3 ms·

All the discussion about gravity is good, but I was equally interested in the slowing of the earth's rotation. 1.8ms per day? It seems like that's large enough

All the discussion about gravity is good, but I was equally interested in the slowing of the earth's rotation. 1.8ms per day? It seems like that's large enough to be noticed with modern clocks/astronomy outside of eclipse records. Is this confirmed outside of eclipse records?

CabSauce 6y ago
Yes. That's why we have leap seconds.
- mgamache 6y ago
  I thought that was just to correct for the initial definition of a second which was inaccurate. But, it looks like it is the reason for leap seconds: https://www.timeanddate.com/time/leap-seconds-background.html#:~:text=Leap%20seconds%20are%20added%20to,slowing%20of%20the%20Earth's%20rotation https://www.timeanddate.com/time/leap-seconds-background.htm....
  - jandrese 6y ago
    Leap seconds can be affected by things like big volcanic eruptions and earthquakes. That's why they are defined "as needed" instead of at regular intervals, and why they are applied somewhat irregularly.
vharuck 6y ago
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://news.ycombinator.com/item?id=16656903 https://news.ycombinator.com/item?id=16656903
- 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.

CabSauce 6y ago

Yes. That's why we have leap seconds.

mgamache 6y ago
I thought that was just to correct for the initial definition of a second which was inaccurate. But, it looks like it is the reason for leap seconds: https://www.timeanddate.com/time/leap-seconds-background.html#:~:text=Leap%20seconds%20are%20added%20to,slowing%20of%20the%20Earth's%20rotation https://www.timeanddate.com/time/leap-seconds-background.htm....
- jandrese 6y ago
  Leap seconds can be affected by things like big volcanic eruptions and earthquakes. That's why they are defined "as needed" instead of at regular intervals, and why they are applied somewhat irregularly.

vharuck 6y ago

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://news.ycombinator.com/item?id=16656903 https://news.ycombinator.com/item?id=16656903

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.