In high latitudes, directions change fast with movement of the observer. Near the poles, meridians converge, and excessive longitudinal curvature renders the meridians and parallels impracticable for use as navigational references. All time zones meet at the poles, and local time has little significance. Sunrise and sunset, night and day, as they are known in the temperate regions, are quite different in polar regions.
At the poles the sun rises and sets once a year, slowly spiralling for three months to a maximum altitude of 23° 27′ and then decreasing in altitude until it sets again three months later. The Moon rises once each month and provides illumination when full, though sometimes the aurora (gives even more light; and the planets rise and set once each sidereal period (12 years for Jupiter, 30 years for Saturn).
Fog is most frequent when the water is partly clear of ice. Low cloud ceilings are prevalent. ‘Whiteouts’ occur from time to time, when daylight is diffused by multiple reflection between a snow surface and an overcast sky, so that contrasts vanish and neither the horizon nor surface features can be distinguished. All these conditions, combined with the ice itself, add to the difficulties of navigation.
Using Polar Charts
Polar charts are baled largely on aerial photography which may be without proper ground control, except in a relatively few places where modern surveys are available, e.g. in the approaches to bases and similar frequented localities. Even then, the conditions under which these surveys have been carried out are such that their accuracy is unlikely to be similar to that of work done in more clement dimes. For these reasons the geographical positions of features may be unreliable and, even when they are correctly placed relative to adjacent features, considerable errors may accumulate when they are separated by appreciable distances. In general soundings, topography and all navigational information are sparse in most polar regions.
Visual and radar bearings, unless of observed objects which are close, require to be treated as great circles. If used on a Mercator chart, bearings should be corrected for half-convergency in the same way as radio bearings.
Natural landmarks are plentiful in some areas, but their usefulness is restricted by the difficulty in identifying them, or locating them on the chart. Along many of these coasts the various points and inlets bear a marked resemblance to each other. The appearance of a coast is often very different when many of its features are marked by a heavy covering of snow or ice than when it is ice-free.
The gyro compass losses all horizontal directive force as the poles are approached and is thought to become useless at about 85° of latitude. It is generally reliable up to 70° but thereafter should be checked by azimuths of celestial bodies at frequent intervals (about every 4 hours and more frequently in higher latitudes). Frequent changes of course and speed and the impact of the vessel on ice introduce errors which are slow to settle out.
The magnetic compass is of little value for navigation near the magnetic poles. Large diurnal changes in variation (as much as 10°), attributed to the continual motion of the poles, have been reported.
In other parts of the polar regions, however, the magnetic compass can be used, provided that the ship has been swung and the compass adjusted in low latitudes, and again on entering high ones.
Frequent comparisons of magnetic and gyro compasses should be made and logged when azimuth checks are obtained.
Learning to Listen to Sounders
The echo sounder should be run continuously to detect signs of approaching shoal water, though in many parts of the polar regions depths change too abruptly to enable the Mariner to rely solely on the sounder for warning.
In some better sounded areas, the depths may give an indication of the ship’s position, or of the drift of the ice, and in these areas ships should make use of all enforced stops to obtain a sounding.
Working in drift ice the echo sounder trace may be lost due to ice under the ship or hull noises, so, if necessary, the ship should be slowed to obtain a sounding.
The Mariner cannot rely on obtaining accurate celestial observations. For much of the navigational season clouds hide the sun, and long days and short nights in summer preclude the use of stars for observations. In summer when, apart from the moon at times, only the sun can be used for observations, transferred position lines must be used, and as accurate dead reckoning in ice is impossible, the accuracy of the resulting positions must always be questioned.
The best positions are usually obtained from star observations during twilight. As the latitude increases twilights lengthen, but with this increase come longer periods when the sun is just below the horizon and the stars have not yet appeared.
In polar regions the only celestial body available for observations may not exceed the altitude of 10° for several weeks on end, so that, contrary to the usual practice, observations at low altitudes must be accepted.
Most celestial observations in polar regions produce satisfactory results, but in high latitudes the navigator should be on the alert for abnormal conditions.