken.

This watch, whose maker may have been active in the 1770s and up to about 1820, belonged to Captain Charles Gordon of Belfast.

Finding a Ship's Position at Sea

Many navigators wanted easier, more accurate methods of determining their position at sea. One of the most important developments in the second half of the nineteenth century was Capt. Thomas H. Sumner's contribution in his book, A New and Accurate Method of Finding a Ship's Position at Sea by Projection on Mercator's Chart. Discovered in 1837, and first published in 1843, by the time of this 1851 Third edition it had become the standard method.


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