let yR = 150;

​

function setup() {

  createCanvas(400, 400);

}

​

function draw() {

  background(220);

  ellipse(width/2, height/2, xR * 2, yR * 2);

  angle = map(mouseX, 0, width, 0, 2*PI);

  x = xR * cos(angle);

  y = yR * sin(angle);

  ellipse(width/2+x,height/2+y,10,10);

}