The EJS Venus Phases model illustrates the phases of Venus as predicted by either the Copernican system of the Ptolemaic system (the Copernican system is the default). One window shows Earth, Sun, and Venus on its Copernican orbit. Venus is shown as a disk with the side facing the sun colored yellow (since it reflects the sun's light) and the other side colored dark gray (since that side is dark). Playing the simulation puts all moving objects (Venus and Earth/Sun) in motion. Another window shows what Venus would look at this position, when viewed from Earth through a telescope. A menu option displays another window showing Galileo's drawings of his telescope observations of Venus. You can modify this simulation if you have Ejs installed by right-clicking within the plot and selecting "Open Ejs Model" from the pop-up menu item.
Quelle: Open Source Physics