 |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
| Before the advent of radio and satellite aids to navigation, polar travelers, once out of sight of visible landmarks, had four primary means of navigating and determining location: (a) dead reckoning; (b) timepiece; (c) magnetic compass; (d) sextant. Although each is described separately below, in actual practice polar travel generally employed a combination of all four. |
|
| A. Dead Reckoning. The term dead reckoning is derived from deductive reckoning [or reasoning]. For example, if one is traveling at the rate of 50 miles per hour, one can "deduce" that in two hours time, one has traveled 100 miles. |
| Shortcomings: Dead reckoning is at best an estimate and depends on the accuracy or validity of the premise on which the "deduction" is based. For example, speed is rarely a constant. Usually an average is used that may or may not be accurate. It should be noted also that most dead reckoning problems are not as simple as the example given above that can be done in one's head. Most involve the use of an algebraic formula to solve. Nevertheless, despite its shortcomings dead reckoning is still one of the most widely used and reliable aids to navigation. |
|
| B. Timepiece. An accurate timepiece, i.e., a watch or chronograph, is necessary for most dead reckoning estimates and for sextant observations. Further, in the absence of a compass, a watch face can be used as a make-shift compass for direction finding in high latitudes. (See The Sun as a Compass in High Latitudes). Additionally, by noting the exact time of local apparent noon, or the time the sun is highest in the southern sky, it is possible to determine longitude. |
| Shortcomings: Older timepieces were highly susceptible to the effects of temperature and motion. Early polar explorers are reported to have kept their watches inside their clothing next to their skin for warmth and to avoid jarring. Before the advent of quartz watches, timepieces had to be re-wound regularly and frequently would gain or lose time, often over a short period of time. |
|
| |
| C. Magnetic Compass. In the northern hemisphere, magnetic compasses point to the Magnetic North Pole, a mineral deposit in the high Canadian Arctic, rather than to True North or the North Geographic Pole. The angular difference between Magnetic North and True North is compensated for by a value known as Magnetic Variation. Magnetic variation is less in lower latitudes and greater in higher latitudes. However, if Magnetic Variation is known and compensated for, magnetic compasses are fairly reliable in the north polar regions; it is in the vicinity of the magnetic pole only that compasses are unusable. (See Magnetic North: the wandering pole). |
| Shortcomings: In addition to variation, magnetic compasses are subject to (a) deviation, or an attraction of the compass needle to any nearby metal objects and could make readings unreliable, and (b) oscillation, or erratic movement of the compass needle that occurs whenever a change in direction, however slight, is made. Further magnetic variation is rarely a constant, but can vary significantly from day to day and over a relatively short distance. This can be a problem particularly in high latitudes where variation is greatest. |
|
| D. Sextant. Various instruments for measuring the height of celestial objects such as the sun, moon and stars above the horizon have been in existence since antiquity. The marine sextant and its predecessor the quadrant date back at least several centuries. Originally, sextants were made of brass and could be quite heavy. Newer versions are made of aluminum, sometimes even plastic, and are lighter in weight. |
| In simplest terms, the sextant measures the altitude, or height, in degrees, minutes and seconds of an arc, of a celestial object (usually the sun) above the natural horizon. Once an altitude is obtained, one is left with a set of numbers that of themselves have little meaning. In order to convert the raw data of an observation to actual location, it is necessary to consult an Almanac for the particular date and time of the observation, apply corrections, and do a set of mathematical computations to convert altitude to actual location in degrees, minutes and seconds of latitude and longitude. In actual practice, locations obtained by sextant would be cross-checked for accuracy and reliability with factors such as last known position, and time, speed and direction traveled. |
| Interestingly, it is possible to "fake" sextant observations. The procedure consists simply of taking a location in terms of degrees of longitude and latitude and working a navigational problem "backwards" to determine the altitude (of the sun) for a given location at any given date and time. There are probably more than merely a few examples of "faked" observations in the annals of polar exploration history. |
| Shortcomings: There are numerous problems associated with the use of the sextant. First, the accuracy of an observation depends on the skill of the person making the observation. Even with experienced navigators, it is common practice to make several observations over a short period of time and use an average of these observations. Further, errors are common in doing the mathematical computations to convert altitude to location. Two additional factors complicate the use of a sextant: (a) Obviously, the celestial object being used, i.e., generally the sun, must be visible. However, it is not uncommon, in any latitude, for there to be two or more days in a row when the sky is obscured. (b) The natural horizon must be visible. However, the natural horizon frequently is obscured by weather phenomena, i.e., snow, fog, haze, etc. In northern latitudes, snow and ice drifts often produce a jagged horizon. Nowadays, radio and satellite navigation has virtually replaced the use of the sextant. |
|
|
