The Earth and Moon form a unique pairing in the solar system, characterized by a satellite that is unusually large relative to its planet. Our Moon’s substantial size fundamentally influences the dynamics of the Earth-Moon system. This relationship is so distinct that some scientists refer to them as a double-planet system rather than a planet and its satellite. To understand the true scale of this difference, it is necessary to quantify the size disparity across multiple physical dimensions.
Comparing the Diameters
The most direct way to compare the sizes of Earth and the Moon is by looking at their diameters. Earth’s mean diameter spans approximately 12,742 kilometers (7,917 miles), while the Moon’s diameter measures about 3,475 kilometers (2,159 miles). This reveals that the Moon is about 27% the size of Earth’s width. This ratio is large compared to other satellites; for example, the largest moon of Mars, Phobos, is less than one-half of one percent of Mars’s diameter. It would take approximately four Moons lined up across the Earth’s equator to span the full distance.
Understanding Volume and Mass
Volume
Analyzing volume reveals the true difference in the amount of space occupied. Because volume increases faster than diameter, the Moon’s volume is far less than 27% of Earth’s. Earth’s volume is approximately 50 times greater than the Moon’s volume, meaning nearly 50 Moons could fit inside the Earth.
Mass and Density
Earth has a total mass of about 5.97 x 10²⁴ kilograms, compared to the Moon’s mass of about 7.35 x 10²² kilograms. This makes Earth roughly 81 times more massive than the Moon, resulting in the Moon’s mass being only about 1.2% of Earth’s. The difference between the volume and mass ratios is explained by density, which is the amount of mass packed into a given volume. Earth has an average density of about 5.51 grams per cubic centimeter, while the Moon’s average density is only about 3.34 grams per cubic centimeter. This lower density suggests the Moon has a smaller iron core relative to its overall size, a characteristic of bodies formed from lighter, outer-mantle material.
Scale Analogies and Visualizing the Difference
Translating these numbers into everyday objects helps visualize the size disparity. If Earth were scaled down to the size of a basketball, the Moon would be comparable to a tennis ball or a softball. A more precise analogy scales Earth to the size of a nickel, which would make the Moon about the size of a coffee bean. These analogies illustrate the difference in surface area. The Moon’s total surface area measures approximately 38 million square kilometers. This is less than the surface area of the continent of Asia, which covers about 44.4 million square kilometers.