William Robert Martin

“ The great development in both size and speed of modern ships enormously increases the responsibilities of those who command and navigate them, and has led to a careful examination of the existing modes of determining a ship’s position at all times by day or night, both when in sight of land and on the open ocean. ”

“ Astronomical observations afford the most accurate means of ascertaining positions at sea, other methods (dead reckoning) being only relied upon when the weather does not admit of the practice of these, though by utilizing twilight and night observations of moon, stars and planets, the navigator in most parts of the world need seldom proceed far without the means of astronomically rectifying his position either in latitude, longitude or both at the same time. ”

“ On elevating the pole to the approximate latitude of the observer, and turning the sphere until the sidereal time is under the fixed meridian, a correct representation of the heavens at the time of observation is obtained; the stars are then easily identified by their bearings and altitudes. This valuable instrument is not merely useful when at twilight, only a few of the brighter stars being visible, the constellations to which they belong are difficult of recognition, but it enables arrangement to be made in advance for such observations as are desired to be taken during the night. ”

“ To assist the navigator in the choice of suitable objects between which to measure the angles, a very useful pamphlet is issued by the Admiralty, from the diagrams in which it can be seen at a glance which combination of objects in sight gives the most favourable result, always remembering as a broad principle that nearer objects are more suitable than distant ones, and that the accuracy of position determined depends on the relative distances of the objects as well as on the magnitude of the angles between them. ”

“ Local or ship time can only be found with fair accuracy from calculations based on altitudes of heavenly bodies, when they are nearly east or west of the observer or technically on the prime vertical. Such times can be approximately seen from the azimuth diagrams or from tables of true bearings of heavenly bodies, and the error involved by uncertainty as to the position of the horizon can be greatly obviated in twilight or at night by taking the mean of results arising from nearly simultaneous observations of bodies bearing both east and west. ”

“ In 1537 Pedro Nunez (Nonius) , cosmographer to the king of Portugal, published a work on astronomy, charts and some points of navigation. He recognized the errors in plane charts, and tried to rectify them. Among many astronomical problems given is one for finding the latitude of a place by knowing the sun’s declination and altitude when on two bearings, not less than 40° apart. Gemma did a similar thing with two stars; therefore the problem now known as a “double altitude” is a very old one. It could be mechanically solved on a large globe within a degree. ”

“ Now there be some that are very inquisitive to have a way to get the longitude, but that is too tedious for seamen, since it requireth the deep knowledge of astronomy, wherefore I would not have any man think that the longitude is to be found at sea by any instrument ”

“ For instance, the position of an observer at the instant of observing a (true) altitude of the sun of 40° 10′ must be somewhere on a portion of the circumference of a circle (usually of such size that the portion considered may be represented on a chart by a straight line) having its centre in latitude equal to the sun's declination, and in longitude equal to the Greenwich apparent time at the instant, the radius of such a circle being equal to the sun's zenith distance of 49° 50′. ”

“ The list of instruments given by Davis as necessary to a skilful seaman comprises the sea compass, cross-staff, chart, quadrant, astrolabe, an “instrument magnetical” for finding the variation of the compass, a horizontal plane sphere, a globe and a paradoxical compass. The first three are said to be sufficient for use at sea, the astrolabe and quadrant being uncertain for sea observations. The importance of knowing the times of the tides when approaching tidal or barred harbours is clearly pointed out, also the mode of ascertaining them by the moon’s age. ”

“ The heavenly bodies commonly observed at sea are: The Sun, Moon, Venus, Mars, Jupiter, Saturn, the Pole star, and the larger (or first magnitude) fixed stars, the positions of all of which in the heavens are given in the Nautical Almanac for fixed epochs at Greenwich, with the requisite data for computing their positions at all other times in all other places.
The chief astronomical observations made at sea are those for ascertaining (1) latitude, (2) time and thence longitude, (3) error of compass, and (4) latitude and longitude simultaneously. ”

“ Even with the larger and more accurate instruments used in astronomical observatories on shore the means of ascertaining latitude were far in advance of those by which longitude could be obtained, and this equally applied to the various heavenly bodies themselves upon which the terrestrial positions depended, the astronomical element of declination (corresponding to latitude) being far more accurately determined than that of right ascension (corresponding to longitude) . ”

“ The association of commercial activity and nautical progress shown by the Portuguese in the early part of the 15th century marked an epoch of distinct progress in the methods of practical navigation, and initiated that steady improvement which in the 20th century has raised the art of navigation almost to the position of an exact science. ”

“ When the king was pressed by St Pierre, Sir J. Moore and Sir C. Wren to establish an observatory for the benefit of navigation, and especially that the moon’s exact position might be calculated a year in advance, Flamsteed gave his judgment that the lunar tables then in use were quite useless, and the positions of the stars erroneous. The result was that the king decided upon establishing an observatory in Greenwich Park, and Flamsteed was appointed astronomical observer on March 4, 1675, upon a salary of £100 a year, for which also he was to instruct two boys from Christ’s Hospital. ”

“ The causes of the changing errors of a compass in an iron ship are described elsewhere (see Compass) , but by making comparisons as above the navigator can at once ascertain what is termed the “total” error, and if he takes from that the portion of error due to the earth, or what is termed variation (known from a chart of such elements) , the remaining error is that caused by the iron of the ship, technically known as deviation. ”

“ The astronomical positions of latitude and longitude (entered as “obs” or “by observation”) are very seldom both determined at noon, but are carried up or back to that instant by calculation from the intervening dead reckoning. The variation allowed is taken from the published variation chart, on which the latest results of such observations are embodied at intervals of about ten years with the annual changes (as far as known) in different localities, thus enabling the navigator to obtain its value at intermediate dates. ”

“ In 1556 Martin Cortes published at Seville Arte de navigar. He gives a good drawing of the cross-staff and astrolabe, also a table of the sun’s declination for four years (the greatest value being 23° 33′) , and a calendar of saints' days. The motions of the heavens are described according to the notions then prevalent, the earth being considered as fixed. He recommends the altitude of the pole being found frequently, as the estimated distance run was imperfect. He devised an instrument whereby to tell the hour, the direction of the ship’s head, and where the sun would set. ”

“ But it is surprising when we read of the progress made among the ancients in fixing positions on shore by practical astronomy that so many years should have passed without its application to solving exactly the same problems at sea, but this is probably to be explained by the difficulty of devising instruments for use on the unsteady platform of a ship, coupled with the lack of scientific education among those who would have to use them. ”

“ Until the establishment of national astronomical observatories it was impossible to accumulate the vast number of observations necessary to fufil the astronomical conditions, and until the invention of the sextant no instrument existed capable of use at sea which would measure the distances required with the necessary accuracy, while even up to the time when the problem had attained its greatest practical accuracy the calculations involved were far too intricate for general use among those for whom it was chiefly intended. ”

“ There is in the museum attached to the Royal Naval College at Greenwich an instrument described as Sir Francis Drake’s astrolabe. It is not an astrolabe, but may be a combination of astronomical rings as invented by Gemma with additions, probably of a later date. It has the appearance of a large gold watch, about 21/2 in. in diameter, and contains several parts which fall back on hinges. One is a sun-dial, the gnomon being in connexion with a graduated quadrant, by which it could be set to the latitude of the place. There are a small compass and an hour circle. ”

“ All observations relating to ocean meteorology are of great service in the compilation and improvement of wind and current charts, and in many ships more extensive meteorological journals are voluntarily kept on forms supplied by the Meteorological Office. A knowledge of the temperature of the surface of the sea is often of great practical use in navigation as giving warning of change in direction of the surface ocean current, especially in localities where there exist near to each other warm and ccld currents setting in different directions, as, for instance, near the edge of the Gulf Stream. ”

“ He gives a sketch of Halley’s quadrant, but without a clamping screw or tangent screw.
To ascertain local time at sea by astronomical observations by the altitude of suitably-situated heavenly bodies was an old, well-known and frequently practised operation, so that a comparison could thus be easily made between such local time and the Greenwich time if known at the same instant. The introduction of timekeepers by which Greenwich time can be carried to any part of the world, and the longitude found with ease, simplicity and certainty is due to the invention of John Harrison. ”

“ A book of Tables Requisite to be Used with the Nautical Ephemeris was published by Maskelyne at the same time as the first Almanac, and ten thousand copies were quickly sold. A second edition, prepared by Wales, appeared in 1781, an octavo of 237 pages, in the preface of which it is stated that it contains everything necessary for computing the latitude and longitude by observation. There are in all twenty-three tables, the traverse table and table of meridional arts alone being deficient as compared with modern works of the liind; dead-reckoning Maskelyne did not touch. ”

“ With all this knowledge the earth is still considered as stationary-although Wright alludes to Copernicus, and says that he omitted to allow for parallax. Wright ascertained the declinations of thirty-two stars, and made many improvements or additions to the art of navigation, considering that all the problems could be performed trigonometric ally, without globe or chart. He devised sea rings for taking observations, and a sea quadrant to be used by two persons, which is in some respects similar to that by Davis. ”

“ Directions are given for making a compass similar to those then in common use, also for ascertaining and allowing for the variation. The east is here spoken of as the principal point, and marked by a cross.
The third part of Martin Cortes’s work is upon charts; he laments that wise men do not produce some that are correct, and that pilots and mariners will use plane charts which are not true. In the Mediterranean and “Channel of Flanders” the want of good charts is (he says) less inconvenient, as they do not navigate by the altitude of the pole. ”

“ He gives instructions for all kinds of sailings, for marine surveying and making Mercator’s chart. There are two good traverse tables, one to quarter points, the other to every 15 minutes of arc; the distance to each is 120 m. There is a table of meridional parts to minutes, which is more minute than customary. Book ix., upon what is now called “the day’s work,” or dead-reckoning, appears to embrace all that is necessary. A great many methods, we are told, were then used for measuring a ship’s rate of sailing, but among the English the log and line with a half-minute glass were generally used. ”

“ The early history of the rise and progress of the art of navigation is very obscure, and it is more easy to trace the gradual advance of geographical knowledge by its means than the growth of the practical methods by which this advance was attained. ”

“ The practical problems involved are precisely those employed at astronomical Observatories, but it is not possible to attain similar accuracy of results, for though the sextant (the instrument always employed at sea in making such observations) is capable of marvellous accuracy, yet, as practically all such observations depend directly upon altitudes measured above the sea horizon, the uncertainty and variability of the true position of this, due to the changing effects of refraction, much affect observations made' at any one time. ”

“ A scale of equal parts, near the centre of the chart, extends from the equator to what is intended to represent 75° of latitude; by this scale London would be in 55° instead of 511/2°, Lisbon in 371/4° instead of 38° 42′. The equator is made to pass along the coast of Guinea, instead of being over four degrees farther south. The Gulf of Guinea extends 14° too far east, and Mexico is much too far west. Though there are many vertical lines on the chart at unequal distances they do not represent meridians; and there is no indication of longitude. ”

“ The errors which he said were inseparable from the dead-reckoning “even in the hands of the ablest and most skilful navigators,” amounting at times to 15 degrees, appear to be overestimated. On the other hand, the equations to determine the moon’s position at time of observation from Mayer’s tables, would, he believed, always determine the longitude within a degree, and generally to half a degree, if applied to careful observations. ”