Single Rotate Method
For one, you don't want to pass a string to setInterval(); better to use a function. You can bind that function to the object's context, so it'll call the right thing.

For another, aim for less ambiguous names. rotation should probably be updateRotation, since that's what it does. And rotate should likely be startRotating (and they you could have an equivalent stopRotating).

That be something like:

this.startRotating = function () {
  var self = this; // to maintain the object's context
  setInterval(function () {
    self.updateRotation();
  }, 100);
};


However, if you're planning to add more planets etc., it would probably be nicer to have one central timer that calls an update function on all your objects.

And I'd also define a "proper" prototype for all this, and extract some general logic/avoid repetition:

// extract this logic, since it's generally useful
function getElement(element) {
  if (typeof element == "string") {
    return document.getElementById(element);
  }
  return element;
}

function Mass(element, angularVelocity) { // added the rotation speed as an arg
  var computedStyle;

  this.element = getElement(element);

  // fetch this once
  computedStyle = window.getComputedStyle(this.element);

  this.width  = parseInt(computedStyle.width, 10);
  this.height = parseInt(computedStyle.height, 10);
  this.top    = parseInt(computedStyle.top, 10);
  this.left   = parseInt(computedStyle.left, 10);
  this.radius = this.width / 2;

  this.origin = {
    x: this.left + this.radius, // you just calculated the radius; might as well use it here
    y: this.top + this.radius
  };

  this.angularVelocity = angularVelocity;
  this.rotation = 0;
}

// make nice prototype
Mass.prototype = {
  update: function () {
    this.rotation = (this.rotation + this.angularVelocity) % 360;
    this.element.style.webkitTransform = "rotate(" + this.rotation + "deg)";
  }
};

// do the timer magic outside the object
var moon = new Mass("moon", 1.0),
    timer;

timer = setInterval(function () {
  moon.update();
}, 100);


fiddle