Keeping Time
How Humanity Learned to Measure the Days, the Hours and the Seconds
The history of time in ten chapters, from notched bones and stone circles to atomic clocks that can feel a millimetre of gravity. Each chapter answers one question, with sources.
- 1. Bones and stonesc. 40,000 – 2000 BCE · How did prehistoric people keep track of time?
- 2. The 24-hour dayc. 3000 BCE – 150 CE · Why are there 24 hours in a day and 60 minutes in an hour?
- 3. Water, sand and firec. 1500 BCE – 1400 CE · How did ancient clocks work? Sundials, water clocks, candles and hourglasses
- 4. The mechanical clockc. 1275 – 1700 · Who invented the mechanical clock?
- 5. The longitude problem1707 – 1773 · Who solved the longitude problem?
- 6. Railway time and time zones1840 – 1918 · How did time zones start? Railway time and the 1884 Meridian Conference
- 7. Einstein and relativity1905 – today · Does time really slow down? Einstein, relativity and time dilation
- 8. Atomic clocks1949 – today · How do atomic clocks work?
- 9. Is time real?1850 – today · Is time real? The arrow of time, entropy and physics
- 10. The future of timeToday and next · What is the future of timekeeping?
Timeline of timekeeping
| When | What happened |
|---|---|
| c. 40,000 years ago | Lebombo bone: 29 notches, possibly a lunar count |
| c. 8000 BCE | Warren Field pits track the moon, reset at midwinter |
| c. 3200 BCE | Newgrange aligned to the winter solstice sunrise |
| c. 2100 BCE | Egyptian star clocks divide the night into 12 hours |
| c. 1500 BCE | Egyptian sundials and water clocks |
| 3rd century BCE | Ctesibius builds a constant-flow water clock |
| 46 BCE | Julian calendar with a leap day every four years |
| 1092 | Su Song's astronomical clock tower in Kaifeng |
| c. 1275–1300 | First mechanical clocks in Europe |
| 1582 | Gregorian calendar |
| 1656 | Huygens builds the first pendulum clock |
| 1714 | Longitude Act offers £20,000 |
| 1759 | Harrison completes H4 |
| 1840s | British railways adopt London time |
| 1883 | US and Canadian railroads adopt standard time zones |
| 1884 | Greenwich chosen as the prime meridian |
| 1905 | Einstein's special relativity |
| 1915 | General relativity |
| 1916 | First daylight saving time (Germany) |
| 1949 | First atomic clock |
| 1955 | First accurate cesium clock |
| 1967 | Second defined by the cesium atom |
| 1972 | UTC with leap seconds |
| 1995 | Kiribati moves the date line, creating UTC+14 |
| 2016 | Most recent leap second |
| 2024 | Thorium nuclear transition excited by laser |
| 2035 | Leap seconds to end |