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Why do you think a time scale has to be aligned with solar day? Are you an astronomer or come from an astronomy adjacent background?

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Of all the definitions and hidden assumptions about time we’re talking about, possibly the oldest one is that the sun is highest at noon.

That's already false except along one line within every timezone (and that's assuming the timezone is properly set and not a convenient political or historical fiction). Let's say your timezone is perfectly positioned, and "true" in the middle. Along its east and west boundaries, local noon is 30 minutes off. Near daylight savings transitions, it's off by about an hour everywhere.

Local noon just doesn't matter that much. It especially doesn't matter to the second.

True, exactly “noon” hasn’t had a solar definition for a while. But whatever time the sun is highest for you, I imagine you still expect that to happen at the same time every day.

The first clock precise enough to even measure the irregularity of Earth rotation was only build in 1934.

Before, it was simply the best clock available.

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