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Earth Records Shortest Day, Putting Clocks (Slightly) Out of Time

posted by janrinok on Tuesday August 02 2022, @12:10PM   
from the fleeting-milliseconds-slip-by dept.

coolgopher writes:

Considering the recent thread on the potential removal of leap seconds, a story in TheAge aussie paper seemed worth adding to the discussion:

Earth had its shortest day since records began last month, with 1.59 milliseconds shaved off the usual 24 hour spin on June 29 - raising the prospect that a negative leap second may soon be needed to keep clocks matched up with the heavens.

The Earth appears to be spinning slightly faster than normal.

Usually, Earth's average rotational speed decreases slightly over time and timekeepers have been forced to add 27 leap seconds to atomic time since the 1970s as the planet slows.

But since 2020, the phenomenon has reversed with records being frequently broken over the last two years. The previous fastest day was -1.47 milliseconds under 24 hours on July 19 2020 and it was almost broken again on July 26, when the day was -1.50 milliseconds shorter. While the effect is too small to be noticeable by humans, it can accumulate over time, potentially impacting modern satellite communication and navigation systems which rely on time being consistent with the conventional positions of the Sun, Moon and stars.

It means that it may soon be necessary to remove time, adding a negative leap second, and speeding up global clocks for the first time ever.

Related stories:
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  5. 4. 3. 2. 1. 1... An Extra Second to See Out 2016

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Original Submission

• Re:Why subtract for a small error? (Score: 3, Informative) by Anonymous Coward on Tuesday August 02 2022, @05:40PM

  by Anonymous Coward on Tuesday August 02 2022, @05:40PM (#1264602)

  But we know (if our general understanding of long-term trends with gravity holds up) that the long-term trend should be toward slowing rotation for the earth, which means adding seconds occasionally.

  So why subtract ONE? Lots of code deals with positive leap seconds. Very little (from what I understand) accounts for the possibility of NEGATIVE leap seconds.

  If the earth's "speed up" trend continues for several years and we end up starting to accumulate a significant difference in that direction, maybe we either need to take action (actually do some negative leap seconds) ...

  But if we are going to have them, it makes sense to use them as corrections to stay on-course with the long-term trend, which (again, assuming we actually understand planetary rotation well) is going to require adding seconds.

  This is basically how the process already works.

  UTC is (currently) specified to be within ±0.9 seconds of UT1. To achieve this, there are provisions for leap seconds in either direction. A negative leap second would only be inserted if the UT1-UTC difference is getting uncomfortably close to +0.9s

  As you mention, the overall trend is a slowing rotation (primarily due to tidal drag from the moon) so a negative leap second has literally never been observed to date.

  On the other hand, we have also never observed the difference going more positive year over year before. But the difference is still negative and rotational speedups would have to continue for many more years before we get close to +0.9s.

  Parent

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