"Most clock faces have a unique animation when you zoom in and out of the clock app with the Digital Crown:
The Solar face animates the watch app icon into the sun when zooming in"
This means that when you are on the home page, and you zoom into the clock app, with the digital crown, it animates so that the clock app icon slowly turns into the sun. Here's an example: http://i.imgur.com/IEdLmph.gifv