The model of the solar system in Figure 10 also illustrates the second type of abstraction: abstraction as encapsulation. The brass cylinder in the middle of the device encloses a clockwork of cogwheels similar to the one in Figure 10.
This mechanism causes the small spheres representing the planets to rotate around the centre at a velocity that is proportional to the speed of the actual planets. The mechanism is hidden from view for the casual user, who is only interested in the movement of the planets. Figure 11 illustrates this other view of the orrery in terms of two layers.
Generally, abstraction as encapsulation involves the two layers shown in Figure 12.
The layer through which the user interacts with the model is called the interface. It hides the detailed workings of the model from the user. The interface sits between the user and the layer at which the model is implemented. The latter is responsible for making the model do what it is supposed to do. This is where the automation of the model takes place.