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

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!