
Concept explainers
The original “clock” used to define the length of the second was the daily rotation of Earth about its axis. Why has this clock been replaced by one based on the oscillation period of light waves emitted by atoms like cesium and rubidium?

The reason clocks based on the daily rotation has been replaced by the oscillation period of light waves emitted by atoms like cesium and rubidium.
Answer to Problem 1CA
The time period of the earth’s rotation is not fixed and to have a proper clock, it requires giving error free time period which is not possible. Hence, it is replaced by the oscillation period of light waves emitted by atoms like cesium and rubidium. The oscillation period of light waves emitted by atoms is fixed. Hence, the clock will give error free time.
Explanation of Solution
Introduction:
The earth’s rotation period is four minutes less than what we call a day. Due to this, the Sun’s position in the sky at noon is roughly fixed, but the stars slowly drift apart. So, the stars which are overhead today will slowly move in the sky until they will be overhead at noon 6 months apart.
If instead, a day was defined to be the time taken for earth to rotate, then according to above stated reason, the sun will be overhead on some day at midnight. The time period for oscillation of light waves emitted by the atoms of cesium and rubidium is always fixed. Hence, they will always give error free time period forever.
Conclusion:
The time given by the earth’s rotation won’t be same everyday as the earth’s rotation speed is not fixed. Hence, the clocks based on it will give wrong time. Hence, we use clocks based on the oscillations of light waves emitted by atoms like cesium and rubidium.
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Chapter 1 Solutions
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