The Moon is substantially smaller than Earth, being only slightly more than a quarter of our planet’s diameter. This dimensional difference influences everything from the Moon’s density to its gravitational pull, revealing a unique relationship between the two bodies that is far from typical in the solar system.
The Core Comparison: Diameter and Volume
The Moon’s diameter is approximately 3,474 kilometers, making it about 27% the size of Earth’s mean diameter of 12,742 kilometers. This linear measurement means that if Earth were the size of a basketball, the Moon would be comparable to a tennis ball, visually demonstrating the one-quarter size relationship. Focusing on diameter alone can be misleading, however, because size in space is best measured by volume, which represents the three-dimensional space an object occupies.
Since volume scales with the cube of the radius, even a small difference in diameter results in a massive difference in total volume. Because the Moon’s diameter is roughly 0.27 that of Earth’s, its volume is only about 2% of Earth’s volume. In practical terms, it would take around 50 objects the size of the Moon to equal the total volume of Earth.
Mass and Density Differences
The Moon is smaller than Earth in size, but it is even smaller when measured by mass, which is the amount of matter it contains. Earth is approximately 81 times more massive than the Moon. This disproportionate difference occurs because the Moon is significantly less dense than our planet.
The Moon’s average density is only about 60% of Earth’s. This lower density is largely attributed to the Moon having a much smaller iron core compared to Earth. This difference stems from the Moon forming primarily from crust and mantle material ejected after a massive impact on the early Earth.
The gravitational pull on the Moon is only about one-sixth (16.6%) of Earth’s gravity. The Moon’s comparatively low mass means it cannot exert the same powerful gravitational force that Earth does to hold objects down.
Planetary Context of the Earth-Moon System
The size ratio between the Moon and Earth is unusual when compared to other planet-moon systems in our solar system. The Moon is the fifth largest satellite overall, but it is the largest in relation to its parent planet. The moons orbiting the other terrestrial planets, like Mars’s Phobos and Deimos, are tiny in comparison to their planet’s size, often having masses thousands of times smaller than their host.
The Earth-Moon pair is sometimes informally referred to as a “double planet” system due to this large relative size and mass. Although the common center of mass (barycenter) lies inside Earth, the Moon is massive enough to cause Earth to noticeably wobble as they orbit their common point. This shared history has led to a system that is distinct from almost every other planet and satellite pairing.