The Moon is a familiar and bright object in the night sky, making it Earth’s largest natural companion. The Moon is neither a star nor a planet. It is a large, rocky celestial body gravitationally bound to Earth, classified based on specific astronomical criteria.
Why the Moon Is Not a Star
The primary characteristic of a star is its ability to generate its own light and heat through thermonuclear fusion. Stars are massive celestial bodies that achieve the immense pressure and temperature required to fuse hydrogen into helium in their cores. The Moon completely lacks the necessary mass to initiate this process.
The Moon is a cold, solid object that does not generate internal heat. The light we see is sunlight reflected off its rocky surface. This reflection makes the Moon visible to us, similar to how a mirror reflects light, contrasting sharply with the self-illuminating nature of a star.
Why the Moon Is Not Classified as a Planet
The International Astronomical Union (IAU) established three criteria for a celestial body to be classified as a planet in our Solar System. A planet must orbit the Sun, possess sufficient mass to achieve a nearly round shape, and have gravitationally cleared the neighborhood around its orbit. The Moon fails to meet the first and third of these requirements.
While the Moon does meet the shape requirement, having enough mass for its gravity to pull it into a spherical form, its orbital path disqualifies it. The Moon orbits Earth, a planet, rather than directly orbiting the Sun as a primary body. Furthermore, the Moon has not cleared its orbital neighborhood, as it is a satellite within the gravitational domain of a much larger body.
The Moon’s True Classification: A Natural Satellite
The correct astronomical classification for the Moon is a natural satellite. A natural satellite is defined as a celestial body that orbits a planet, dwarf planet, or other smaller Solar System body. Earth’s Moon is the fifth-largest natural satellite in the entire Solar System.
The Earth-Moon system is unique because of the Moon’s size relative to its host planet, which is unusually large compared to other planet-moon ratios. This size difference sometimes leads to the system being described as a “double-planet” system. Despite this dynamic, the Moon’s orbital path around Earth places it firmly in the category of a natural satellite.