A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
From: Frank Reed
Date: 2018 Jan 23, 11:57 -0800
I wrote yesterday:
"Remember that the gnomonic projection can be understood very simply by thinking of a crystal globe with a lightbulb at the center. The lat/lon coordinate lines and continent lines are etched into the globe. We place that globe on a flat table with some point of tangency --for example, 30°N, 60°W. Now turn on the light. The coordinates and continent lines are all projected onto the flat table, and if we trace them, we have our gnomonic projection."
Something occurred to me just now that I have not thought of before. If you project from the center of a "crystal globe" like this, you won't quite get a normal gnomonic projection. That projection with a normal globe will be a mirror-image, east/west reversed. So you have to flip the chart over and trace from the other side if you want it to "look right". Naturally it works just fine either way. Indeed any chart or map can be printed as a mirror image, and it will be fit for all purposes. It's just "freaky" seeing everything backwards. :)