Bob Terwilliger wrote: > > Dear Friends. > > I have received a request for information about an interesting device. > > == Quoted Message == > > I am interested by sundials. With a friend we search for information about > a type of sundial used by US ARMY in north Africa during the world war two. > My friend possess one exemplary (see the picture) but we would like to now > more information about utilization, history ... This object is a "Sun > Compass" his name is "PROTRACTOR" it was built by the company "ABRAMS ? ? > LANSING MICHIGAN" Thank you to send me all information (pictures, text ...) > Thanks also to diffuse this message to all amateurs of sundials you know. > Thank you very much for your help. > > FranGois PINEAU > 6 jardin Ockeghem > 37000 TOURS > FRANCE > > [EMAIL PROTECTED] > > ==================== > > He sent a good photograph and I have put it on line: > > http://www.shadow.net/~bobt/stuff/suncompass.jpg > > If any readers have information about this sun compass they can respond > directly to FranGois; please copy to the list. > > Bob Terwilliger
Bob, I have no particulars on the company of ABRAMS, Lansing Michigan, but the device is simply a universal analemmatic dial. To use it effectively, one would have to know the latitude of the place, the date and the time of day (with the usual eot correction to solar time). Knowing these things, the sun comapss is turned until the shadow falls across the latitude line at the known time (by watch or radio signal). When dial time agrees with calculated local solar time for the given location, then the axis of the dial forms a north south line, Also, the gnomon and the other post can be used to take bearings on objects or landmarks, with reference to north. The Abrams instrument is obviously made for use in the low latitudes, but I am not able to make out the finer markings from the photo. My guess is that it is intended for use between 0° and about 35° North or South. Note, on the abrams device, the latitide ellipses, while having a common center, have different major and minor axes, so that the gnomon offset for any given date will be the same for all indicated latitudes. A similar sort of device, the D. H. Baldwin Solar chart was used by the USGS (United States Geological Survey), circa 1920, for the field orientation of the Plane Table as used by topographic surveyors of the time. The Baldwin Solar Chart is also a multi-latitude analemmatic dial, but differs from the Abrams version in that its latitude ellipses all have the same major axis. This means that the gnomon offset for any given date will be different for different latitudes of use. The Baldwin Solar Chart has an interesting, albeit complex, nomograph for setting the correct gnomon offset for any date or latitude. My belief is that the Abrams Compass would be far easier to use than the Baldwin chart. The long out of print Bulletin 788, of the United States Geological Survey, 1928, gives a complete description of the use of the Baldwin Solar Chart, as well as a detailed drawing of it on Plate 14 of that Bulletin. I can make available a redrawing of the Baldwin Solar Chart, done in DeltaCad 3.1, either as a DeltaCad .dc file, or as a .gif file, but I do not have the means to scan the original drawing, or to scan the pages of instructions for its use (and I am not gointg to re type them). Also, my researches into the workings of the Baldwin Chart led me to the conclusion that a better approach would be along the lines of the Abrams device. Thus, a couple of months ago I drew a universal analemmatic dial for use between latitudes 30 and 65, which looks remarkably similar to the one in the photo posted by Bob Terwilliger. This too is available as a DeltaCad .dc file or .gif Tom McHugh [EMAIL PROTECTED]
