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The title here is a little misleading. A signal that repeated once per hour, as in 60 minutes, would be pretty astounding, and might be a good way for a civilization with enough information about us to say ‘hi’ in a way we’d recognize. It would certainly be very strange to see a natural phenomenon ticking away the hours at a precise rate.
53.8 minutes, on the other hand, is a bit less attention grabbing.
It’s more that their knowing what an hour is would be impressive. Our selection of the hour as a measure of time is arbitrary outside of its specific context. It’s just 1/24th of our planet’s rotational period. We could just as easily split the day up into 10ths or 15ths or 7ths or whatever.
To broadcast a signal that’s exactly an hour long to a planet that uses the hour as a measure of time might potentially imply someone trying to reference our way of measuring time. A signal that repeats every 53.8 minutes is on a timer that isn’t specifically relevant to Earth in the same way an hour exactly would be.