For most of history, an hour in summer outlasted one in winter
Ancient timekeepers split daylight into twelve parts and night into twelve more, whatever the season. A summer afternoon hour therefore stretched long while a winter one shrank. These seasonal hours lasted for millennia, and it took mechanical clocks, which hated being readjusted every dawn and dusk, to make every hour the same length.
The Egyptians seem to have started it. The Nile flood that sustained their farming coincided with Sirius rising before dawn, and twelve constellations crossing the night sky, so they carved the night into twelve intervals and later did the same for the day. Greece and Rome inherited the scheme. Early Greeks divided only the day into twelve, keeping three or four watches at night; Hipparchus of Nicaea is probably the first to have split the full day and night into twenty-four, and in the first century BCE Athens gained the Tower of the Winds, which tracked a twenty-four-hour day with sundials and mechanical indicators.
The sixties come from even further back. Splitting an hour into sixty minutes and each minute into sixty seconds is a Sumerian legacy, taken up through Babylonian astronomy by medieval scholars such as al-Biruni. Where counting began varied too. Many cultures started at sunrise; so-called Italian hours began at sunset, running from 1 to 24, so in Lugano the December sun rose during the fourteenth hour. Medieval Europe's working day followed church bells ringing prime, terce, sext, nones and vespers at roughly three-hour steps.
Mechanical clocks changed the equation. Showing unequal hours meant altering a pendulum's length every morning and evening, while equal hours required adjustment only a few times a month. Revolutionary France briefly tried going further, decreeing a day of ten hours, each of 100 decimal minutes, from 1794; mandatory use was suspended within months, though a few places clung to it until 1800.
Today an hour is fixed at 3,600 atomic seconds, detached from Earth's spin. Yet the planet still intrudes: because the Moon's tidal drag gradually slows Earth's rotation, civil hours of Coordinated Universal Time have run to 3,601 seconds on 27 occasions since 1972. The word itself descends through Old French and Latin from a Greek term for any span of time, including seasons, making hour a distant cousin of year.
Source: Hour
