mklotz
Well-Known Member
In a recent thread (which I don't want to hijack further), I outlined a simple method to manually (i.e., no indexer, no CAD) divide a circle into 3,6,12,24,48... divisions.
It occurred to me that many folks may be interested in ten divisions. Ten divisions are trivial if one can do five divisions. So the question becomes, "How to construct a pentagon with only straightedge and compass?" - a so-called Euclidian construction.
[Aside: Not all regular polygons have a Euclidian construction. Gauss proved that certain ones (e.g., the heptagon - with seven sides) absolutely cannot be so constructed. Fortunately, the pentagon does not fall into this forbidden group.]
For most modelmakers there will be easier methods available to subdivide a circle. Still, it's nice to know how our craftsmen forefathers managed in the days before indexers and computers. And there will be times when the job is too big or awkward to fit into more conventional indexing devices.
The diagram below should be self explanatory. If you want to verify the technique, you can use trigonometry to prove that the angle AOE is indeed 360/5 = 72 deg.
It occurred to me that many folks may be interested in ten divisions. Ten divisions are trivial if one can do five divisions. So the question becomes, "How to construct a pentagon with only straightedge and compass?" - a so-called Euclidian construction.
[Aside: Not all regular polygons have a Euclidian construction. Gauss proved that certain ones (e.g., the heptagon - with seven sides) absolutely cannot be so constructed. Fortunately, the pentagon does not fall into this forbidden group.]
For most modelmakers there will be easier methods available to subdivide a circle. Still, it's nice to know how our craftsmen forefathers managed in the days before indexers and computers. And there will be times when the job is too big or awkward to fit into more conventional indexing devices.
The diagram below should be self explanatory. If you want to verify the technique, you can use trigonometry to prove that the angle AOE is indeed 360/5 = 72 deg.