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.
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.”