The Moon is significantly bigger relative to Earth than most other satellites are to their planets. The question of how much smaller the Moon is compared to Earth depends on the specific measurement used for comparison, whether that is a linear dimension, the total space occupied, or the amount of matter it contains. Breaking down these metrics provides a comprehensive understanding of the Moon’s size disparity.
Linear Comparison: Diameter and Circumference
The simplest way to compare the size of two spheres is to measure their diameters. Earth’s mean diameter is approximately 12,742 kilometers, while the Moon’s diameter is about 3,474 kilometers. This means the Moon is less than a third the width of Earth, or about 27% of Earth’s diameter. Approximately four Moons would fit across the Earth’s diameter. The Moon’s equatorial circumference is also proportionally smaller, measuring about 10,917 kilometers compared to Earth’s 40,075 kilometers. This roughly one-quarter ratio gives a straightforward but incomplete picture of the overall size difference.
Spatial Comparison: Volume and Scale
While the diameter comparison is easy to grasp, the true scale of the difference emerges when comparing volume, which measures the amount of three-dimensional space an object occupies. Since volume scales by the cube of the radius, a small difference in diameter results in a much larger difference in volume. The Moon’s volume is only about 2% of the Earth’s volume. The Earth’s immense size means that roughly 49 to 50 objects the size of the Moon could be packed inside the Earth’s total volume. This cubic relationship between diameter and volume explains why the Moon appears so much smaller in scale than the simple 1:4 diameter ratio suggests.
Gravitational Comparison: Mass and Density
The comparison shifts when considering mass, which is the measure of the total amount of matter in an object, directly determining its gravitational influence. Earth is vastly more massive than the Moon, with a mass ratio of approximately 81:1. This means Earth has 81 times more material than the Moon, a much larger disparity than the 50:1 volume ratio. This difference is due to density, which is the amount of mass packed into a given volume. Earth has an average density of about 5.5 grams per cubic centimeter, while the Moon’s density is only about 3.34 grams per cubic centimeter. The Earth’s high density is largely attributed to its substantial iron-nickel core. The Moon, in contrast, has a much smaller core relative to its size and is composed of materials more similar to Earth’s outer mantle, making it a much lighter world for its given size.