Titan and Ganymede are indeed moons or natural satellites, despite their relatively large size compared to Mercury. The classification of a celestial body as a planet or moon is not solely based on size but also on its relationship to the star it orbits.
In our solar system, planets are defined as celestial bodies that:
- Orbit the Sun directly.
- Are not satellites of other planets.
- Have cleared their orbit of most other debris, meaning they have become gravitationally dominant in their vicinity.
Mercury meets these criteria, as it orbits the Sun directly and is not a satellite of any other planet. It is considered a planet because it satisfies the specific criteria set by the International Astronomical Union (IAU).
On the other hand, Titan and Ganymede are both moons because they orbit other planets within our solar system. Titan is the largest moon of Saturn, and Ganymede is the largest moon of Jupiter. Despite their substantial sizes, they are still considered moons because they meet the criteria of being natural satellites of another celestial body.
It is worth noting that the term "moon" does not imply any inferiority or less significance compared to the term "planet." Moons play important roles in the dynamics of their planetary systems, and they are fascinating objects of study in their own right.