A compass is of great value to keep one's course between observations on the sun; and an aneroid barometer and a thermometer make possible a more accurate correction for refraction. A pedometer, also, or some other form of distance meter, would be useful to estimate the distance traveled.
Although the methods of determining one's geographical position would be the same near either pole, there are slight differences in their applications; for instance, the solid land of the Antarctic continent precludes drift, and therefore this disturbance is absent. Moreover, when Lieutenant Shackleton reached his farthest south in the beginning of January, 1909, the sun was about 25 degrees above the horizon; at this altitude the refraction is not large and its value is well enough known not to introduce any great error. Near Lieutenant Shackleton's base camp, at the foot of Mount Erebus, the north pole of the compass needle pointed about 30 degrees east of south. Along the most southerly part of his route, on his dash toward the pole, the north end of the needle pointed very nearly to the south pole.
On the return trip Lieutenant Shackleton could have been guided by his compass, by the mountain range which ran very nearly parallel with his route, or by other landmarks, and, perhaps, to some extent, by his tracks; so that he found it unnecessary to make many astronomical observations. Commander Peary was guided, to a great extent, on his return by his tracks and those of his supporting parties; and Dr. Cook seems to have relied entirely on his astronomical observations.
Note.—For the sake of simplicity the sun has been generally taken, in this article, as the heavenly body on which observations are made. But the stars could serve equally well, and, for some observations, better. If the pole should be approached when the stars were visible, the altitudes of two stars lying on meridians about 90 degrees apart would determine one's position without delay; moreover, stars could be selected whose altitudes were sufficiently great to exclude errors due to refraction; or this correction could be determined by observations on a pair of stars having about the same altitude and lying on opposite sides of the zenith.
The sun's apparent motion around the earth is not uniform, and therefore a correction, known as the equation of time, must be applied to all observations on the sun; but this correction is accurately known and leads to no error.
