Over the years, humans have found increasingly efficient ways to ration the day into smaller—and more precise—units.
3500 through 1500 B.C.
The earliest clocks
Mechanism: The sun’s movement across the sky, water dripping from a vessel, candles burning down through marked increments
Error: Extremely large and variable
Accurate enough to: Schedule religious ceremonies
1200s through 1920s
Mechanical clocks
Mechanism: Balance wheels, weights, or pendulums
Error: From 2 hours per day in the 1200s to 1 second per year (1921)
Accurate enough to: Coordinate military activities and standardize trains
1920s through the present
Quartz clocks Mechanism: Electrically induced vibrations of quartz crystal
Error: From 10 seconds per day for a cheap watch to 0.3 second per year
Accurate enough to: Measure variations in the rotation rate of the Earth
1949 through the present
Atomic clocks Mechanism: Microwave resonance of atoms
Error: From 1 second every 300 years in 1955 to 1 second in 70 million years (2001)
Accurate enough to: Synchronize GPS satellites
Future
Optical atomic clocks Mechanism: Ultraviolet resonance of atoms
Error: Less than 1 second in 1 billion years
Accurate enough to: Improve space navigation
Lizzie Buchen
