The question of whether the Moon orbits inside Earth’s atmosphere highlights the difference between the common public understanding and the technical scientific definition. While the atmosphere is often envisioned as a thin, dense blanket of air, its true outer boundary is far more complex and diffuse. The answer depends entirely on the criteria used to define where the air ends and space begins. When considering the vast, outermost reaches of gas molecules gravitationally bound to Earth, the Moon is often found to be within this extended envelope.
The Traditional Boundary of Earth’s Atmosphere
For most practical purposes, Earth’s atmosphere is a dense, layered structure supporting life, weather, and aviation. The layers traditionally recognized are the troposphere, stratosphere, mesosphere, and thermosphere, where the bulk of the gas molecules are concentrated. The most widely accepted marker for the beginning of outer space is the Kármán line, which sits at approximately 62 miles (100 kilometers) above sea level. This boundary represents the point where air density is too low for conventional aerodynamic flight.
Given that the Moon orbits at an average distance of about 238,900 miles (384,600 kilometers) from Earth, it is definitively outside this conventionally defined, dense part of the atmosphere. Based on the concept of the Kármán line, the immediate answer to the question is a clear and simple “No.”
The Vast Extent of the Geocorona
The scientific understanding of the atmosphere extends far beyond the Kármán line into the outermost layer called the exosphere. This region is so thin that its atoms rarely collide, instead following ballistic trajectories under the influence of gravity. The visible component of this exosphere is the geocorona, a vast cloud of neutral hydrogen atoms surrounding Earth.
The geocorona is formed when water vapor molecules break apart in the upper atmosphere, causing hydrogen atoms to drift outward. These atoms scatter a specific type of ultraviolet light called Lyman-alpha radiation. Recent analysis of data collected by the Solar and Heliospheric Observatory (SOHO) shows this diffuse hydrogen envelope reaches immense distances, stretching out to a maximum of about 391,500 miles (630,000 kilometers).
Since the Moon orbits at 238,900 miles, it is comfortably enveloped within the outer atmosphere for at least part of its orbit. Even at this distance, the gas density is measurable, though incredibly sparse, at about 0.2 hydrogen atoms per cubic centimeter. A telescope placed on the lunar surface by Apollo 16 astronauts in 1972 captured an image of the geocorona. Based on the technical definition of the physical presence of neutral gas molecules gravitationally bound to Earth, the Moon is indeed flying through our atmosphere.
Atmosphere Versus Gravitational Influence
The physical presence of the atmosphere, even in the form of the tenuous geocorona, must be distinguished from Earth’s purely gravitational influence. Earth’s gravitational domain, often defined by the Hill sphere, represents the region where Earth’s gravity is the primary force acting on an object, dominating the Sun’s pull. The Earth’s Hill sphere extends out to approximately 930,000 miles (1.5 million kilometers). The Moon’s orbit is well within this gravitational sphere, which is why it orbits Earth and not the Sun independently.
Similarly, the Earth’s magnetosphere, the region of space controlled by the planet’s magnetic field, is also distinct. The magnetosphere protects Earth from the solar wind but consists of charged particles, not the neutral hydrogen atoms that constitute the geocorona. While the Moon is fixed within Earth’s gravitational and magnetic influence, its location within the atmosphere is determined specifically by the physical limits of the geocorona’s gas.