Abstract: The communications synchronous satellite should move exact synchronous track and it's aerial should be suit to the needs of the earth,while keeping attitude stabilization. The article supposes that the earth is a uniform ball which is surrounded by the seawater, analyzes the force of the sun and the moon to seawater in the earth's core reference frame, calculates the tide force causing the sun tide and the moon tide by the law of gravity, and draw a conclusion that asymmetry gravitation causing the tide is ...