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Flat Earth SubGenius Society

03 December 2025 16:59

When people start talking about astrolabes, sextants, and navigation as if they’ve always been based on a globe, I genuinely have to laugh. I don’t “claim” they were built for a flat plane — it’s documented history, and the geometry is impossible to argue with. Every ancient navigation instrument ever created was designed around straight lines, flat baselines, and planar trigonometry. Not one of them required, measured, or even hinted at curvature.
The astrolabe — one of the oldest scientific instruments on Earth — is literally a flat disc. Its mathematics are entirely planar: straight chords, straight angles, flat projections. The early mariner’s astrolabe, the kamal, the quadrant, the backstaff — all of them assume a flat water surface and a level horizon. They wouldn’t even work on a curved baseline. The fact they guided ships thousands of miles with perfect reliability is already enough to end this argument.

Then you get to the sextant, and this is where it becomes embarrassing for anyone still repeating “but spherical trig!”. A sextant only works because the Earth is treated as a flat plane. The fundamental formula:

requires a straight, level horizon and a planar triangle. There is no curvature in that geometry. None. The entire device is built on the assumption that the line from your eye to the horizon is straight and the baseline underneath it is flat. That’s why “dip correction” even exists — because the sextant only works on a flat plane, so later globe believers had to insert a correction to force a flat instrument to mimic a curve it wasn’t designed for. The same applies to refraction correction, height-of-eye correction, semi-diameter correction, parallax correction… every one of these patches was bolted on after the fact to retrofit spherical assumptions onto a flat-working device.

If the Earth truly had curvature, there would be no need for dozens of corrections trying to bend flat math into spherical expectations. The original sextant tables worked flawlessly without any of these adjustments — spherical corrections were introduced centuries later, once people started believing in a globe and needed the instruments to match the belief.

And even today, every real-world navigation system still uses flat geometry. Aviation uses flat Cartesian coordinates and never applies curvature corrections. Surveyors use flat grids for everything except massive regional projects, and even then they use linear approximations, not true curvature. GPS trilaterates using straight lines in a 3D Cartesian grid — not spherical trigonometry. Nautical charts are Mercator projections — flat by definition. Nobody flies, sails, builds, maps, or engineers using a curved baseline. The only place curvature exists is in textbook illustrations and imagination.

So yes — I’m very aware of how astrolabes and sextants work, and I’m very aware that people only start talking about spherical trig after the fact to defend a model that the instruments themselves never needed. The historical record is simple, the geometry is unambiguous, and the engineering is brutal: navigation was built on a flat plane, works on a flat plane, and still operates on flat-plane math to this day.