Measurement of Time

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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.

But time, was perhaps the earliest experience of all living beings. 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.

Time possibly was the first entity to be measured by humans. The cycle of day &amp; 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 &amp; nights. The seasons also had a cycle which could be related to the lunar &amp; the daily cycle. All these patterns 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.

Persistence of non-decimal systems

Since the flow of time was intricately related with human life, there have been different ways of measuring various periods of time. Though the decimal system has been adopted in almost all measurements, it has still not changed the way we measure time. We still have 60 seconds in a minute, 60 minutes in an hour, 24 hours in a day and 365 or 366 (approximately) days in a year!

Above a year we measure time in the decimal system in terms of centuries &amp; 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!

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.

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 &ldquo;local&rdquo; 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.

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.

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