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Published Nov 17, 2022

Leap Seconds

Gary Arndt delves into the complex world of leap seconds, unraveling their role in harmonizing atomic clocks with Earth's fluctuating rotation while discussing the historical evolution of the second and its impact on precise timekeeping.
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  • Historical Definitions

    explains that the concept of a second has evolved significantly over time. Initially, a second was defined based on the Earth's rotation, with Islamic scholar Abu Rahan Muhammad ibn Abad al Biruni formalizing the division of a day into 24 hours, each hour into 60 minutes, and each minute into 60 seconds 1. However, as scientific instruments became more precise, it became clear that the length of a day could vary slightly, necessitating a more accurate definition 2.

    The modern second was created around the year 1000 by the Islamic scholar Abu Rahan Muhammad ibn Abad al Biruni. Al Biruni codified the system of dividing a day into 24 hours and then dividing an hour into 60 minutes and a minute into 60 seconds.

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    This led to the adoption of an atomic definition of a second, which is still in use today.

       

    Modern Atomic Time

    The shift to atomic timekeeping marked a significant advancement in precision. Scientists defined a second as the duration of 9,192,631,770 vibrations of a cesium-133 atom, a method chosen for its accuracy and reliability 2. This atomic definition closely matches the astronomical definition, differing by only one part in ten billion.

    This definition of a second using cesium atoms was very close to the astronomical definition to within one in 10 billion.

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    Despite this precision, the need for leap seconds arises to reconcile atomic time with the Earth's variable rotation.

       

    Challenges

    Even with the precise atomic definition, challenges remain in timekeeping. The Earth's rotation is influenced by factors like tidal friction and seismic events, causing slight variations in day length 3. These variations necessitate occasional leap seconds to keep atomic time and universal time in sync.

    The leap second is used to ensure that universal time and coordinated universal time don't get too far away from each other.

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    Since 1972, 27 leap seconds have been added, highlighting the ongoing need for adjustments in our timekeeping systems 2.

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