4. Radiolab: The Length of a Day

Jad Abumrad and Robert Krolisch talk about this idea: that days were shorter in the past. There were roughly 410 days a year millions of years ago (oh, and the moon was ten times closer). Days at the beginning of the Earth took 6 hours a day.

The questions I like to think about this include: was it that days were shorter? Or was it perhaps this: days stayed the same, and years and hours were longer? Which was it really? What is the standard between these we’re using to judge which measure of time — days, hours and years — changed and which stayed the same? And why?

Here is another way to open this question. Say a time traveller went back to that time,  and they experienced those days as being as long as the equivalent days now. Not only that but their watches run such that they count twenty-four hours with each spinning of the Earth. This is possible, especially if we consider the following point: if experience and the watch alter in relation to the year as described above, these means by which the traveler measures time are related to the day as coral growth is related. Radiolab introduces the differences in the growth rate between prehistoric and modern coral as their first piece of evidence for why paleontologists think the ancient year is longer.

So, the traveller — through their experience and through their watch — comes to the following judgement: it is the years which are different — they, like Westeros in the Game of Thrones, take longer between summers and winters. Would the traveller be wrong? (Compare with Poincare’s mathematical thought experiment about climbing to the edge of the universe).