3 ms·

Somewhat related, but the Earth's rate of rotation changes somewhat randomly over the centuries due to tidal friction, tectonic motion, and climate change. The

Somewhat related, but the Earth's rate of rotation changes somewhat randomly over the centuries due to tidal friction, tectonic motion, and climate change. These random perturbations add up, so over the course of a few thousand years, the rotation angle of the Earth can be uncertain by up to 70 degrees or so. But because total solar eclipses occur at times that we can calculate and are only visible from specific locations on the Earth, geologists have been able to use ancient accounts of solar eclipses to constrain the long-term change in the Earth's rotation rate.

pfortuny 4y ago
Just a question: 70 degrees in just several thousand years? Is that a mistake?
- antognini 4y ago
  Those were the numbers I saw when I was looking into this a few weeks back. If you think about it, a few millennia corresponds to ~1,000,000 days. So to know the rotation angle to within 10 30 degrees or so, you need to know the length of the day to better than one part in 10^7. But each year the length of the day changes by a few tenths of a millisecond, which is about 3 x 10^-8 of the length of a day. So you have to know the change in the length of the day reasonably well not only today, but at all times going back 2000 years. Those few tenths of a millisecond add up over thousands of years!
  - pfortuny 4y ago
    Ah, I see where your number comes from. However, those deviations should happen in both senses (adding and subtracting)? But thanks for the computations!
    - ls612 4y ago
      They do but in the long run because of the tidal forces of the moon days become inexorably longer.
      - pfortuny 4y ago
        Wow, thanks for the details!
- deleted 4y ago
  [deleted]
- vikingerik 4y ago
  He means 70 degrees of rotation within the daily cycle, so roughly 5 hours of rotation. (He's not talking about the rotational axis orientation being off by 70 degrees.) 5 hours over thousands of years is known to be within the margin of unpredictable irregularities. Nowadays we compensate for these irregularities by altering our timekeeping by way of leap seconds.
  - pfortuny 4y ago
    I got this idea late, thanks. My mistake. That makes sense.

pfortuny 4y ago

Just a question: 70 degrees in just several thousand years? Is that a mistake?

antognini 4y ago
Those were the numbers I saw when I was looking into this a few weeks back. If you think about it, a few millennia corresponds to ~1,000,000 days. So to know the rotation angle to within 10 30 degrees or so, you need to know the length of the day to better than one part in 10^7. But each year the length of the day changes by a few tenths of a millisecond, which is about 3 x 10^-8 of the length of a day. So you have to know the change in the length of the day reasonably well not only today, but at all times going back 2000 years. Those few tenths of a millisecond add up over thousands of years!
- pfortuny 4y ago
  Ah, I see where your number comes from. However, those deviations should happen in both senses (adding and subtracting)? But thanks for the computations!
  - ls612 4y ago
    They do but in the long run because of the tidal forces of the moon days become inexorably longer.
    - pfortuny 4y ago
      Wow, thanks for the details!
deleted 4y ago
[deleted]
vikingerik 4y ago
He means 70 degrees of rotation within the daily cycle, so roughly 5 hours of rotation. (He's not talking about the rotational axis orientation being off by 70 degrees.) 5 hours over thousands of years is known to be within the margin of unpredictable irregularities. Nowadays we compensate for these irregularities by altering our timekeeping by way of leap seconds.
- pfortuny 4y ago
  I got this idea late, thanks. My mistake. That makes sense.