How can I gradually make an element less opaque while I am dragging?

In my program there are actually two suns, but one sun aka "dark_sun" is just behind the other aka "sun". I want "sun" to gradually fade out or gradually become less opaque halfway across the arc (which is the direction the sun is going in on my program), so only "dark_sun" will be visible by the end of the arc. How do I do that? This is the program URL: http://ift.tt/1ke141D