What are the half-moons in the HUD in Prince of Persia: The Sands of Time?
Solution 1:
These are called power tanks, which you need for certain special abilities.
The white crescent symbols next to the Sand Tanks indicate your Power Tanks. Shove your dagger into sixteen enemies and Retrieve their sand to add another Power Tank (you can't have more than ten). Most of your special moves require energy from the Power Tank.
Source