4 ms·

This struck me, too. So days were 4 hours shorter 2500 years ago? The evidence our ancestors worked fewer hours a day just keeps piling up [0]! [0] https://n

• jaycroft 6y ago
  Perhaps I'm missing some sarcasm, but the article states that two clocks, one synchronized to the earth's rotation, and the other keeping atomic time, they would drift by a total of 4 hours over 2500 years, not drift by 4 hours per day. Those are completely different things.
• pdonis 6y ago
  > So days were 4 hours shorter 2500 years ago? No. The cumulative clock difference between two clocks, one of which is adding leap seconds due to the slowing of Earth's rotation, the other of which is not, is not the same as the rate of slowing of the Earth's rotation. The current rate of slowing is about 1.4 milliseconds per 100 years. That means that, 100 years ago, the length of 1 day was 1.4 milliseconds shorter than it is today. So let's suppose that 100 years ago, the day was exactly 86,400,000.0 milliseconds long. Then today would be 86,400,001.4 milliseconds long. Now add up all those days over the last 100 years, i.e., 36,525 of them, and suppose the rate of increase in the day is linear. Then the average extra milliseconds per day is half of 1.4, or 0.7 milliseconds, and the total number of milliseconds in those 36,525 days is 3,155,760,025,567.5 instead of the 3,155,760,000,000.0 that it would have been if the length of the day had been constant. That's an extra 25,567.5 millliseconds over 100 years, which is a lot bigger than 1.4. That's the difference between the cumulative clock difference and the rate of slowing.