The red time ball on top of the Flamsteed House is one of the world's first visual time signal. It was installed in 1833 to enable navigators on ships in the Thames to check their marine chronometers
The time ball drops daily at 1300 GMT in winter and British Summer Time (an hour ahead of GMT) in summer. It is raised halfway up the mast at 1225 as a preparatory signal and to the top 2 minutes before it drops.
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