Units of Time
Like other numbers, time can be added or subtracted. However, due to how time is defined, there exist differences in how calculations must be computed when compared to decimal numbers.
| Unit | Definition |
|---|---|
| Millennium | 1,000 years |
| Century | 100 years |
| Decade | 10 years |
| Year (average) | 365.242 days or 12 months |
| Common year | 365 days or 12 months |
| Leap year | 366 days or 12 months |
| Quarter | 3 months |
| Month | 28-31 days (Jan, Mar, May, Jul, Aug, Oct, Dec: 31 days | Apr, Jun, Sep, Nov: 30 days | Feb: 28/29 days) |
| Week | 7 days |
| Day | 24 hours, 1,440 minutes, or 86,400 seconds |
| Hour | 60 minutes or 3,600 seconds |
| Minute | 60 seconds |
| Second | Base unit |
| Millisecond | 10-3 second |
| Microsecond | 10-6 second |
| Nanosecond | 10-9 second |
The History and Measurement of Time
Development of the Second, Minute, and 24-Hour Day
The Egyptian civilization is often credited as being the first civilization to divide the day into smaller parts, due to documented evidence of their use of sundials. The earliest sundials divided the period between sunrise and sunset into 12 parts. Since sundials could not be used after sunset, Egyptian astronomers noticed patterns in a set of stars and used 12 of those stars to create 12 divisions of night. Having these two 12-part divisions of day and night is one theory behind where the concept of a 24-hour day originated.
The divisions created by the Egyptians, however, varied based on the time of the year, with summer hours being much longer than those of winter. It was not until later, around 147 to 127 BC, that Greek astronomer Hipparchus proposed dividing the day into 12 hours of daylight and 12 hours of darkness based on the days of the equinox. This constituted the 24 hours that would later be known as equinoctial hours, resulting in days with hours of equal length. Despite this, fixed-length hours only became commonplace during the 14th century along with the advent of mechanical clocks.
The Sexagesimal System
There are two distinct forms of measurement typically used today to determine time: the calendar and the clock. These measurements of time are based on the sexagesimal numeral system, which uses 60 as its base. This system originated from ancient Sumer within the 3rd millennium BC, and was adopted by the Babylonians.
Base 60 is used due to the number 60's status as a highly composite number having 12 factors. For example, 1 hour (60 minutes) can be evenly divided into 30, 20, 15, 12, 10, 6, 5, 4, 3, 2, and 1 minute, illustrating the reasoning behind the sexagesimal system's use in measuring time.
Early Timekeeping Devices
Early devices for measuring time were highly varied based on culture and location. Oil lamps and candle clocks were used to mark the passage of time from one event to another. The water clock, also known as a clepsydra, is arguably the most accurate clock of the ancient world, functioning based on the regulated flow of water into a container.
The first pendulum mechanical clock was created by Christiaan Huygens in 1656; it was the first clock regulated by a mechanism with a "natural" period of oscillation. Today, atomic clocks are the most accurate devices for time measurement. They use an electronic oscillator to keep track of passing time based on cesium atomic resonance.
Concepts of Time in Physics and Philosophy
Aristotle, Newton & Leibniz
The ancient Greek philosopher Aristotle defined time as "a number of movement in respect of the before and after." Sir Isaac Newton tackled the concepts of space and time as absolutes, arguing that absolute time exists and flows without any regard to external factors, which he called "duration." Contrary to Newton's assertions, Gottfried Leibniz believed that time only makes sense in the presence of objects with which it can interact (relational time).
Einstein and Relativity
Albert Einstein introduced the idea of spacetime as connected, rather than separate concepts of space and time. Einstein posited that the speed of light, c, in a vacuum, is the same for all observers, independent of the motion of the light source. To put it simply, if an object moves faster through space, it will move slower through time, and if an object moves slower through space, it will move faster through time.