Robert Bueno, Jr.,, age 10, of E1 Cajon, California, for his question:
Why does the earth have more gravity than the moon?
Judging from Andy's mail, the moon's gravity is just about the most puzzling topic of our times. Some young spaceagers even think that the moon has no gravity at all. They wonder how the astronauts can stand and even walk on the lunar landscape with no force of gravity to hug them down to the surface. However, most of us know that the moon definitely has gravity. But we naturally wonder why its share is weaker than the earth's.
Gravitation is the force that holds together the whole universe and everything in it. It is built into every tiny particle of matter, part and parcel of the earth and moon and every other heavenly body. The invisible force must belong to a material object every large or small object made of solids, liquids, gases or a mixture of all three. This invisible force of gravity in every object reaches out and pulls at all other objects, near and far. Gravity attracts gravity.
The matter in an object is its mass and gravity is related to mass. The small mass of a speck of dust has a small, weak quota of gravitational force. The massive earth has a mighty quota of gravity. Our globe is about 49 times bigger than the moon. Besides, the moon is made of lighter materials. So it must be less massive than the earth. Naturally the moon has a smaller, less powerful, quota of built in gravity to match its smaller mass.
Actually the earth is almost 50 times bigger than the moon and more than 80 times more massive. These figures make it a bit tricky to compare their quotas of gravity. Does the earth have 50 or 80 times more gravity? The answer is neither. To get at the truth, we must compare these two figures with something rather odd that the force of gravity always does.
Its pulling power seems to reach outward from a spot in the very center of mass. The earth's center of gravity is pinpointed in the middle of the planet. The moon's center of gravity is down inside the middle of the moon. From the center, the force of gravity grows stronger until it reaches the surface. Then it spreads on out and gets weaker as it goes.
The earth's gravity stretches about 4,000 miles to reach the surface. In the smaller moon, its distance is only about 1,000 miles mass and size together help us to figure out the strength of surface gravity. When these tricky details are computed, the answer may not be what we expect. Actually the surface gravity of the earth is only six times stronger than the moon's surface gravity.
Weight is the measure of the pull between gravity and every object on the surface.. On the moon, this pull is only one sixth as strong as it is on the earth. If on earth you weigh 100 pounds, on the lunar landscape you weigh only about 16 1/2 pounds. You also can jump six times higher and take six times longer strides.