FIRSTLIGHT25 SPIRAL COORDINATE SYSTEM (FL25)
θ = 2²⁵ · r (mod 360°)
r = colatitude from the South Pole (0° at the pole, 180° at the North Pole). One
fixed spiral, 16,777,216 windings, starting at the South Pole. θ0 = 8.5°E, from the
1246 CE equinox/perihelion derivation — the "first light" the name refers to.
The position needs essentially all 16 digits shown — truncating it isn't a small
rounding loss here, it's the difference between meters and hundreds of kilometers.
click anywhere on the map
Green marker: where you clicked. Amber marker: the nearest point on the spiral (same
longitude, closest winding by latitude). Position runs 0.0 at the South Pole to 1.0 at the
North Pole — the Equator sits at essentially exactly 0.5.