This wall is lit through a 20 cm square of clear acrylic whose exit face carries a computed relief, 5.8 mm from lowest ripple to highest. Sunlight entering the flat face is bent by that relief — nothing else — and the bending has been arranged, by gradient descent through a differentiable refraction model, so that the scattered light reconvenes on a wall 50 cm away as a luna moth.
What you are dragging is the sun. Here is the surprise the physics insisted on: a thin pane bends light almost the same way at every angle, so off the design angle the moth does not dissolve — it walks. It slides 8.8 mm along the wall per degree of sun, still 94% correlated with itself out to ±10°. All afternoon it crawls across the room at about two millimetres a minute, and for roughly nine minutes it stands exactly where it was drawn. The simulation you are watching is the same ray optics the design was verified with: refraction at both faces, Fresnel losses, and the real 0.53° width of the sun, which is why the moth is soft — sunlight cannot draw sharper than the sun.
This is a buildable object. The relief is smooth, slope-limited to 28°, and sized for a CNC mill or resin print in clear material. Files below.
The pane as printable/millable geometry, and the raw prescription. 200 mm square, 8 mm base, relief on top. Acrylic (n≈1.49), design wall distance 500 mm, normal-incidence sun. If you mill it, polish well — scattering is the enemy.
