The circle with a dot in the middle represents the Yoni, the female sexual organ.
What is Female Principle ?
This circle with a spot in the middle should not be confused for the symbol of sun which has a blue or black dot in the middle. This sun is seen like a moon because it has a white light with a dark spot. The circle was designed with a drainage outlet with the black spot replaced by the lingam or phallus, the male principle.