Once every 28 days approx. The moon orbits the earth. When the moon is on the opposite side of the earth with respect to the sun it is full moon because the sunlit side of the moon is facing us. As it travels within the next two weeks approx, it shows its phases as we can only view a limited degree of the sunlit side. As the moon comes between the earth and the sun (new moon) the sunlit side is turned away from us so we do not see it at all. Then during the next two weeks approx the phases show the same but in an opposing manner. The solar eclipse happens when at new moon the moon just happens to be in a perfect line between the sun and earth and so obscures the sun for a time.