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It took centuries for the people on Earth to decide on a common meridian to measure longitude from, but other planets also need everyone to agree about the origins of their mapping systems. In the case of the terrestrial planets a single bright spot was chosen in the early stages of exploration, and as maps improved the exact location is defined with increasing accuracy. For tidally locked moons the meridian is defined based on orientation relative to the parent body, but even then there’s a lot of room for improvement as data improves.
Finally some bodies are just not suited to spherical coordinated, because they’re not particularly spherical.