You need to provide more details. It depends on what you want to do in your game, what other tiles you lose, potentially what civ you're playing, what map type, and probably more. For example, are you aiming for a peaceful early game with a focus on a late game CV such as culture/science? Then IF you can settle an observatory capital with GOOD tiles then do so.
But if you're not getting good tiles as well or your strategy needs as strong early game as possible then don't move out of your way to settle your capital next to a mountain, because observatories do not kick in until much later and Astronomy is also somewhat out of the way (on a Pangaea map). If you find a good location next to a mountain you can always settle an expansion there instead.