How the floor
was made to turn
FOUCAULT is a single-canvas instrument: a long pendulum seen from above, its plane of swing precessing by the sine of your chosen latitude, a ring of pins falling one by one on each bearing, and a sidereal clock keeping the compressed day. No photographs — every mark is drawn.
An 1851 demonstration, rebuilt as an instrument
Léon Foucault hung a 67-metre wire from the Panthéon in Paris and let a brass bob swing over sand. Because a free pendulum holds its plane fixed against the stars, the plane appeared to wheel across the floor as the Earth rotated beneath it — proof of rotation you could watch, without a telescope. FOUCAULT is that apparatus for a curious visitor: set the latitude, watch the plane precess, and count the pins topple. Its single job is to make an invisible cosmic motion legible in a few seconds.
Marble, brass, engraved ink
Argued from the object: a stone floor, a brass bob and steel wire, letters cut into rock. The bright brass is reserved for lines and marks; small text uses a dedicated dark engraved-ink so it clears 4.5:1 on the warm stone.
Two faces, doing different work
One canvas, real precession maths
The whole apparatus is drawn on a single <canvas> in 2D — no images, no libraries, no WebGL. It renders top-down so the precession and the ring of pins read at a glance.
The signature — a precessing plane that topples pins
Each frame the Earth's rotation advances at a fixed rate (a full sidereal day compressed to ~60 seconds, about 1440× real time). The swing plane rotates by that increment times sin(latitude) — so at the equator it never moves and at the pole it turns a full circle each day, exactly as the physics demands. The bob oscillates on a fast visual cycle along the current plane; a ring of 24 pins stands just past the swing, and each pin stores the bearing at which the plane will reach it. When the accumulated sweep passes a pin's bearing, it topples with an easeOutBack fall. After a half-turn the pins restand and the demonstration loops.
Second read — the sand rosette
The bob scribes a faint line into a persistent trace layer every frame; as the plane precesses, the overlaid chords build a slowly-fading rosette in the sand — the same star-pattern Foucault's audiences watched accumulate. A sidereal clock hand and a draggable latitude protractor (pointer + arrow-key accessible) complete the panel.
Resting state
The simulation is pre-advanced ~7.5 seconds before first paint, so a still capture already shows a mid-precession plane, a half-toppled ring and a running clock — never a blank floor waiting for input.
Three passes
Spacing, contrast, the compass rose
Tightened the type scale to a deliberate ratio and set prose in Cormorant at 500 for a cut-stone feel. Reserved brass strictly for marks after checking contrast — steel and brass both failed as text over marble, so all copy moved to engraved-ink. Rebuilt the rose with graded ticks (5°/30°/cardinal) and mono degree labels, and gave the floor a raking-light radial gradient plus faint veining so it reads as stone, not a flat disc.
The rosette, the loop, the clock
Added the persistent sand-trace layer so the bob's path accumulates into a fading rosette — the second-read detail. Made the pins restand after each half-turn so the demonstration loops indefinitely instead of dying once emptied. Wired the sidereal clock hand and digital readout to the same Earth-rotation accumulator that drives precession, so clock and plane can never disagree. Refined the topple to an easeOutBack with a lengthening cast shadow.
375px, reduced motion, driving the signature
Verified headlessly by driving the apparatus: set the latitude to the pole and confirmed the plane sweeps and pins fall fast; set it to the equator and confirmed the rate reads 0.0°/hr and nothing topples. Capped DPR at 2, re-laid the trace canvas on resize, and paused the rAF loop on document.hidden. At 375px the instrument cards stack to one column and the dial stays draggable. prefers-reduced-motion renders a single settled frame — plane mid-precession, pins half-down, clock parked — with no loop. Removed a redundant outer ring from the rose (Chanel rule) to keep the engraving calm.