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Date: Tue, 11 Jul 2000 A little clarification on the steering method used for the 1st 161 miles (until Marvin's 1st sight). On the previously well-mapped coastline (Nares, Greely, Peary); pick two prominent headlands, say Pt. Moss and Cape Hecla. On the chart, the line from Pt. Moss to Cape Hecla is about 100 degrees clockwise from due north (true bearing = 100). Now stand at Moss and take a compass bearing on Hecla. According to Peary's instructions to Mac (in Mac's book) the bearing would have been 201 degrees clockwise from magnetic north (i.e., compass heading S 21 W, or 21 degrees west of south). Subtract the 100 degrees from 201, so that the 201 degrees is composed of the following: 101 degrees clockwise from magnetic north is true north, another 100 degrees clockwise is the bearing to Cape Hecla. So the variation of the compass is 101 degrees west, which is right about on for Cape Hecla (65 W. longitude, more or less, as I recall). If you were departing from Hecla, your magnetic bearing of true north would be 101. If you depart from 5 degrees of longitude further west, add 5 degrees to the course to get 106. (This would not be exactly right, because the direction the needle points would be slightly different over at Columbia, but not by much.) If you keep following 106 in your best boy scout method (compass and time of day & direction of shadow), your true course will gradually curve slightly to the left, as the magnetic variation changes as you head north. Maybe this is why Peary used 112 (According to Mac) rather than 106. Or, maybe he used 112 because he figured he was west of 75 due to drift (he was, but not by that much). Or maybe Peary's compass variation numbers were correct and our current models are wrong. Whatever. In any case, if Peary followed 112 for the whole 161 miles, his error due to compass variation changes would be no more than a few miles. He would have had errors of about the same order (but not necessarily in the same direction) due to failure to set his course accurately. With care, these errors could be very small. Finally, he would have errors due to drifting of the ice. The predictable ice drift (based on wind data) for this portion of the trip was relatively small (I forget - its in our report - but I think it was about 12 miles). All of this would have been completely satisfactory. A drift of 20 miles would only have increased the total distance to the pole by 2 miles. 40 miles would increase the total distance to the pole by 8 miles. The key is then to make a compass course correction when Marvin took his first sight. Bartlett (in a letter to Grosvenor) says they did - that they set their course by the sun at 1:00 after Marvin's sight. If they checked it one or two times thereafter, that would be more than sufficient. Rawlins makes a big stink about the compass being unreliable. He uses data from a discussion in a publication for people driving big steel ships. All that steel, plus the motion of the ship certainly would cause problems in areas of low magnetic horizontal field strength, but should not have been a problem for a person standing still on the ice. There is a web site that has magnetic field data, and I am going to see how the conditions Amundsen faced compare with Peary's. No one doubts that the magnetic compass worked just fine for Amundsen. Date: Tue, 31 Oct 2000 The guy who verified Peary's location of Cape Morris Jesup was Robert Lillestrand. Since Rawlins says he confirmed Peary's location within a mile, this is not an urgent issue. He does say others had it wrong, and Lillestrand might have written something on that, but this does not strike me as a high priority. Doug |
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| © 2002 by Douglas R. Davies. All rights reserved. No part of this text may be used without written permission from Douglas R. Davies. Email request |