It's been a while since I wondered why are our days divided into 24 hours. An hour, unlike the day or the year, does not correspond to a physical phenomenon. It seems to be an artificial unit.
We can easily imagine how the day arose as a unit of time: it is the time until the sun returns to the same point in the sky. The year also arises from a similar physical argument: it is the time until the stars return to the same location in the night sky (at a fixed time during the day), or, macroscopically, until the earth has completed one full revolution around the sun. Even the month arises (roughly) from a physical phenomenon: one month is almost the time between full moons.
If anything, you would want to divide the day into 20 hours (10 hours of day and 10 hours of night). However the hour, 1/24th of the day, does not seem to arise from a similar physical phenomenon. Or does it?
I found the likely answer in the book "Sundials: Design, Construction, and Use" by Denis Savoie.
The idea originated around 2000BC in Ancient Egypt. The Egyptians used to divide the year into 12 months of 30 days each (12x30=360 total), to which they added the 5 so-called "epagomenal" days. Critical fact: The 30-day months where divided into 10-day "weeks".
(Sidenote: Numbers like 60 or 360 were very convenient for the ancients, since due to the lack of calculators they needed numbers that could be divided easily in many parts. That's why a circle is also divided into 360 degrees.)
So the Egyptian priests needed some way to determine the exact time for the nighttime prayers. They obviously thought to use the stars to achieve this. They marked the end of the night when a certain star was just rising when the dawn was breaking. As the days pass, the same star can mark the end of the night, but each day it rises a little earlier in the night sky, seen against an increasingly darker sky (that's due to the motion of the earth around the sun).
By that time, another star will have the property of just rising when the dawn breaks, marking the end of the night. After a few more days, another star would be selected, and so on.
Each star was naturally selected to mark the end of the night for a "week", or a 10-day period. Over the course of the year, a total of 36 stars would be appointed as timekeepers, each star marking the end of the night for a 10-day period. These 36 stars would be evenly spread in the night sky, dividing it into 36 "hours".
So how many of these stars would be rising during the course of the night in the sky? The answer is not 18, because of the diffusion and scattering of the sunlight from the atmosphere.
You'll have probably noticed that the sky is bright long before the sun rises, and it is not completely dark for quite a while after the sun sets. Even at the equinoxes, where the sun is above the horizon exactly for half a day, there is light for more time than half a day due to this effect.
In addition, the amount of daylight varies throughout the year, and in the summer the amount of timekeeper stars rising would be definitely less than 18.
It turns out that around the summer solstice (where the night time is minimum), if we also include the effect of sunlight diffusion, only 12 stars will be seen to rise during nighttime. As a result, the night was divided into 12 hours, as was the day.
PS: It is worth noting that the 24-hour days ultimately originate from the familiar number 10 again. If the Egyptian weeks consisted of 7 days instead of 10, we would have 30 or 32 hours in a day.
