function* circles(now) {
const t = now / 60, cx = width / 2, cy = height / 2;
yield* circle(cx, cy, 120, 96, -0.001 * t);
yield* circle(cx, cy, 30, 10, 0.03 * t);
yield* circle(cx, cy, 60, 3, -0.05 * t);
yield* circle(cx, cy, 15, 4, -0.02 * t);
yield* circle(cx, cy, 0, 1, -0.02 * t);
yield* circle(240 + cx, -120 + cy, 80, 4, -0.02 * t);
yield* circle(240 + cx, -120 + cy, 0, 1, -0.02 * t);
yield* circle(280 + cx, 120 + cy, 40, 8, 0.02 * t);
yield* circle(280 + cx, 120 + cy, 20, 8, -0.02 * t);
yield* circle(280 + cx, 120 + cy, 0, 1, 0.02 * t);
}