Mercury is indeed bigger than the Moon, a fact that may surprise many who are accustomed to seeing the Moon as a large, bright object in the night sky. The celestial body orbiting Earth is a natural satellite, whereas Mercury is the smallest of the eight planets in our solar system. Although both are small, airless, rocky worlds, the differences in their physical scales are significant enough to fundamentally change how they function in space.
Comparing the Scale
Mercury has an equatorial diameter of approximately 4,880 kilometers, which is significantly wider than the Moon’s diameter of about 3,474 kilometers. To put this difference into perspective, the Moon is roughly 71% the size of Mercury across its equator.
The disparity in mass is even more pronounced due to Mercury’s dense composition. Mercury has a mass of about 3.3 x 10^23 kilograms, which is more than four times the Moon’s mass of approximately 7.35 x 10^22 kilograms. This difference stems from Mercury’s high density (5.4 grams per cubic centimeter) compared to the Moon’s density (3.3 grams per cubic centimeter). Mercury’s interior is dominated by an enormous iron core, giving it the second-highest density in the solar system after Earth.
The Consequences of Size
The vast difference in mass and density dictates the physical conditions on the surface of each body, particularly regarding gravity. Mercury’s greater mass creates a much stronger gravitational pull than the Moon’s. Surface gravity on Mercury is about 38% of Earth’s gravity, while on the Moon, it is only about 17%.
This stronger gravity allows Mercury to retain a slightly more substantial, though still extremely tenuous, exosphere composed of elements like sodium, potassium, and oxygen. Both bodies technically lack a true atmosphere, instead possessing an exosphere. The Moon’s weak gravity means that any gases released from its surface quickly escape into space, resulting in a near-vacuum environment.
Placement in the Solar System
The reason the Moon often appears larger and more prominent than Mercury is purely a matter of location. The Moon is Earth’s close companion, orbiting at an average distance of only 384,000 kilometers. This proximity makes it the second-brightest object in our sky and allows us to easily observe its full size.
Mercury is the innermost planet, orbiting the Sun at an average distance of about 58 million kilometers. Its distance from Earth varies significantly, ranging from about 77 million to over 222 million kilometers. Mercury is also visually challenging because it is always seen close to the glare of the Sun, appearing only briefly as a “morning star” or “evening star.”