Earth's spin has slowed. But we still may need a negative leap second.

Oct 21, 2019
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Quote from article "When astronomical time, based on Earth's rotation, deviates from UTC by more than 0.4 seconds, UTC gets an adjustment in the form of a "leap second." Sometimes leap seconds are added, as last happened on New Year's Eve 2016, when a second was added at 23 hours, 59 minutes and 59 seconds of Dec. 31. Scientists have added a leap second about every 18 months on average since 1972, according to the National Institute of Standards and Technology (NIST). When seconds are subtracted, it's known as a negative leap second. There has never been a negative leap second in international timekeeping, but 2020 raised the possibility that one might be needed. That year, Earth's rotation sped up, breaking the previous record for shortest day, set in 2005, 28 times. The shortest day in 2020 occurred on July 19, when the planet completed its rotation 1.4602 milliseconds faster than the average of 86,400 seconds. "

Whilst small variations in the Earth's rotational speed do occur in both directions i.e both speeding up and slowing down, the above quote shows that the overall trend is for the Earth's rotation to be slowing down; this is no surprise as the systematic effect of tidal drag caused by the Moon is to slow the Earth's rotation. The Earth's angular momentum that is lost by this slowing down is transferred to the Moon, which as a result moves the Moon's orbit a small amount (just under 4cm) further away from Earth each year.
 
May 14, 2021
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Likely, a negative leap second won’t necessary. More than likely, the Earth will return to its normal slowing down, and we’ll just wait ‘til we need to add a second, just be a bit longer.
 

rod

Oct 22, 2019
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The slow down in Earth's rotation is documented using solar eclipses, including ancient solar eclipse records. Historical eclipses, https://ui.adsabs.harvard.edu/abs/1982SciAm.247d.170S/abstract
Reference paper, Historical Eclipses, https://www.scientificamerican.com/article/historical-eclipses/, October 1982. "Reliable records of solar and lunar eclipses go back as far as 750 B.C. They bear on such questions as whether the sun is shrinking or the earth is not spinning as fast as it once did."

My observation. "Calculations based on this and several other observations reveal an average rate of day lengthening since ancient times to be 1.78 + or - .11 milliseconds per century." My note. The Earth's LOD is slowing down and solar and lunar eclipse measurements supports this. The rate used in this report is 1.78 x 10^-3 s/100 years. That works out to be 1.78 x 10^-5 s/yr rate of slow down. In 4 billion years, the Earth's LOD slows down 7.12 x 10^4 seconds or 19.78 hours, thus Earth's LOD 4.22 hours using linear rate of change and extrapolation, 4 billion years ago. Using the present Earth mass and radius, spin rate at equator ~ 2.64 km/s.

The Giant impact model for the origin of the Moon, continues being tweaked :) TWO IMPACTS, NOT JUST ONE, MAY HAVE FORMED THE MOON, https://skyandtelescope.org/astronomy-news/two-impacts-not-just-one-may-have-formed-the-moon/
Reference paper cited, Collision Chains among the Terrestrial Planets. III. Formation of the Moon, https://iopscience.iop.org/article/10.3847/PSJ/ac19b2, 23-Sep-2021.

My observation. The paper has interesting comments like, “If the Earth–Moon system can shed angular momentum, then perhaps the proto-Earth could have been spinning 10 times faster than today. In this case the target's equator would be already almost escaping owing to centrifugal forces, so that Moon formation could be an "impact-triggered fission," the scenario proposed by Ćuk & Stewart (2012)..." My note. Thus, the early earth day would be about 2.3 hours in this model. Indeed, the changing LOD for Earth is a clock too :)
 

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