Midnight is not an obvious answer. It is a choice, and for most of human history it was not the most popular one.
A day has to start somewhere, and there are only really three candidates: when the sun comes up, when it goes down, or at some arbitrary point in the dark when nobody is watching. Every one of the three has been an official national standard, and two of them still are.
CAIRO — where the twenty-four hour day was invented.
Egyptian astronomers divided the sky into 36 star groups called decans and used them as a night clock. Twelve were visible across a full night, so the night got twelve hours — and the day got twelve to match.
That is where the 24-hour day comes from, and it has never been revised. The same system produced a 365-day civil calendar with no leap day at all; when Ptolemy III proposed adding one in 238 BC, the priests refused. The calendar then drifted backwards through the seasons for 1,460 years before returning to where it started.
→ Cairo
ISTANBUL — where the day began at sunset, until 1926.
Under the Ottoman alaturka system, twelve o'clock was the moment the sun went down. Which means the moment moved every single day, and every clock in the country had to be corrected every few days, all year, forever.
That job could not be centralised. It required muvakkithanes — timekeeping rooms attached to mosques — staffed by specialists who calculated and adjusted continuously so the city stayed roughly in agreement with itself.
Law 697, passed in Ankara on 26 December 1925, abolished it from 1 January 1926. The muvakkithanes were only formally closed in 1952.
NAIROBI — where the day still begins at sunrise.
Swahili time counts from dawn. Seven in the morning by international reckoning is saa moja — hour one. The offset is exactly six hours, and the day and night run as two separate twelve-hour cycles.
It is in daily use across Kenya, Tanzania and Uganda. Multilingual East Africans switch systems depending on which language they are speaking.
It works because Nairobi sits at 1.3° south, where sunrise barely moves all year.
→ Nairobi
ADDIS ABABA — the same principle, plus a different year.
Ethiopia also begins its day at dawn: six in the morning internationally is twelve o'clock in Ethiopia. Alongside that it runs a calendar of thirteen months — twelve of thirty days plus a short thirteenth called Pagumē — which is currently seven to eight years behind the Gregorian.
The structure of that calendar is the ancient Egyptian civil calendar, inherited through the Coptic church. The framework Cairo built five thousand years ago is still running here.
PRAGUE — the clock that refuses to choose.
The Orloj has been running on the wall of Prague's Old Town Hall since 1410. It displays, simultaneously:
- Old Bohemian time, in which the day begins at sunset — the same principle as Ottoman Istanbul
- Babylonian time, in which the day begins at sunrise and the daylight hours are divided into twelve regardless of length — the same principle as Nairobi and Addis Ababa
- Central European time — fixed hours, counted from midnight
- Sidereal time — measured against the stars rather than the sun
One clock face. Four answers to the question of when a day starts. All running at once. All correct.
Everywhere else in this series, a city argued about these systems and picked one. Prague built a machine that shows all of them and left the decision to whoever is looking.
→ Prague