The museum opens the roof when the museum opens in the morning. And they close the roof at noon, and reopen it after few minutes. They closes the roof when the museum closed in the afternoon. The actual time, you can check on the museum web site. But, be aware of if it is a windy day, the roof will not open! Check the weather.
They open at 10 every day and close at 5 (except Thursday, which are at 8). They also flap at noon every day.
I didn't know there was a roof that opened/closed.
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