I'd actually never seen that before! Thank you for showing me, it's a lovely piece!
I was inspired primarily by this gorgeous drawing by Flying-Fox! It seems to be a common theme for Celestia and Luna, same with the title (I did a quick search on dA alone and there's more princess drawings called 'eclipse' or 'eclipsed' or something similar that I gave up trying to count )
The image is reminiscent of the Taoist yin-yang, symbolic for balance, while the title of "eclipse" implies one celestial body obstructing another. I was wondering if this juxtaposition was intentional.