Every point on Earth, expressed in one number, sub-meter accurate - with a reference meridian derived as nearly as possible from astronomical considerations, and privileging no nation or culture.
Twice a year, an equinox splits day and night evenly everywhere on Earth. Twice a year, a solstice marks the moment when the South Pole is most tilted toward the sun and most tilted away. And once a year, Earth reaches perihelion — its closest point to the sun in its elliptical orbit.
These events drift slowly relative to one another, because the calendar year and Earth's orbital year aren't quite the same length. Roughly every 21,000 years, that drift completes a full cycle — and passes through a moment where the December solstice and perihelion land at exactly the same instant.
The most recent such moment is well documented: December 1246 CE. At that instant, the South Pole was tilted at its most extreme toward the sun (southern midsummer) at precisely the same time Earth's orbit brought it physically closest to the sun. Both effects point the same direction and stack.
This isn't a symbolic description. It's the literal physical consequence of two unrelated astronomical cycles briefly agreeing with each other — reproducible by anyone with orbital mechanics and no observatory required.
The September equinox that year is the moment the sun first becomes visible at the South Pole after six months of continuous polar night — literally, first light, after the longest possible darkness.
Standing at the South Pole at that instant, there is exactly one meridian you could point toward the rising sun along — not a cardinal direction (every direction from there is north), but a specific line of longitude, fixed by the sky, not by any nation's survey.
Computing that meridian requires care: the equinox's exact clock time has to be converted from a uniform time scale into Earth's actual historical rotation state, using measured ΔT from the eclipse-record literature (Morrison & Stephenson, 2004) — not assumed to be zero, the way a casual calculation might.
Colatitude from the South Pole (r, 0° at the pole to 180° at the North Pole) and the First-Light-relative azimuth (θ) are, on their own, just an ordinary two-number coordinate pair — a rotated, re-anchored version of ordinary latitude/longitude.
To collapse that pair into a single number, tie θ to r through a winding spiral: pick how many times the spiral wraps around the globe as it travels from pole to pole, and a location's position along that one spiral becomes its entire address.
Higher k means a more tightly wound spiral, which means finer resolution — but also a spiral that's harder to actually draw, and a fraction that needs more digits to stay trustworthy. The demo below lets you add windings one at a time and watch both effects happen together.
k = 25 means 16,777,216 windings — enough that the gap between adjacent wraps drops under a meter, while still comfortably inside what a standard 64-bit float can represent without falling apart.
Click anywhere on the map below. You'll get a single decimal — 0.0 at the South Pole, 0.5 at the Equator (where it crosses the First Light Meridian), 1.0 at the North Pole — that pins the location to within less than a meter, verified against the actual click, not just claimed.
Every country boundary on Earth — 177 countries, 289 rings, 10,654 vertices, from Natural Earth's public dataset — converted into FL25 addresses and back, rendered with no basemap underneath: nothing on screen except lines decoded from single 16-digit numbers, live, in JavaScript.