Next we saw these two windows:

In the upper Universe all stars move at the same rate, while in the lower, some stars move more slowly than others.

The fact that distant objects appear to move more slowly than nearby objects is known as motion parallax. Motion parallax is a property of optics, specifically arising because any image has an angle of view.

To understand the import of an angle of view, we can draw a line at any depth that turns the angle into a triangle. In the figure to the left, the dotted lines w1 and w2 reveal the width of the triangles at distances d1 and d2 respectively. Notice that w2 is wider than w1. The greater the depth the wider the view.