Where were shadow clocks first used?, In ancient Egypt., What did shadow clocks measure?, The position and length of a shadow., What made water clocks more reliable than shadow clocks?, They worked both day and night., What did oil clocks use to measure time?, A burning candle or lamp with markings., When did mechanical clocks appear in Europe?, In the 13th century., How did early mechanical clocks signal the time?, By ringing a bell., What invention did Christiaan Huygens create in 1656?, The pendulum clock., Why are atomic clocks the most accurate?, They use atomic energy transitions..

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