The question of why Earth’s gravity is so much stronger than the Moon’s gravity is answered by two factors: Earth’s significantly greater mass and its much higher average density. Gravity is a fundamental force of nature, an attraction that exists between any two objects possessing mass. The more matter an object contains, the stronger its gravitational influence. Understanding the physical laws that govern this attraction clarifies the vast difference between the two worlds.
The Rules of Gravitational Attraction
The strength of gravity acting on an object is determined by two primary variables: the mass of the attracting body and the distance from that body’s center. The gravitational force is directly proportional to the mass, meaning if the mass of a planet doubles, its gravitational pull also doubles. This relationship establishes mass as the single most important factor when comparing the gravity of two celestial objects.
The second factor, distance, is equally important but acts inversely, specifically following an inverse square law. This means that if the distance between two objects is doubled, the gravitational force between them drops to one-fourth of its original strength. When calculating the gravity felt on the surface of a planet or moon, the relevant distance is the radius, measured from the surface point to the object’s center of mass.
For any spherical body, the entire collective mass acts as if it were concentrated at the central point. Surface gravity is a measure of the acceleration an object feels at the body’s surface, calculated using the object’s total mass and its radius squared. This relationship shows how the physical size of the body, represented by its radius, slightly counteracts the effect of its mass, since a larger radius means the surface is farther from the center of mass.
Earth vs. Moon: The Mass and Density Comparison
When applying the rules of gravity, the immense difference in mass between Earth and the Moon becomes the most significant factor. Earth has approximately 81 times the mass of the Moon. This overwhelming disparity in the amount of total matter is the primary reason for Earth’s superior gravitational field.
While Earth is larger, with a radius about 3.7 times that of the Moon, this size difference is not enough to offset the huge difference in mass. Although the Moon’s smaller radius inherently increases surface gravity by bringing the surface closer to the center of mass, its lack of mass means the total amount of matter pulling on an object is far less than on Earth.
The root cause of the mass difference lies in the objects’ internal composition and density. Density is a measure of how tightly matter is packed into a given volume. Earth has an average density of about 5.5 grams per cubic centimeter, making it the densest large body in the Solar System. This is largely due to Earth’s massive iron and nickel core, which compresses the surrounding layers.
The Moon, by contrast, has an average density of only about 3.3 grams per cubic centimeter, closer to the density of Earth’s crust and mantle. This lower density is thought to result from the Moon’s formation, created from lighter debris ejected after a massive impact on the early Earth. Lacking a dense, large metallic core, the Moon does not contain enough tightly packed matter to generate a gravitational pull comparable to Earth’s.
Measuring the Difference in Surface Gravity
The combined effect of Earth’s greater mass and higher density results in a measurable, quantifiable difference in surface gravity. On average, the acceleration due to gravity on the Moon’s surface is approximately 1.62 meters per second squared. This is only about one-sixth, or 16.6%, of the acceleration due to gravity experienced on Earth’s surface, which is about 9.8 meters per second squared.
This ratio explains why an object’s weight on the Moon is one-sixth of its weight on Earth, as weight is the force exerted by gravity on a given mass. For example, a person weighing 180 pounds on Earth would weigh only 30 pounds on the Moon. This low surface gravity also results in secondary effects, such as the Moon’s inability to retain a substantial atmosphere.
The Moon’s weak gravitational force allows lighter gas molecules to escape into space over time. Earth’s strong gravitational field, conversely, is sufficient to hold a thick, life-sustaining atmosphere close to the surface. Furthermore, the Moon’s small size and low mass caused it to cool much faster than Earth, resulting in a geologically mostly inactive body.