In reading d’Iberville’s Gulf Journals translated and edited by Richebourg Gaillard McWilliams, I was perplexed by longitude readings recorded in d’Iberville’s Journals. Being a sailor myself, I was familiar with the range of longitudes spanning from the Exumas in the Bahamas to the western reaches of the Gulf of Mexico. So, when Iberville recorded a longitude of 301 degrees at 10 Leagues (approximately 30 nautical miles) west of his starting point of Leogane, Haiti (also referred to as St Domingue by the French) for his first voyage to the Mississippi Territory in 1699 I was somewhat puzzled. This Brief gets into the “weeds” somewhat but a basic understanding of the concepts and utility of latitude and longitude was (and is) critically important to the navigation of vessels by both early and contemporary seaman.

Starting with the basics, longitude is defined as “a geographic coordinate that specifies the east–west position of a point on the Earth's surface.” It is an angular measurement from a reference meridian, usually expressed in degrees and minutes.

My experience was in areas west of the Prime Meridian (0 deg longitude) located at Greenwich, England. Thus, the longitude of points west of the Prime Meridian are designated as XX deg W. However, by convention, positive longitudes are east of the Prime Meridian and negative longitudes are west of the Prime Meridian. Thus, the longitude of a point west of the Prime Meridian can be correctly stated as -XX deg. In terms of distance, by definition, one minute of longitude at the equator corresponds to one nautical mile. The circumference of the earth at the equator is 131,482,560-ft. Given that the circumference can be divided into 360 degrees or 21,600 minutes, it can be determined that one nautical mile corresponds to a distance of 6,087 ft. Despite what appears to be an accurately determined value, the internationally accepted number of feet in a nautical mile is 6,076 ft. Thus, a nautical mile equals 1.15 statute miles. At a latitude of 30 deg, one minute of longitude corresponds to 0.87 nautical miles. As you proceed northward, the diameter of the earth decreases; thus, the physical distance represented by one minute of longitude diminishes.

Now that you hopefully understand longitude, we can return to an analysis of the apparent discrepancy in d’Iberville’s longitude estimate for Leogane. Based on current sources referenced to the Prime Meridian at Greenwich, the longitude of Leogane is 72.624 deg W or - 72.624 deg. Alternatively, as measured easterly, the longitude could be stated 287.376 deg E or simply as 287.376 deg. But now we have a discrepancy of 13.624 deg compared to the 301 degrees reported by d’Iberville. Note the difference in precision of the currently reported values compared to the value reported by d’ Iberville. A footnote provided in the translation of d’Iberville’s Journal suggests the source of this discrepancy; that is, at the time of Iberville’s voyage, Tenerife in the Canary Islands was used as the Prime Meridian. Tenerife has a longitude of -16.629 deg based on the current Prime Meridian at Greenwich. Tenerife being west of Greenwich, the positive value of its Greenwich-based longitude of Tenerife (-16.629 deg) should be added to the Greenwich-based longitude of Leogane (287.376 deg) yielding 304.005 deg. While closer to the longitude of 301 deg reported by d’Iberville, in terms of eastwest distance, the points thus located would be some 171 nautical miles apart (at Leogane’s latitude of 18.514 degrees, a degree of longitude corresponds to a distance of 57 nautical miles). In evaluating the possible source of error in the longitude maybe we need consider the assessment of d’Iberville as stated in the translation of his Journal on the 8th of January 1699- “I do not, by any means, find that all these lands are shown in their true longitude.” Also, recognize that at the time of Iberville’s voyage, estimates of longitude were based on dead reckoning and not accurate celestial observations and measurement of time. Dead reckoning involved estimates and/or measurements of speed, direction, drift, and distance travelled that were highly dependent on the skills and experience of the Navigator or the Master of the vessel. Still an error of approximately 1 % considering the crude method of estimating longitude is somewhat amazing! Accurate determination of longitude is dependent on an accurate measurement of time, and it wasn’t until the early to mid-18th Century that a reliable and accurate chronometer was developed for ocean going vessels. The principal motivation for developing such a chronometer was the loss of an English fleet along the coast of Sicily because an erroneous estimate of longitude. As a result, in 1714, the British offered a “longitude prize” of varying amounts dependent on the accuracy of a chronometer at sea. However, it was not until 1730 that John Harrison (shown here), a Yorkshire carpenter, submitted a project that was not completed until 1735. Given that prototype, Harrison completed two sea timepieces in 1741, a third in 1759 and a fourth in 1761. The first three timepieces proved to be too inaccurate for the conditions at sea. The first recorded use of the fourth Harrison time piece for determination of longitude at sea was by the French military officer Charles-François-César Le Tellier, marquis de Montirelin aboard Aurore in 1767.

Reading d’Iberville’s Journals, you will note that he reported his positions at noon each day, a time at which a fairly accurate determination of Latitude was possible based on celestial observations of the Sun. Latitude is defined as the position of a point either north or south of the Equator. Several notable latitudes have been established as identified in the diagram. In terms of distance, one degree of latitude corresponds to approximately 60 nautical miles (69 statute miles or 111 kilometers). Unlike longitude, the distance corresponding to one degree of latitude remains constant irrespective of position.

Methods for determining latitude by either measuring the height of Polaris above the horizon or the angle of the Sun above the horizon at Noon were reasonably well established and used in the 15 th -17th centuries. Inclination measurements were made by a mariner’s astrolabe. While conceptually simple, accurate measurements were not always possible from the rolling deck of a vessel in rough seas.

Given its geographical coordinates in terms of longitude and latitude uniquely defines the location of a point on the earth’s surface. Several nearby examples:

Tchefuncte River Entrance Channel Marker 2- 30 deg 22.052 min N 90 deg 10.363 min W

Outer Marker, Gulfport Shipping Channel- 30 deg 07.192 min N 88 deg 52.654 min W

Entrance, Pensacola Bay- 30 deg 19.578 min N 87 deg 18.281 min W

Entrance, Mobile Bay- 30 deg 13.727 min N 88 deg 03.408 min W

Entrance, South Pass of Mississippi River- 28 deg 59.926 min N 89 deg 08.352 min W

Given the comparatively small differences between the coordinates of some of the major features along the northern Gulf Coast and the rudimentary measurement tools of early Gulf and Gulf Coast explorers, it’s likely that upon navigating the Straits of Florida early explorers of the Gulf and its coast simply steered northward and proceeded until land was sighted, at which time they could make their way westward along the coast. In anticipation of possible future voyages to the Gulf Coast, they could have recorded the longitude and latitude of entrances to bays and any prominent landmarks. However, there was no assurance that they could return to that exact longitude because both their original and subsequent estimates of longitude were subject to errors and approximations. Also, as we learned earlier, comparatively small differences in coordinates can correspond to substantial differences between the actual locations of features. What is clear is the fact that these difficulties did little to deter early explorers of the Gulf Coast.

Sources

Iberville’s Gulf Journals, Translated and Edited by Richebourg Gailland McWilliams, The University of Alabama Press, Tuscaloosa, 1981.

John Harrison- John Harrison - Wikipedia

Geographic coordinate system- Geographic coordinate system - Wikipedia

Mariner’s Astrolabe- Mariner's astrolabe - Wikipedia

History of Latitude- History of latitude - Wikipedia