Measurement of Time

< 24.2 Story of Measurement 1 | Topic Index | 24.4 Story of Measurement 2 >

Earliest Measured Entity

Time, was perhaps the earliest "ever-changing" experience of all living beings.

It possibly was the first entity to be measured by humans. The cycle of day & night was an unavoidable fact of life. The waxing and waning of the Moon was also seen to have a pattern which in turn were seen to be related to the cycle of days & nights. The seasons also had a cycle which could be related to the lunar & the daily cycle.

Since the flow of time was intricately related with human life, they must have motivated humans to keep track of them with tally marks, knots on ropes etc etc.

It is possible that the idea of counting may have also emerged from these observations.

Day, Hour & Second

Though the decimal system has been adopted in almost all measurements, we still measure time using non-decimal units. We divided the year into 365/366 days; the hour into 60 minutes & the minute into 60 seconds.

The "minute" was called "pars minuta prime" or the primary minor part of the hour. The "second" was called "pars minuta secunda" or the secondary minor part of the hour.

Acknowledging the advent of the digital era, we use the decimal system for measuring time periods above a year or below a second. We also use units like centuries & millennia.

Extremely Small Time Periods

For much of history, the second was adequate as the shortest unit of time. It could possibly be related to the blinking of the eye.

But how long is a second? Initially it was taken as the fraction of a day, which was the time taken for the Earth to rotate once.

The day was found to fluctuate, and then the second was defined as a fraction of an year, the time the Earth takes to revolve around the Sun once. Even this was known to fluctuate.

But scientific &amp; technological advancements have required the measurement of ever smaller intervals of time. Today launching of satellites in space using rockets and coordination of their movement for calculating positions accurately using GPS have become common occurrences. Hence there was need for an extremely accurate measure of the second!

Another interesting fact is that the artificial satellites used for GPS move at sufficiently high speeds for the relativistic effects on time to take place. Hence GPS software has to take account of "shortening" of time durations due to the Theory of Relativity!

Hence scientists started looking at the vibration time of the electrons in certain atoms. Thus atomic clocks were invented.

For this the decimal system has become handy. Now routinely extremely short parts of the second are also coded in milli (10-3) &amp; nano (10-9) seconds.

In 1967 the second was defined as “the duration of 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium-133 atom.”

In 1968, after one year of the ICWM definition, Atomic clocks (which their timekeeping mechanism is based on the Cesium-133 frequency) started to be commercially available, and used to this day.

The shortest possible time interval that can be measured using classical physics is the Planck Time. It is the time it takes for a photon to travel a distance equal to the Planck length: = 1.62 × 10^-35 m.

Meter and Second are closely Connected

In one of the rare unexplained coincidences, the meter & the second seem to be closely connected.

The value of &pi; is closely related to that of the gravitational constant "g". &pi;^2 is almost equal to g which 9.80665!

Because of this, there is a relation closely connecting the length & time measurement units.

The time period of a pendulum which is 1m long is almost 2 seconds, for a complete swing. Such a pendulum is called a "Seconds Pendulum".

Historical Time 

Today we follow a system of dating international events which is uniform all over the world. Hence events in history can be arranged chronologically. This was not always so.

In ancient times, each region had its own way of denoting the year. Years were denoted by events associated with famous kings or calamities. So arranging historical events chronologically was a very difficult task.

But Selucus Nicator, who succeeded Alexander, started the Selucid Era in around 323 BC and ensured that, in the areas under his control, all years &amp; events were marked by one number. All subsequent calendars like the Gregorian, Islamic etc followed this system. Incidentally this was the same Selucus who gave his daughter in marriage to Chandragupta Maurya.

Today the entire world, at least for political and commercial transactions, follows a calendar which is supposed to start with the birth of Christ. The time before the birth of Christ is called BCE, Before Common Era, originally called BC and the time after the birth of Christ is called CE or Common Era, originally called AD.

No Year 0

The Anno Domini calendar that we use was invented in the 5th century A.D when the concept of 0 had not been introduced in Europe. Hence in this calendar there is no year 0. 1 A.D follows 1 B.C.

The Hindu & Buddhist calendars had an year 0.

AM &amp; PM

The term &ldquo;day&rdquo; is used in 2 senses. The layman&rsquo;s idea of a day is when the Sun is shining and it is 12 hours by the clock. The scientists take the day as consisting of 24 hours. Using AM &amp; PM allows both systems to coexist without confusion.

Arithmetic of the clock can be very confusing to young children, since here the base is 12 when dealing with hours and 60 when dealing with seconds. Do we really need to put young children through all this mental trauma in the earlier classes? They can learn it is a few hours at the middle school level.

Time Zones 

Scientifically the time at any place is unique and dependent on the longitude. But in practical terms countries, even if they span several hundred degrees of longitude, need common time standards. Hence as a compromise most countries follow a time standard depending on the "local" time of a particular longitude which passes through it. Some large countries like the US &amp; Russia have more than one standard time. The entire world also has accepted that their time standards will be related to the time at the Greenwich meridian. Modern airlines have to make their flight schedules keeping in mind the time standards of the various countries where they fly.

Criticality of Time Measurement to Shipping

Ships cross oceans which have no clear landmarks. They need to find their positions in terms of latitudes & longitudes. Latitude is easy to find from the inclination of the Sun's rays.

But locating longitudes needs comparison of the "local time on the ship" to a standard time. Local time on the ship can be worked out from the position of the Sun. But comparing it to a "standard time" needs a clock which is "in sync" with a clock showing the "standard time". This needed a clock on the ship which can be "synched" to a standard time before sailing and which would keep accurate time in spite of the constant "movement" of the ship on the ocean.

This problem was considered so critical that the British government announced a huge prize to any one who could build such an accurate clock.

Learning About Time

One of the best ways for children to understand time is to experience its passage. They can be asked to close their eyes for a minute. They can be taught to count numbers to keep track of the passing of a second.

They can even be enabled to experience that a minute seems too long when they keep still and seem so short when they are playing a game. A taste of the relativity of time!

Flexible Use of Time-Related Words

Because words related to time have been in use for a very long time, they have also entered our daily-vocabulary with very flexible meanings. "Wait a second" or "I will be there in a minute" are such words which are not meant to be interpreted in a strict manner.

Is Time Real?

The passage of time is one of the least understood of physical phenomena. Einstein says that time, space & matter are an integrated whole and that we cannot understand any one of them without the other.

Scientists, philosophers and others have pondered the nature of time. And although we've learned a lot about time, such as the reality of time dilation and the possible connection between time and entropy, we haven't been able to come up with a complete description of what time is.

Some philosophers and physicists have argued that what we experience as time is just an illusion, an artefact of our consciousness.

We experience it first through our memory. We remember things that happened in the past and realise time is flowing.

Later historical & archaeological remains reminds that there was indeed a past.

A Philosophical View of Time

Every time, time is determined by your feelings and your psychological conditions and not by clocks.

Time is Slow when you wait!

Time is Fast when you are late!

Time is Deadly when you are sad!

Time is Short when you are happy!

Time is Endless when you are in pain!

Time is Long when you feel bored!

Time is Most Beautiful when you are in love!

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