Time, ever thought about it? What is it? We can't see it or even sense it, it just happens. Collins English Dictionary defines time as 'the continuous passage of existence in which events pass from a state of potentiality in the future, through the present, to a state of finality in the past'. We often hear the expression 'Since time began' but when did it begin? Well the accepted theory at present is that it began about 15 billion years ago when the Big Bang is suspected to have taken place, before that, nothing! That is the longest time span imaginable and at present the shortest period of time, which can be measured accurately, is 1 picosecond or one-trillionth of a second. (How do they measure something like that?) In between there are time spans that we can relate to more easily.
- 1 millisecond (one-thousandth of a second) ' the fastest time for the exposure of film in a camera that will normally freeze all human motion.
- 1 centisecond (one-hundredth of a second) ' the length of time it takes for a lightning stroke to strike.
- 1 decisecond (one-tenth of a second) ' the blink of an eye.
- 1 second ' the average beat of a healthy heart or the Earth travels 18.5 miles in its orbit.
- 60 seconds ' one minute, a long time on the 'phone, as in 'hold the line a minute please'.
- 60 minutes ' an hour, which is about as long as a person can sit in a lecture without resting their eyes or an adult male sheds 600,000 particles of skin.
- 24 hours ' a day, the amount of time it takes the Earth to rotate once on its axis or 10 pints of blood makes 1,000 complete circuits of the human body.
- 7 days ' one week, newborn Guppies (tropical fish) double in size.
- 365.24 days ' one year, the amount of time it takes the Earth to complete one orbit of the Sun, or 16 million thunderstorms occur on Earth.
- 75 years ' the typical lifespan for a human being.
- 5,000 years ' the extent of recorded history.
- 50,000 years ' the length of time homo sapiens have existed as a species.
- 65 million years ' the length of time dinosaurs have been extinct.
- 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.
How long is a day? It's the amount of time it takes the Earth to do one complete rotation on its axis, but how long does it take the Earth to rotate? This is where things become completely arbitrary. The world decided to standardise on the following increments. A day consisting of two 12 hour periods, an hour of 60 minutes and a minute of 60 seconds. The seconds are then divided on a decimal system into tenths or hundredths of a second.
Quite a bizarre way to divide up a day. Divide it in half, then divide the halves by 12ths, then divide the 12ths into 60ths, then divide by 60 again, then convert into a decimal system for the smallest increments. No wonder we all had trouble learning to tell the time! Why are there 24 hours in a day? We don't really know but the tradition goes back as far as the 8th century BC and is another story.
Modern time is based on the second, a day is 86,400 seconds and a second is officially defined as 9,192,631,770 oscillations of a Caesium-133 atom in an atomic clock. (Now I know how they measure a picosecond)
We, as astronomers, use light years to measure distance but unfortunately the 'popular press' tend to use it to mean a very long time, which can cause confusion. We know that the definition of a light year is the distance light will travel in space in one year. That is, the speed of light in metres per second multiplied by the number of seconds in one year and divided by 1000, means that a light year is 9.46 X 1012km. but in astronomy, time and distance are bound together.
When we observe a celestial body not only are we looking at an object which is an unimaginable distance from us, but we are looking back into time and seeing that object as it was x years in the past.
'Time has only a relative existence' Carlyle.
Acknowledgements are due to How Stuff Works for the factual data.