Don't Re-Draw Background

// sketch.js

var xC;  // orbit center
var yC;

var rV = 120;  // orbit radius Venus
var rE = 180;  // orbit radius Earth

var aV;  // angle Venus
var aE;  // angle Earth

var xV; var yV;  // coord venus
var xE; var yE;  // coord Earth

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

  background(0, 0, 255);

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

  aV = -PI/2;
  aE = -PI/2;

  ellipseMode(RADIUS);
} //setup

function draw() {
//  background(0, 0, 255);

//  drawCentralPoint();
//  drawOrbits();

  aV = aV + PI/40;
  aE = aE + PI/60;

  drawVenus();
  drawEarth();

  strokeWeight(3);
  stroke(255, 255, 0);
  line(xV, yV, xE, yE);
} //draw

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

function drawOrbits() {
  strokeWeight(1);
  fill(0, 0, 0, 0);
  ellipse(xC, yC, rV, rV);
  ellipse(xC, yC, rE, rE);
} //drawOrbits

function drawVenus() {
  xV = xC + rV*cos(aV);
  yV = yC + rV*sin(aV);
  fill(255, 0, 255);
  ellipse(xV, yV, 7, 7);
} //drawVenus

function drawEarth() {
  xE = xC + rE*cos(aE);
  yE = yC + rE*sin(aE);
  fill(0, 255, 255);
  ellipse(xE, yE, 7, 7);
} //drawEarth