Agis Porikos
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If you should find the National Observatory of Athens closed, when you arrive, during your visit, as I did in August. Rather than leave, explore a little.
You may find that the grounds surrounding the Observatory itself are open to the public, providing the opportunity to view Athens from an unobstructed vantage point.
If you're looking for a quiet spot, offering a great view of the city. Without having to wait in line in the city center, this is a great choice.