I am looking for the use of the second (outer) dial numbered from 0 to 7 on the Silva Polaris compass. This second dial rotates independently of the inner dial.
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Please upload a better quality picture that is in focus. I can’t see any example of a Polaris compass with two dials on Google images.– DarrenAug 19, 2020 at 15:28
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BTW, the “dial” is called a “bezel”.– DarrenAug 19, 2020 at 15:33
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Thanks. I’m intrigued by this. Neither me or my friend (a very experienced mountaineer) have ever seen this or can think what it would be for. Maybe connected to days of the week, but we don’t know how.– DarrenAug 19, 2020 at 17:32
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Although he suggests emailing Silva as they’re quite responsive. If you get an answer, please post it here.– DarrenAug 19, 2020 at 17:34
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1I received a response from Silva : "The Silva Polaris was a Johnson Outdoors manufactured Silva compass that has been discontinued for some time now. Johnson Outdoors is no longer associated with Silva. I am sorry but I do not know the purpose of the second dial. As a long shot, Liberty Mountain in the US used to distribute Johnson Outdoors Silva compasses and now distributes the Swedeish designed ones now. If you reach out to them they may have carried this model at one point. Liberty Mountain" My Quest continue!– La raisonAug 19, 2020 at 21:25
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2 Answers
It is used to measure distance. You turn it on its side and roll it along a path on the map. This means it has to correspond to a particular map scale (my first guess is 1:50,000, but that looks a bit off). For a different scale you could do a simple conversion.
Can you tell me what the arc length of one major division is? From the picture, it kinda looks like a little more than one inch (maybe 1.25" ?? ). I can get the associated scale from that (i.e. what is the arc length between 6 and 7)
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If you look at the smaller divisions, at the top in the image, they seems to be 1/4 inch, so major division one inch.– Tomas ByAug 20, 2020 at 20:17
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Well done. It is indeed 1 inch between two marks. Thank you for the answer. Aug 20, 2020 at 23:29
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Dang! I was hoping for a slightly different number. The best guess that i have (total WAG) is that the scale is 1:40000 and the major division is for a kilometer, which cant be right because the 1/4 increments don't make any sense for metric. So I still say it is to measure distance, but I am stumped on the scale.– MikeAug 21, 2020 at 2:12
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1@Mike - if it is 1" scale will be 1:63,360 or inch per mile - used to be standard in the Commonwealth (e.g. Canada, NZ, AU, UK) before metric conversion. Not sure if the UK still uses this scale as they still use mph on cars AFAIK. I thin US survey is still mile based too.– bob1Aug 21, 2020 at 2:30
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I did just find a reference that says "Most of Alaska has been mapped at 1:63,360." which would fit spot on. And I see that Bob beat me to it which makes way more sense.– MikeAug 21, 2020 at 2:31
A couple of additional items:
Angular units - Note that, for the full circle, it actually goes from 0 to 8. Each division is 1/8 of a circle. Each division is further divided into quarters, so the total number of subdivisions is 32, which is exactly how many points are in a compass. See Points_of_the_compass
Distance measurement - It doesn't really matter what the circumference or arc length is! If you know how many turns and fractions of a turn your measured map distance is, just reproduce the same distance down the side of the map's grid, or along its bar scale, and you have the scaled distance! You can also measure a map distance using a piece of string. You don't need to know the length of the string other than to lay it down along the map grid.
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It seems pretty clear that the whole idea of this additional thingy is to measure inches. It would make more sense to claim it does not matter it is exactly eight in the full circle. I agree it does not really appear to be an optimal solution to the problem.– Tomas ByAug 21, 2020 at 19:44
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@TomasBy - if you look more closely at the image you will see that the 1/4 divisions go past 7, so as MartinF says - it goes from at least 0-7.75, or if you do a full circle 0-8 (actually back to 0 again).– bob1Aug 21, 2020 at 23:11
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I should have waited a bit before choosing Mike's answer. my excuses... I believe that the fact that each division is one inch, divided into quarters, tilts scales on the side of a ruler-wheel. The division into eight is unusual, I agree. Perhaps this is a reason for its little distribution? But, then why is the system not found with other more practical divisions? Experienced mountaineers could perhaps judge the usefulness of the device. Aug 22, 2020 at 16:51
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@Laraison the size seems appropriate since it extends outside the square base of the compass on three sides. It could have been slightly smaller, but what for.– Tomas ByAug 22, 2020 at 19:51