Navigation

Although no generally practicable method of determining longitude at sea existed until the end of the eighteenth century, the determination of latitude was relatively simple. This was done by measuring the angular distance of the sun above the horizon at noon—the meridian altitude. It was also possible to determine latitude by observing the pole-star, but not in the latitudes frequented by the Trinity . These facts governed the navigational methods used during the Trinity 's voyage.

As there was no way of checking east-west progress once out of sight of land, position had to be estimated by dead reckoning—by keeping an hour-by-hour check on courses and distances sailed (possibly, though unlikely in the Trinity , aided by some form of speed-measuring log), taking into account such factors as the strength of the wind, currents, leeway, and so forth. This is illustrated in Fig. p. 36, which is copied from Captain Sharp's log-book, with the actual courses and distances sailed each day resolved into their north-south and east-west components ("northing," "southing," "easting," and "westing"). The log includes a periodical check on the totals of each of these components since the last land seen (the "departure" point—in this case, Juan Fernández), a check that was maintained until the next landfall.

East-west progress could only be estimated, but north-south progress could be actually measured daily, weather permitting. To do this, the navigator needed two things: an instrument for measuring the sun's me-

Image not available.

Page from a fair copy of Bartholomew Sharp's log for December 1680, showing how dead reckoning 
was computed by resolving the courses and distances made good each day into their north-south 
and east-west components. The "Meridian Altitude" column actually records, not the sun's altitude
 above the horizon, but its observed zenith distance at noon (altitude + zenith distance = 90°); this in
 turn was used to compute the "Lattitude by Observation" column. 
(From Naval Historical Library MSS. 4.)

ridian altitude; and a table giving, for every day of the year at noon, the sun's declination—its angular distance north or south of the celestial equator. (At the equinoxes, the declination is zero; at northern midsummer, it is 23°.5 north, at midwinter, 23°.5 south.)

All but one of the angle-measuring instruments available at the time can be seen in Fig. p. 37, left, which shows William Hack's title page to the first translation of the "great book" captured from the Rosario (BL MS. K.Mar. VIII 15). At top left is the Davis quadrant, or backstaff, with which the navigator, his back to the sun, makes the measurement using the sun's shadow. Though not easy to use when the sun is high (as at noon in the tropics), it would probably have been the instrument preferred by the buccaneers. They might not have had such an instrument, however; it is unlikely one would have been carried on the march across the Isthmus of Darien, and, as the backstaff was not popular with Spanish seamen, it was probably not among the equipment of the Trinity or the Mayflower .

In the printed version of Ringrose's journal (but, curiously enough, not in the holograph version), the entry for August 2o, 1680, states that he had "finished two Quadrants; each of which were two foot and a half radius" (JP3 , 68). On October 1, 1681, he records: "This day I finished another Quadrant, being the third I finished in the Voyage" (JP3 , 176). In using the term quadrant , he could have been referring either to the Davis quadrant or to the simple mariner's quadrant, such as that at bottom left in Fig. p. 37, left. It is difficult to decide which. The first is a complex piece of joinery, and dividing the scales is difficult without a jig and special tools; the second is simpler to make but had virtually gone out of use at sea by that date.

Image not available.

Title page of the first of William Hack's manuscript South 
Sea Waggoners, dedicated by Bartholomew Sharp to
 King Charles II in 1682. It shows contemporary navigational
 instruments: top left, backstaff; top right, cross-staff; bottom 
left, quadrant; bottom right, globe and dividers. 
(From  British Library MS. K.Mar. VIII 15.)

The instrument shown at top right in Fig. p. 37, left, is the cross-staff. This simple wooden instrument was used by pushing one of the cross-pieces, or transoms, to and fro on the staff so that, with the eye on the butt of the staff, the transom's bottom was on the horizon and its top on the heavenly body being observed; the angular distance was then read off scales engraved on the staff. (Transoms of three different lengths were supplied to cater for different ranges of angles, only one being used at a time.) Because of the glare, this was not a good instrument for use with the sun but was excellent for finding latitude by the pole-star; however, the Trinity was never in a latitude from which satisfactory polestar observations could be obtained (north of, say, 15° north), although the buccaneers would certainly have found at least one cross-staff on board the Trinity when they captured her.

The contemporary angle-measuring instrument not shown in Fig. p. 37, left, was the mariner's astrolabe, the method of use being demonstrated in Fig. p. 37, right. One or more of these would certainly have been found in the Trinity when she was captured. Though very difficult to use in a ship with any movement on her, the mariner's astrolabe was an accurate instrument for measuring altitudes ashore. On October 18, 1681, at the Duke of York's Island in Chile, Sharp wrote in his journal (J11 ): "Being Tuesday we had a clear day and we observed by our Astrolobes on shore & found our selves to be in the Latitude of 50°37' South. High land, mountanous & barren." Cox makes much the same entry, as does Ringrose in his journal, though the latter does not mention the type of instrument used (JP3 , 181).

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Observing the sun with a mariner's astrolabe. 
(From Pedro de Medina,  Arte de Navegar  [Valladolid, 1545].)

On November 2, in the same place, Ringrose tells us that he has determined the south polar distance (SPD: the angular distance from the south celestial pole) of "the South star in the Cock's foot" and made it 28°25' (JP3 , 184). This was probably the third-magnitude Alpha Tucanae, whose actual SPD in 1680 was 28°10'. He probably did this by observing the star's meridian altitude and then applying the latitude found a fortnight earlier. At that time, the positions of the southern stars were very imperfectly known, so he presumably took this observation simply to add to the corpus of astronomical knowledge, as he would himself have had no navigational use for it.

As for that other requirement for finding latitude, the solar declination table, we have only one reference: on November 15, 1680, Ringrose says: "Our latitude by observation we found to be 23D.25S. I took now the

Declination-Table used and made by the Cosmographer of Lima " (JP3 , 101)—which had, perhaps, been captured at Ilo a few days earlier.

One other navigational instrument was mentioned in the accounts of the voyage, an azimuth compass said by Ringrose to have been used south of Cape Horn to find magnetic variation: "In the evening of this day [November 27, 1681] we had a very exact sight of the Sun, and found above 30d . variation of the Needle. From whence ought to be concluded, that it is very difficult to direct a course of Navigation in these parts. For in the space of only twenty five leagues sailing, we have experimented [sic ] eight or nine degrees difference of variation, by a good Dutch Azimuth Compass" (JP3 , 134). Other navigational apparatus which the mariners were bound to have had was a lead and line for sounding the depth of water, and possibly, but not certainly, a log-ship, log reel, and half-minute sandglass for measuring speed through the water.

On September 12, 1680, there occurred a very rare event, an annular eclipse of the sun visible in the south Pacific,^[2] which Ringrose seized on as the only opportunity to measure his longitude astronomically. In his journal entry for September 13, he says: "Yesterday in the Afternoon we had a great Eclipse of the Sun, which lasted from one of the clock till three after dinner. From this Eclipse I then took the true judgment of our longitude from the Canary Islands , and found my self to be 285 D.35 [east of the lie de Fer, the modern Hierro, or 92°19' west of Greenwich] in Lat 11 D.45 S" (JP3 , 84).

Alas, we have no means of checking the accuracy of that longitude, as the Trinity was on a long tack some eight hundred miles into the Pacific on passage between Guayaquil and Coquimbo. The order of magnitude is certainly right, however (see Fig. p. 12), proving that Ringrose must have had considerable navigational training even to attempt such an observation. He would of course have needed some sort of almanac with predictions of the times of the various parts of the eclipse. This was most likely a Spanish manuscript almanac—perhaps taken from one of the prisoners—based on the meridian of the Ile de Fer.

The documents that have survived prove that there were at least three expert navigators in the Trinity: Bartholomew Sharp, John Cox, and Basil Ringrose; John Hilliard, the Trinity 's master who died, was presumably a competent navigator as well. Undoubtedly, the best educated of these was Ringrose, but Captain Sharp's predawn sighting of Barbados at two and a half leagues, without any significant alteration of course to search for the island—and after nearly three months with no glimpse of land—was a remarkable feat of navigation, even if he was following the accepted contemporary practice of finding the island by "running down the latitude." Whatever opinion one might have of these people as pirates, one cannot but admire them as magnificent seamen and navigators.

[2] This was an annular eclipse whose central path crossed about 500 miles north of the Trinity , so it would have appeared very nearly total (see Oppolzer 1887).