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Answer: Manhattanhenge is a semi-annual occurrence when the setting sun aligns itself with the direction of Manhattan's grid system. The title is a reference to the stonehenge whose is suspected to have been built for a similar reason of alignment with the sun. AJ, Raghu, tsp, Akshay, Karthik, Ameya, Matti Tapaninen, Vetti, Jaggi, Krithi, Kr!sh, Rajesh K, Kamal Rathi, Renuka, anon and abstract randomness got it. Well done.