In recent years, long-term observations and research by scientific researchers have shown that: (1) Earth's rotation has seasonal changes and periodic changes in half a year and year cycles; (2) Earth's rotation speed has slowed down in the long run; (3) Sun's orbital movement is strongly correlated with changes in the Earth's rotation rate; (4) Earth's core has a fast rotation opposite to the direction of the Earth's rotation; so what are the causes of these phenomena? This paper analyzes the gravity between planets, proposes the equivalent sphere of the planet's graviton emitted gravitons and the gravitational action point of the planet, and explains the separation phenomenon of the gravitational action point and the center of mass. Taking the Sun-Earth system as an example, the Earth's action point under the gravity of the Sun is at a 0.5 radius near the Sun side. Due to the separation of the Sun-Earth gravitational action point on the Earth and the center of mass of the Earth, the Sun's gravity acts on the Earth's gravitational action point, which will cause the Earth to produce a centripetal force orbiting the Sun, and the Earth will produce a reverse rotation force. For the rotation of the earth, since the gravitational action point is relatively fixed, the center of mass of the earth rotates inversely relative to the sun-earth gravitational action point. Under the combined action of the inertial force of the earth, the centripetal force of the sun, and the rotation of the earth in the opposite direction about the gravitational action point, the earth's rotation will form an elliptical orbit. This paper simulates the elliptical orbit of the earth orbiting the sun. Through calculation and simulation, 99.99% of the earth's gravity is used for the earth's rotation. This essentially finds the internal reason for the earth's rotation changes. This article believes that the earth's rotation changes seasonally and periodically, slows down for a long time, is related to the strong changes in the solar orbit, and the rapid rotation of the earth's core in reverse are all related to the separation between the sun's gravitational point and the center of the earth, and are the inevitable result of almost all of the sun's gravity used for the earth's rotation.