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Relativity says it’s impossible for two observers to agree on things like simultaneity, distance, duration, etc. Without reference to astronomical landmarks (observed within a given proximity of the solar system, at low speed) how could anyone agree on the meanings of time intervals?

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I believe the cosmic microwave background provides a reasonably universal rest frame to anchor your definitions of time. It should at the very least be a lot less local than using the rest frame of a particular solar system.

Second isn't defined in terms of astronomy, it's atomic. Shouldn't matter how fast you go and where. https://en.wikipedia.org/wiki/Second#"Atomic"_second

https://en.wikipedia.org/wiki/Hafele%E2%80%93Keating_experim...

Duh. Thanks. Not my brightest moment.

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