Gravitation the force that acts among all objects is because of their mass. Because of gravitation, objects that are on or near the earth are pulled toward it that is why all the stars orbit around the center of the galaxy. (World Book Encyclopedia Eric lerner, an Internet yahoo search)
The effects of gravity are easy to see, on explanation for gravitational force has puzzled people for centuries. ( World Book Encyclopedia Eric lerner) The ancient astronomers studied the motions of the moon and the planets. (Eric lerner) But these motions weren't correctly explained until the late 1600's, when the English scientist Sir Isaac Newton showed that there is a connection between the force that attracts objects to the earth and the way the planets move. (Internet -Yahoo search And Find World book Encyclopedia)
Newton's theory of gravitation says that the gravitational force between the two objects is proportional to the size of their masses. That is, the larger either mass is, the larger the force is between the two objects. (Newton) The theory refers to mass rather than weight because the weight of an object on the earth is really the strength of the earth's gravity on that object. On different planets, the same object could have different weights, but its mass would always be the same. Also the gravitational force is inversely (oppositely) proportional to the distance between the centers of gravity of the two objects squared.
Having researched about gravity has left me in awe. I have learned a lot about gravity and what affects are on all the planets, especially Earth and the moon. What I have learned has led me to a very interesting experiment and things you can do with gravity. The experiment is as fallows, first you tie a string to a ball or an apple then you hold the string in your hand and rotate thus producing the effect of gravity. To find out what happens when gravity is lost you then carefully cut the string with a pair of scissors and the ball or apple loses its gravity.