const b = 75;

let theta = 0;

const points = [];

​

function setup() {

  createCanvas(480, 480);

  angleMode(DEGREES);

}

​

function draw() {

  background(255);

  strokeWeight(3);

  noFill();

  stroke(255, 0, 0);

  circle(width/2, height/2, a*2);

  stroke(0, 0, 255);

  circle(width/2, height/2, b*2);

  const x1 = width/2  + a * cos(theta);

  const y1 = height/2 + a * sin(theta);

  const x2 = width/2  + b * cos(theta);

  const y2 = height/2 + b * sin(theta);

  stroke(128);

  strokeWeight(8);

  point(x1, y1);

  point(x2, y2);

  strokeWeight(3);

  line(x1, 0, x1, height);

  line(0, y2, width, y2);

  stroke(0, 204, 0);

  strokeWeight(8);

  point(x1, y2);

  points.push(createVector(x1, y2));

  strokeWeight(3);

  beginShape();

  for (const p of points) {

    vertex(p.x, p.y);

  }

  endShape();

  theta += 1;

  if (theta == 360) {

    points.splice(0);

    theta = 0;

  }

}