With this app you can learn visually about the waxing and waning of the moon based on the positions of the moon earth and sun.

The upper screen is a chart of the celestial sphere from which you can check the positions of the sun and moon as seen from earth.
Change to "tracking" mode to see from the perspective of a person looking at the moon. Move the slider to change the angle of the celestial sphere.

The lower screen is a chart from which you can see the relationships between the positions of the moon earth and sun.
(To allow for easy understanding sizes and distances are different from actual proportions.)
The moon as seen from earth at the current time is shown in the top left. Move the slider to change the angle of the figure.

*The celestial sphere chart shows the perspective from the northern hemisphere (around Japan).