Mark a Central Point

// sketch.js

var xC;  // orbit center
var yC;

function setup() {
  var cnv = createCanvas(640, 400);
  cnv.position(24, 140);

  background(0, 0, 255);

  xC = width/2;
  yC = height/2;
} //setup

function draw() {
  strokeWeight(3);
  point(xC, yC);
} //draw