This instrument draws on the fact that the sun´s shadow from the tip in the middle of a disk describes different hyperbolas at different times of the year. When you have the hyperbola representing 62° and the four weeks around summer solstice, you don´t have to know the time of the day in order to find the general directions. All you have to do is rotate the disk until the shadow of the tip falls on the hyperbola, and the general directions are given with an accuracy of a few degrees. One of the ingenious things about navigating with this instrument is that if you should choose the wrong gnomon curve and get a course that is a little too much north in the morning, this will be corrected in the afternoon by a slightly south bound course-and your average direction will be correct.
Ya know, I'm such a moron when it comes to numbers and three dimensional thinking that I'm not sure I fully understand the above. But it sure is interesting. And it makes a nice tattoo!
And if this tattoo looks familiar that may be because the fabulous Bjork has this tattoo on her arm.