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How do you know whether the sun is up or down in a place when all places have the same time?

I see people bringing this question up as a counter-argument for a unified time.

But how do you know whether the sun is up or down in any given place on Earth right now? You probably don't, if the place is a few thousand miles East or West from where you are. You'd have to look up its time zone. And doing that might not be that much different from looking up at what hour the solar noon is somewhere else.

You could do that, sure. Whenever I want to schedule a meeting with someone in Zurich, I could reason “15:00 UTC is about an hour before solar noon for me and — looking it up — about four hours past for him. Reasonable time for a meeting.” This would prove so useful in practice that people would rapidly start referring to times that way when talking to people from abroad: “sorry I couldn’t take your call, it was seven hours past solar noon for me; I was having dinner.” Everyone would memorize their location’s offset from UTC and use it colloquially all the time.

Oh wait, we’ve reinvented time zones!

Basically, the point of time measured in hours and minutes is to be useful to humans in daily life. And it’s useful to have a common language for things relevant to humans.

It’s fundamentally the same lookup.

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