Understanding Time: Units, History, and Measurement

Common Units of Time

Time can be added or subtracted just like numbers, but its calculations follow unique rules. Here are standard time units:

Millennium – 1,000 years
Century – 100 years
Decade – 10 years
Year (average) – 365.242 days
Common Year – 365 days
Leap Year – 366 days
Quarter – 3 months
Month – 28–31 days (varies by month and leap years)
Week – 7 days
Day – 24 hours (86,400 seconds)
Hour – 60 minutes (3,600 seconds)
Minute – 60 seconds
Second – Base unit
Millisecond – 10⁻³ seconds
Microsecond – 10⁻⁶ seconds
Nanosecond – 10⁻⁹ seconds

Historical Concepts of Time

Ancient Greece – Aristotle defined time as a measurement of change tied to motion, believing it infinite and continuous.

Newton vs. Leibniz – Newton proposed absolute time, flowing independently of events, while Leibniz argued for relational time, existing only in relation to objects and events.

Einstein – In his theory of relativity, Einstein connected space and time into spacetime, showing that time is relative to speed and gravity. The faster you move through space, the slower you move through time.

Measuring Time: Calendar & Clock

Today, time is tracked using the calendar (years, months, days) and clock (hours, minutes, seconds). These systems are based on the ancient sexagesimal system (base 60) from Sumer and Babylon, chosen for its many divisors.

Development of the 24-Hour Day

Egyptians used sundials dividing daylight into 12 parts; night was divided using star positions.
Hipparchus (147–127 BC) standardized 12 hours of daylight and 12 of night, leading to the 24-hour day.
Claudius Ptolemy divided degrees into minutes and seconds, giving us the terms still used today.

Early Timekeeping Devices

Sundials – Measured daylight hours.
Water clocks (clepsydra) – Tracked time with flowing water.
Hourglasses – Used sand to measure fixed intervals.
Pendulum clocks – Invented by Christiaan Huygens in 1656, accurate to within seconds a day.
Atomic clocks – The most precise today, measuring time using cesium atomic resonance.