When, how and why did we decide that the day should start from midnight instead of 6am?
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As others have stated, midday is when the sun is highest, but more importantly, that time doesn’t move around.
You asked why not 6am? I’ll counter that with: what’s so special about 6am? Because of the tilt of the earth and the orbit around the sun not being perfectly circular, sunrise and sunset wander earlier and later than that over the course of a year. If you measure a day as being from one sunrise to the next, “a day” is longer in the autumn (when the sunrises are getting later and later) and shorter in the spring (when the sunrises are getting earlier and earlier). When a day starts then also varies based on your latitude: the closer you are to the poles, the more extreme it gets, to the point that once you reach the Arctic or Antarctic circles, you suddenly find the sun stops setting and rising during parts of the year, leaving our “1 day = time between two sunrises” system as now having some *really* long days.
Okay, that’s a good argument against using sunrise, but why not just arbitrarily pick 6am? Because 6am is really hard to observe without complex timekeeping tools. These days, devices that keep time with millisecond precision are ubiquitous, but if you’re in an village that’s crowing about their latest technological achievement, the windmill, you’re lucky if your hours are measured accurately. What you *can* measure easily, though, is when the sun gets to its highest point.
Making that the *divider* point, though, is really awkward, as it means something you do in the morning is done on a different date to something you do in the afternoon. But if you make that the *midpoint* of the day, with an equal portion before and after it, that messy seam gets hidden in the night time, when people aren’t doing as much. Sunsets on one day, they go to bed, sun rises on another day when they get up. When did the date change? Eh, somewhen during the night, can’t be quite sure, but conceptually, that moment was as far from yesterday’s noon and it is from today’s noon, and it’s not drifting over the course of a year, nor will it change if you travel north or south.
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