This symbol is commonly known as a standard planetary symbol for Earth. Logically, the circle symbolizes the globe; the Earth itself. The cross has variable symbolic meaning (four "corners" of the earth, four directions, four seasons, four clans of mankind, four virtues, etc).
the Seal of Solomon. The symbol is representative of the combination of opposites and transmutation. By combining the alchemical symbols for fire (upwards triangle) and water (downwards triangle), the alchemical symbols for earth and air are also created. The downwards facing triangle is divided along the center by the base line of the opposite triangle.