From his 1909 base at Cape Columbia, the latitude and longitude of which was known, Peary’s plan as conceived and executed was to head due north until his estimated mileage, corrected by observations for latitude, indicated that he should have reached the Pole. He would then take a series of sun sights to locate the Pole from that position and make whatever final excursion was necessary to assure that he had indeed “nailed” the location and thereby attained his goal.

His method called for finding the direction to the Pole from the sun as often as feasible. By always heading straight for the Pole itself, however he had been diverted, he compensated as he went along for the effects of ice drift, changing magnetic variation, and detours to the east or west due to open water leads and insurmountable high pressure ridges. His was a zigzag course with his heading intermittently corrected to true north, not a beeline up a given meridian. 

Peary’s compass always pointed to the magnetic pole and not true north. However, he was able to set his compass course by the sun, which, when visible on the trip north, lay exactly due south at “local apparent noon,” the moment when the sun reaches its highest altitude above the horizon. To make an observation for apparent noon, a sextant is used to measure the angle between the sun and the horizon. Since the frozen Arctic Ocean with its ice ridges does not provide a clear horizon, an instrument called an artificial horizon--a small wooden pan covered with glass and filled with liquid mercury--is used with the sextant. The mercury acts as a perfectly level mirror, and the sextant measures the angle between the sun and its reflected image.

Marvin or Bartlett (and finally Peary) would stretch out behind the pan and train his sextant on the image of the sun in the mercury; what he saw were the two images of the sun, which would very slowly approach each other as the sun rose. He would watch, over the course of 10 to 15 minutes, for the sun to reach culmination, its highest point or “local noon,” at which moment the two images would stop approaching and start to recede. The direction of the sun was then due south, and, of necessity, the observer would have been looking back along the northbound trail. Any significant deviation from that trail would be immediately apparent. The reverse direction was due north. In truth, after taking his sight, all he had to do was stand up and turn around; his shadow would point true north.

North of about 88 degrees, however, the rising and setting of the sun is too slight to provide accurate direction by this method. Thus Peary relied on dead reckoning rather than additional sights for his last five marches.

When taking a sun sight, the observer would note the tie of the culmination on his watch, and it is important to note that the time indicated by the watch did not have to be the “correct” time. Much has been made of the fact that the Roosevelt’s chronometers were fast when Peary set his watches from them upon departing for the Pole. We have determined that the most probable error of the watches was less than one minute, but whatever the error was, it did not matter, since the time of the sun’s maximum altitude determined the time oflocal noon. Whatever his watch read when the maximum altitude was achieved was regarded as local noon on subsequent days when noon sights were not taken, but direction of the noon sun was used whenever it was visible. We are persuaded that Peary’s system of navigation was adequate to get him to the near vicinity of the Pole without taking longitude observations along the way. 

Continued...