We generally take the force of gravity to be conservative, but what if the source of gravity is moving through space? Then the force would only be conservative relative to the source, correct? As another example, consider someone in a balloon ascending with constant speed relative to earth. This person takes the balloon as his reference frame. For him a fixed point in space would be one that is fixed relative to him. If he chooses one such point, he will notice that the gravitational field at this "fixed" point is subject to change with time. Could we say gravity is non-conservative relative to this observer? Lastly, when we say a force field is conservative, are we saying that it is conservative "relative to the source"?