text: why earth moves round the sun?   every body of mass produces a force of gravity. the larger the body, the more gravity it produces. the sun is large enough to produce enough gravity to pull earth towards it. however, at the same time, earth is in motion. the motion would be straight, but because the sun's gravity pulls earth towards the sun, the earth's path of motion is curved around the sun. it is like swinging a ball on the end of a string, with gravity acting like an invisible string.