Lesson plan
second
The second (symbol: s) is a unit of time derived from the division of the day, first into 24 hours, then into 60 minutes, and lastly into 60 seconds, for a total of 24 × 60 × 60 = seconds per day. That definition, based on of a rotation of the Earth, is still used by the Universal Time 1 (UT1) system. The current and formal definition in the International System of Units (SI) is more precise: The second [...] is defined by taking the fixed numerical value of the caesium (Cs) frequency, Δν Cs , the unperturbed ground-state hyperfine transition frequency of the Cs-133 atom, to be when expressed in the unit hertz, which is equal to s −1 . This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks. As the speed of Earth's rotation varies and is slowing ever so slightly, a leap second is added at irregular intervals to civil time to keep clocks in sync with Earth's rotation.
Article summary
The second (symbol: s) is a unit of time derived from the division of the day, first into 24 hours, then into 60 minutes, and lastly into 60 seconds, for a total of 24 × 60 × 60 = seconds per day. That definition, based on of a rotation of the Earth, is still used by the Universal Time 1 (UT1) system. The current and formal definition in the International System of Units (SI) is more precise: The second [...] is defined by taking the fixed numerical value of the caesium (Cs) frequency, Δν Cs , the unperturbed ground-state hyperfine transition frequency of the Cs-133 atom, to be when expressed in the unit hertz, which is equal to s −1 . This current definition was adopted in 1967 when it became feasible to define the second based on fundamental properties of nature with caesium clocks. As the speed of Earth's rotation varies and is slowing ever so slightly, a leap second is added at irregular intervals to civil time to keep clocks in sync with Earth's rotation.
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Discussion questions
- Summarise the article in your own words. Which single claim would you verify first, and why?
- Which claim in this article is most likely to be revised as the sources update? How would you check its current status?
- If two sources disagreed about one of these facts, what would you need in order to trust one over the other?
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