Webster's New World College Dictionary (Fourth Ed.) defines time as:
At its core, time is fairly elusive. We can't see it or sense it -- it just happens. Human beings have therefore come up with ways to measure time that are totally arbitrary and also fairly interesting from a historical perspective.
The day is an obvious starting point for time. A day consists of a period of sunlight followed by night. Our bodies are tuned in to this cycle through sleep, so each morning we wake up to a new day. No matter how primitive the culture, the concept of a day arises as an obvious and natural increment.
We use clocks to divide the day into smaller increments. We use calendars to group days together into larger increments. Both of these systems have very interesting origins that we'll find out about in the course of this article.
The measurement of time covers an incredible range. Here are some common time spans, from the shortest to the longest.
- 1 picosecond (one-trillionth of a second) - This is about the shortest period of time we can currently measure accurately.
- 1 nanosecond (one-billionth of a second) - 2 to 4 nanoseconds is the length of time that a typical home computer spends executing one software instruction.
- 1 microsecond (one-millionth of a second)
- 1 millisecond (one-thousandth of a second) - This is the typical fastest time for the exposure of film in a normal camera. A picture taken in 1/1,000th of a second will usually stop all human motion.
- 1 centisecond (one-hundredth of a second) - The length of time it takes for a stroke of lightning to strike
- 1 decisecond (one-tenth of a second) - A blink of an eye
- 1 second - An average person's heart beats once each second.
- 60 seconds - One minute; a long commercial
- 2 minutes - About as long as a person can hold his or her breath
- 5 minutes - About as long as anyone can stand waiting at a red light
- 60 minutes - An hour; about as long as a person can sit in a classroom without glazing over
- 8 hours - The typical workday in the United States, as well as the typical amount of sleep a person needs every night
- 24 hours - One day; the amount of time it takes for the planet Earth to rotate one time on its axis
- 7 days - One week
- 40 days - About the longest a person can survive without food
- 365.24 days - One year; the amount of time it takes for the planet Earth to complete one orbit around the sun
- 10 years - One decade
- 75 years - The typical life span for a human being
- 5,000 years - The span of recorded history
- 50,000 years - The length of time Homo sapiens has existed as a species
- 65 million years - The length of time dinosaurs have been extinct
- 200 million years - The length of time mammals have existed
- 3.5 to 4 billion years - The length of time that life has existed on Earth
- 4.5 billion years - The age of planet Earth
- 10 to 15 billion years - The suspected age of the universe since the big bang