When, how and why did we decide that the day should start from midnight instead of 6am?
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Midnight being the start/end of the day is because midday/noon when the sun is highest in the sky is the middle of the day, so, the exact opposite of that, midnight, became the point at which we decided to say the day starts/ends.
Now, this is completely arbitrary, some cultures see days as going from sunset to the next sunset.
The middle of the day, when the sun is at its highest, is close to, though not exactly at, noon. It depends on exactly where you are and the way time zones work, so it is not at 12pm for everyone, but still close to it. If we started the day at 6am, then the middle of the day would be at 6pm in the evening, which would obviously be crazy. So it makes sense that the day starts at midnight, reaches the middle when the sun is at or near its highest point and finishes at midnight.
It probably became more common as people migrated away from the equator, where sunrise and sunset were more consistent, as were the lengths of days.
Time itself, though, continued to be fairly arbitrary until the development of the railways. Because trains would arrive and leave at a specific time, everyone needed to know what that time was, hence the whole of the UK ran to a clock determined from Greenwich in London.
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.
Well, noon is 12 hours before and coincides with the point where the sun is highest in the sky, so midnight is 12 hours later.
Ancient peoples from around the world came to many similar conclusions based on patterns they saw. Every 365 days or so, there is a repeating pattern of short cold days in winter, and long warm days in summer. Its bad enough when you are a hunter/gatherer, but once you start planting crops, storing them in clay jars, and setting up a permanent village, these patterns remain important.
Knowing the time of year tells you when to plant crops. Too late and they may not be ripe before an occasional early winter frost. Too early and they may die in a late spring frost. Knowing the time of year also tells you when to travel to hunt migrating herds.
There are two days when the night is equal to the day, the equinox in Latin is “equal night”. One day in spring and one day in fall. If you build two markers that are inline on those two days, then the shadows at sunrise will tell you when you can celebrate the spring equinox. The fall equinox tells you when winter will arrive soon and you must prepare.
The winter solstice in December is the shortest day, and the longest night. It is a celebration because from that day forward, the days will start getting longer.
The year-long pattern and the day-long pattern seem to be similar in several ways, so the middle of the night is the opposite of the middle of the day, and that is an analogue of the winter and summer solstices.
There are roughly 12 moon cycles per year, so 12 is a sacred number to them. I would have guessed that ancients would have “naturally” made twelve longer hours to cover both the night and day, but our ancestors decided that the day should have 12 divisions of time, and the night should have its own 12 divisions.
Most of this came from the Babylonians, but many things have evolved over time.
Iirc, it’s sort of based on ancient guard rotations. The first rotation at sun up, second at sun high, third at sun down, fourth at a random spot in between down and up
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