Where is north is actually intensely tied to the question of What is a day?. At least we wind up defining it in much the same way(s) as we define the day. In the previous post, I gave a definition for north/south. Namely, the line of a shadow cast by the sun at solar noon (itself define by the fact that it's the shortest shadow of the day) is north/south.
As happened for the day, we find our most accurate definition from examining the stars other than the sun. The 'pole star' isn't actually one that we use for this. It's almost a full degree away from the actual pole of the earth's rotation.
What we do instead is look for day to day differences in the location of stars passing overhead or nearly so. This approach dates farther back than Seth Carlo Chandler, in the 1890s. But we'll be coming back to Chandler. If the earth has wobbled a little to the north, then the star will pass the zenith a little to the north of where it did yesterday. If you've got a good telescope and other instruments, you can observe this to pretty good precision. Chandler was working with accuracy of 1 second of arc or somewhat better for single measurements. Because of the power of using multiple measurements he was able to examine earth wobbles that were less than 0.1 seconds of arc.
This turned out to be quite useful, as the earth wobbles by about 0.3 seconds of arc. It's how he discovered what was promptly called the Chandler Wobble. This translates to about 3 meters motion in where the pole is.
The orientation of the earth is, as with the rotation rate, tied to where the mass is and where it moves to. The earth's orientation is believed to have changed by some millionths of an arc second due to the earthquake. The variations of a few tenths of a second of arc are caused by ... other things. Atmosphere and ocean circulations are what I'm most concerned about, but also the earth's inner core, and the moon, and .... It's a messy business.
As for the length of day, your scientific source for observations is the International Earth Rotation Service. Which, itself, owes something to Chandler.
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