The Royal Greenwich Observatory 24-hour clock
Eli Coory/Fotolia
The terms in bold link to topics in the AQA, Edexcel, OCR, WJEC and CCEA A-level specifications, as well as the IB, Pre-U and Highers/Advanced Highers exam specifications.
The definition of the second involves a regular repeating motion such as the Earth’s rotation, a pendulum or an electronic oscillation. Many derived SI units rely on having a reliable standard for the second, which is now based on the absorption of radiation by excitation of electrons between atomic energy levels.
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