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Finding one's position at sea in the 18th Century, without a GPS was a challenging business. Fortunately, there were many tools to aide those scurrilous scalawags, and other seamen

How to Navigate

Finding latitude
The easier part of navigation has always been determing one's position in latitude - how far north or south one is. To do this the navigator merely needs to determine the elevation of the sun at high noon, or at night the elevation of a star of known declination such as the North Star. The elevation is the angle between the horizon and the bottom of the celestial object. The navigator would then have written tables which would cross-reference the measured elevation by the date to give a latitude. A number of tools have existed over the centuries to measure the elevation of the sun or stars: astrolabe, back staff, cross staff, and sextant. By the age of pirates, the 1700s, most navigators at sea would likely have been using the sextant.

Finding longitude
The more difficult problem in ocean navigation had always been determining one's position east and west - the longitude. Many different theories for determining longitude were tried, and failed. The key to the mystery of longitude was time. Soon after explorers and scientists had finally figured out and proved the earth was, in fact, round, they also discovered that the time is not the same in every place. On land they figured out that for every 15 degrees you go east or west there is a one hours difference in the local time. The trick was using that knowledge at sea. It wasn't until British clockmaker John Harrison devised a truly acccurate chronometer, capable of withstanding the rough treatment and rigours of life at sea, that navigators were able to accurately measure longitude.

Longitude is determined by maintaining two clocks. One is kept permanently set to Greenwich Mean Time - the time at the Prime Meridian. The second clock is reset every day at precisely high noon to determine the local time. By comparing the times on the two clocks, and using the above mentioned ratio that one hour is equal to roughly 15 degrees of longitude, the exact longitude may be determined.

Dead reckoning
There is another way to navigate, and determine your position, if you are not particularly good at celestial navigation or don't have the clocks to determine longitude. This method is called "dead reckoning". However to use dead reckoning, one has to have a very accurate set of charts or maps, one needs a compass, a way of determining one's speed, and one has to know exactly where one is starting out from. If you have all of those, you can navigate, and determine your position, by simply tracking your progress on the charts. From a given point you determine which direction you are sailing, how fast you are sailing, and for how long you've been sailing at that speed. Do the math, draw the line on the map, and there you go.

The big problem with dead reckoning occurs if you hit a storm or other situation that alters your course and/or speed without you being able to follow and chart the changes. Then you may have a problem if you can't at least figure out your latitude using the other methods.

Determining speed
An important part of dead reckoning, and of running a ship in general, was being able to determine the speed at which you were sailing. In the age of pirates they of course did not have fancy electronic gadgets that could tell them their nautical speed. The universal method for determining speed for many centuries of sailing was the chip log, or log line. Simply put, a piece of wood about 18 inches long was tied to the end of a long line which was wound around a spool. The line had knots tied in it at equal distances along its entire length. To determine speed, a sailor would heave the log over the side into the water while another held the spool. When the log hit the water, it would begin to drag and pull line off the spool. The sailor holding the spool would count the number of knots in the line that were pulled out during a period of 30 seconds. The faster the ship was going, the more line would be pulled off the spool and the more knots the sailor would count. The speed of the ship was then given in 'knots', a term we still use today for the speed of a ship. The distance of the knots was very specific - about 47'3". That distance was mathematically related to a calculation of feet per second, which could then be converted to nautical miles per hour, so the speed could be used to determine distance travelled.

Tools -

Astrolabe: One of the older tools of maritime navigation (over 200 years old), the astrolabe came in a number of different forms. At its most basic, the Mariner's Astrolabe simply measured the altitude of the sun during the day, or any given celestial body with a known declination at night. The navigator would have a table or almanac that gave the altitude or declinations from the Prime Meridian for that date. Then, with a relatively simple mathematical calculation, the navigator could determine the latitude.

More skilled navigators, and astrologers, used a more complex version of the astroblabe which dsiplayed a complete picture of the celestial bodies of the heavens. The edge of the astrolabe would be marked with the days and months of the year. Moving components on the face of the astrolabe could be rotated to line up with a particular date and time. The moving parts would show what should be visible in the heavens for a given date and time. Mariners could use the astrolabe to locate important navigational stars, find out the exact time of day, or determine the timing of a celestial event such as sunrise.


A simple mariner's astrolabe.


Charts: A map giving depth of water in fathoms, and showed the outlines of land masses, towns, ports, etc.

Compass: A device with an magnetized iron needle which points to magnetic north, and gives an impression of the direction of travel.


Rope: Used to find speed at sea, by throwing a log overboard with a rope tied to it speed cold be determined by measuring how many knots tied at specific points passed over the side in thirty seconds.

Sextant/Octant: precision instrument used to measure the angle of the sun above the horizon.