# Why does the difference between terrestrial times and universal time, $\Delta T$, have such a strange behaviour?

I was reading about ancient astronomy, and one of the main phenomena described is eclipses. Now, I read this 2016 paper by Stephenson, Morrison and Hohenkerk, in which they determine the values of the parameter $$\Delta T=UT-TT$$. For the pre-telescopic era, that is, from 700 BC to AD 1600, the parameter is simply a smooth quadratic function, while during the telescopic era, from 1600 to the present, it suddenly becomes erratic and oscillatory. I want to know why is this, what causes this parameter to have such a strange behaviour.

• Related: en.wikipedia.org/wiki/%CE%94T_(timekeeping) $\Delta T$ has always been irregular, but the data from before the telescope era isn't sufficient to make precise estimates on short time scales, but we do know its long-term behaviour from solar eclipse data. Sep 30, 2021 at 22:47
• The Moon has nothing to do with short term (up to decades, perhaps even several millennia) variations in the Earth's rotation rate, but it has a whole lot to do with very long term (many millennia or longer) variations in the Earth's rotation rate. Oct 1, 2021 at 15:12
• An example of a long term variation that has nothing to do with the Moon is the ongoing redistribution of mass that started with the end of the last glaciation, about 12000 years ago. There are parts of the far north that are still recovering from the weight of all of the ice that had built up during the last glaciation. Oct 1, 2021 at 15:17

I want to know why is this, what causes this parameter to have such a strange behaviour.

TL;DR: Better observations.

Measurements were absolutely lousy before the telescope era, and remained fairly lousy throughout much of the telescope era. It has only been the recent several decades where very long baseline interferometry (VLBI) measurements have made measurements of the Earth's rotation rate precise. We are seeing apparently chaotic behavior because of those vastly improved measurements, rather than because the Earth's rotation rate suddenly became chaotic at the same time VLBI techniques were developed. The Earth's rotation rate has always been chaotic to some extent.

$$\Delta T$$ is dependent upon the rotation of the Earth, which is affected by multiple factors.

Some of these factors are known and can be calculated/predicted, such as the gravitational pull of the Moon, the Sun, the planets, etc., but some take place inside the Earth itself: mantle currents, for example, are main contributors.

Since we can’t predict or calculate mantle currents, this is why $$\Delta T$$ seems erratic and “unpredictable.”

• Mantle currents are main contributors? Citation needed. Daily variations are mostly due to weather. Seasonal variations are due to the uneven distribution of land and ocean between the northern and southern hemispheres. For example, the snow that accumulates over winter in the northern parts of the northern hemisphere does not have a counterpart in the southern hemisphere. Yearly to decadal changes are due partly to climate, partly to ocean currents. Longer term variations (decades to centuries) have largely been attributed to interactions between the Earth's core and the mantle/crust. Oct 1, 2021 at 14:54
• I suppose that even longer term variations might be attributable to mantle currents, but a citation would be nice. Oct 1, 2021 at 14:56
• I was writing out of memory, and you’re right, I was a little off… However, mantle flows do affect Earth’s tilt… See for example jpl.nasa.gov/news/… Oct 2, 2021 at 2:35