Earth days are getting longer – by 1.8 milliseconds per century

I thought it was getting dark later and later

We know the Earth’s rotation is slowing, hence leap seconds, but by how much? Now we have a new number: 1.8 milliseconds per century for the last 2,720 years.

That's the average figure the solar day has increased by since 720BC according to researchers at the Durham University and the UK’s Nautical Almanac Office.

The boffins reached their conclusion by crunching data on ancient and medieval lunar eclipses from 720 BC to 1600 and of lunar occultations of stars from the years 1600 to 2015.

The result means while the Earth’s days are getting longer, they are not increasing by as much as had been thought.

The rotation of our 4.5-billion-year-old planet is slowing down because of the friction caused by tidal movements. Using tidal flow, a figure of 2.3 milliseconds – 0.0023 seconds - had been calculated.

But the Durham and Almanac Office team reckon celestial beats tidal because the records are more accurate and span a longer period of time.

In their paper, here, the team wrote:

While the tidal component of the Earth’s acceleration can be derived from recent high-precision observations, the actual long-term acceleration, which is the sum of the tidal and other components, cannot be measured directly from modern data because it is masked by the relatively large decade fluctuations. Instead, observations from the historical past, albeit crude by modern standards, have to be used.

Eclipses were seen as the most accurate means of measuring the Earth’s rotation before the advent of telescopic observation and atomic time around 1620. After this it became a matter of working out the Earth's position in relation to the timing of lunar occulations of stars.

It was possible to get a Universal Time (UT) measurement of eclipses in the ancient world by translating local time systems into Universal Time (UT). Observations that were not timed, meanwhile, could be calculated using the Earth’s rotational frame to also produce a UT figure.

The celestial research has updated earlier work on this topic.The new study includes the addition of more data on Babylonian eclipse observations, a revision of data from Chinese star watchers to remove inaccuracies, and a re-working of lunar calculations from 1623 onwards to include NASA JPL data on lunar movements and star positions.

“These developments significantly improve the reliability of the determination of decade and longer-term fluctuations in the Earth’s rotation in the period 720 BC to AD 2015,” the team said.

“We present our results in graphical and tabular form to assist future investigations of the underlying geophysical mechanisms driving changes in the Earth’s rotation.” ®

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