For centuries, people have looked up at the night sky and understood Earth to have a single natural satellite. However, in an age of advanced astronomical observation, the question arises: does Earth truly have only one moon, or are there other objects that might also fit this description?
Earth’s Primary Moon
Earth’s well-known companion, Luna, is a significant astronomical body. It orbits our planet at an average distance of about 384,399 kilometers. With a diameter of 3,474 kilometers, Luna is roughly one-quarter the size of Earth, a proportionally large size compared to other moons in our solar system relative to their parent planets. This substantial size means the Earth-Moon system behaves almost like a double-planet system.
The Moon’s gravitational pull is a primary driver of Earth’s ocean tides, and it also plays a role in stabilizing Earth’s axial tilt, which contributes to our planet’s relatively stable climate. The Moon is tidally locked with Earth, meaning the same side always faces us due to gravitational forces. Scientific theories suggest our Moon formed approximately 4.5 billion years ago from debris ejected into orbit after a Mars-sized object collided with early Earth.
Objects Orbiting Earth
While Earth has only one permanent natural satellite, other celestial bodies can temporarily interact with our planet’s gravitational field. These include quasi-satellites and temporarily captured objects.
Quasi-satellites, like asteroid 469219 Kamoʻoalewa, orbit the Sun in a special orbital resonance with Earth. From Earth’s perspective, these objects appear to loop around our planet, but they are primarily orbiting the Sun and are not gravitationally bound to Earth in a stable, long-term way.
Temporarily captured objects, or mini-moons, are asteroids briefly pulled into Earth’s orbit. These small bodies can orbit Earth for months or even a few years before escaping back into a solar orbit. For example, asteroid 2024 PT5 was a temporary mini-moon for about two months in late 2024, and 2020 CD3 orbited Earth for approximately three years before departing in 2020. Such captures are rare and require precise conditions of speed and trajectory.
What Defines a Moon
Astronomically, a “moon” is defined as a natural satellite that orbits a planet, dwarf planet, or another celestial body. A key characteristic of a true moon is a stable, long-term orbit around its parent body. Unlike the temporary companions, a moon’s orbit is primarily governed by the gravitational pull of the planet it orbits.
While there is no strict lower size limit, traditional moons are large enough to be gravitationally rounded, although smaller, irregular natural satellites also exist. The definition distinguishes natural satellites from artificial satellites or temporary cosmic visitors. Therefore, while Earth may occasionally host transient celestial guests, only Luna fits the enduring astronomical criteria for a true moon.