試してみて、太陽や月を回転させるコードを書いてください。
var rotationSpeed = 0.01;
var rotation = 0;

function rotateSunAndMoon(){
 rotation += rotationSpeed;

 context.save();
 context.translate(canvas.width / 2, canvas.height / 2);
 context.rotate(rotation);
 context.drawImage(sun, -sun.width / 2, -sun.height / 2, sun.width, sun.height);
 context.restore();

 context.save();
 context.translate(canvas.width / 2, canvas.height / 2);
 context.rotate(- rotaion);
 context.drawImage(moon, -moon.width / 2, -moon.height / 2, moon.width, moon.height);
 context.restore();

 requestAnimationFrame(rotateSunAndMoon);
}