In 1610, Galilei observed the change in the visible phase of the planetary disk for the first time, using a telescope he invented. The mechanism of Venus' phase changes is similar to those of the Moon. Individuals with sharp eyesight may sometimes discern the small crescent of Venus with a naked eye.
The model's left-side window displays the Sun with Venus rotating around it. Venus is observed from a point that is stationary in respect to the stars, and is situated at the distance of 5 astronomical units from the Sun.
Press "Run" button. Venus will begin to rotate around the Sun, and the share of its illuminated surface that is inverted in respect to the observer will change. The observer that is using a telescope will see the alternating phases of Venus as shown in the right-side window. Note that the visible portion of the planetary disk undergoes changes. This occurs due to the fact that Venus either approaches the observer or travels away from him.
Press "Stop" button to suspend the animation. Press "Reset" to return the model to its initial state. A special window displays Earth years and months - the time it takes Venus to go through the cycle of its phases.