The concept of needing to divide up the day seems second
nature to even the smallest kid who asks, “is it snack time”. The reality is, even though we’ve decided
that there is a need to divide up time, the actual process and the way we go
about it has been changing for millennia.
The cruel irony is that even though we know we need to measure time,
there has never been a consensus on what time really is.
Throughout all of history there have been two main
schools of thought on what time is, and even many more opinions on how we
should measure it. The first concept of
time is one that most current physicists tend to subscribe to, and that is time
is a fundamental dimension in the universe.
The 4th dimension in which the other three dimensions of space (length
width and height) can move through in sequence. The second concept of time argues against
the idea that it is a dimension, but rather an intellectual concept that allows
people to sequence and compare events.
That time does not exist on its own, but is a way in which we represent
things.
While many physicists tend to view time as a dimension, I
assume because they are trying to hold fast to Einstein’s theories on
Space-Time, I prefer to view it as a tool.
This is because our universe is constantly changing. From one moment to the next, it is always in
motion. From electrons moving around
atomic nuclei, to the Basketball player trying to get their shot off before the
game-clock runs out, everything in our universe is in motion. To be able to understand it, we need a
tool. If you view the universe as a car
and time as a very important tool in a toolkit, you can see how time would not
be a dimension. You need tools to take apart
a car and just like the socket set is needed to take apart and understand all
the inner-workings of that automobile, so too time is needed to take apart and
understand the change in our universe from one moment to the next. But just like the socket set will never be a
part of the car, so too time will never be a part of the universe, just a
needed tool to understand it.
Whatever your position on what time actually is, one
constant has always remained; how do you measure it? In chronometry (The science
of the measurement of time) there are two distinct forms of measurement, the
calendar and the clock. The calendar is
used to measure the passage of extensive periods of time, and the clock is used
to count the ongoing passage of time and is consulted for periods of less than
a day. We obviously will focus on
periods of less than a day, because if we go into the calendar debate, we would
inevitably decide our world was ending in 2012!!
Today the most widely used numerical system is a base 10
system (decimal). This seems appropriate
given we all have 10 fingers and toes, so grade-schoolers and myself, after a
few beers, can do math easily!
Unfortunately for us, the pre-Dewey Decimal civilizations either never
tried to count their sheep drunk, or just plain hated their kids, but all
seemed to use other more complicated systems like a base 12 (duodecimal), or
base 60 (sexagesimal)
The first society credited with separating the day out
into smaller parts was the Egyptians.
They divided a day into two twelve hour sections; night and day. The clock they used to measure time was the
sundial. The first sundials were just
stakes in the ground and you knew what time it was by the length and direction
of the suns shadow. Advances in technology, namely a t-shaped bar placed into
the ground, allowed them more accurately measure the day in 12 distinct
parts. (Damn duodecimal system!!) It was
thought that one explanation for this base system was that one could get to
twelve easily by counting the knuckles on all four fingers with their
thumb. (Apparently they did not have DUI
patrols for drunken camel driving and ancient cops performing field sobriety
tests having folks touch their thumbs to their fingers; otherwise, they would
realize that this method for counting was not a good idea!)
The drawback to this early clock was that at night there
was no real way to measure time.
Egyptians, like us, still needed to measure time after dark. After all, how else would we know when the
bars close? So their early astronomers
observed a set of 36 stars, 18 of which they used to mark the passage of time
after the sun was down. Six of them
would be used to mark the 3 hours of twilight on either side of the night and
twelve then would be used to divide up the darkness into 12 equal parts. Later on, somewhere between 1550 and 1070 BC,
this system was simplified to just use a set of 24 stars, of which 12 were used
to mark the passage of time.
There were many other methods, in ancient times, for
measuring the passage of time after dark. The most accurately known clock was a
water clock, called a clepsydra. Dating
back to approx. 1400-1500 BC, this device was able to mark the passage of time
during various months, despite the seasons.
It used a slanting interior surface that was inscribed with scales that
allowed for a decrease in water pressure as the water flowed out of a hole at
the bottom of the vessel.
Since the day and night could now be divided up into 12
equal parts, the concept of a 24 hour day was born. Interestingly enough, it wasn’t until about
150 BC that the Greek astronomer Hipparchus suggested the idea of a fixed set
of time for each hour was needed. He
proposed dividing the up the day into 24 equinoctial hours observed on equinox
days. Unfortunately for the bean-counters
in charge of overtime hours, most laypeople continued to use seasonally varying
hours for several centuries to come. It
wasn’t until about the 14th century, when mechanical clocks were commonplace,
that a fixed length for an hour became widely accepted.
Hipparchus himself, and other astronomers, used
astronomical techniques they borrowed from the Babylonians who made
calculations using a base 60 system.
It’s unknown why the Babylonians, who inherited it from the Sumerians,
originally chose to use 60 as a base for a calculation system. However, it is
extremely convenient for expressing fractions of time using 10, 12, 15, 20 and
30.
The idea of using this base 60 system as a means of
dividing up the hour was born from the idea of devising a geographical system
to mark the Earth’s geometry. The Greek
astronomer Eratosthenes, who lived between 276-194 B.C., used this sexagesimal
system to divide a circle into 60 parts.
These lines of latitude were horizontal and ran through well-known
places on the Earth at the time. Later,
Hipparchus devised longitudinal lines that encompassed 360 degrees. Even later, the astronomer Claudius Ptolemy
expanded on Hipparchus’ work and divided each of the 360 degrees of latitude
and longitude into 60 equal parts. These
parts were further subdivided into 60 smaller parts. He called the first division “partes minutae
primae”, or first minute. The subdivided
smaller parts he called “partes minutae secundae”, or second minute, which
became known as the second.
Once again, these measuring techniques were lost on the
general public until around the 16th century.
The first mechanical clocks would divide the hour into halves, quarters,
or thirds. It wasn’t practical for the
layperson to need the hour divided up into minutes.
Advances in technology and science over the centuries
have required that there be a more precise defined value for the measurement of
a second. Currently, in the
International System of Units (SI), the second is the base unit for time. This then is multiplied out to get a minute,
hour, day, etc. etc.
The first accurately measurable means of defining a
second came with the advent of the pendulum.
This method was commonly used as a means of counting time in early
mechanical clocks. In 1956, the second
was defined in terms of the period of revolution of the Earth around the Sun
for a particular epoch. Since it was
already known that the Earth’s rotation on its axis was not a sufficiently
uniform standard of measurement, the second became defined as; “The fraction
1/31,556,925.9747 of the tropical year for 1900 January 0 at 12 hours ephemeris
time.”
With the development of the atomic clock, it was decided
that it was more practical and accurate to use them as a means to define a
second, rather than the revolution of the Earth around the Sun. Using a common-view
measurement method based on the received signals from radio station, scientists
were able to determine that a second of ephemeris time was 9,192,631,770 ± 20
cycles of the chosen cesium frequency.
So in 1967 the Thirteenth General Conference on Weights and Measures
defined the second of atomic time in the International System of Units 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
cesium-133 atom.”
Unfortunately for laypeople, scientist with their
constant need to be correct and absolutely accurate, found the effects of
gravitational forces cause the second to differ depending on the altitude at
which it was measured. A uniform second
was produced in 1977 by correcting the output of each atomic clock to mean sea
level. This, however, lengthened the
second by about 1×10−10. This correction was then applied at the beginning of
1977.
Today, there are atomic clocks that operate in several
different frequency and optical regions.
While state-of-the-art cesium fountain atomic clocks seem to be the most
widely accurate, optical clocks have become increasingly competitive in their
performance against their microwave counterparts.
What seems to remain true is that as technology becomes
more and more advanced, the need to more accurately measure time will continue
to evolve. What remains true for most of
us however is that we get to use easy ghetto math and simply know that there
are 60 seconds in a minute, 60 minutes in an hour, and 24 hours in a day!
Bonus
Factoids:
1. Because the second is based on the number of times the
cesium atom transitions between the two hyperfine levels of its ground state
compared to ephemeris time, and the fact that the earth’s rotation is slowing
down, it becomes necessary to add periodic “leap seconds” into the atomic
timescale to keep the two within one second of each other.
2. Since 1972 to 2006 there have been 23 leap seconds
added, ranging from one every 6 months to 1 every 7 years.
3. The International Earth Rotation and Reference Systems
Service (IERS) is the organization which monitors the difference in the two
timescales and calls for leap seconds to be inserted or removed when necessary.
4. Although it is not a standard defined by the
International System of Units, the hour is a unit accepted for use with SI,
represented by the symbol h.
5. In astronomy, the Julian year is a unit of time,
defined as 365.25 days of 86400 SI seconds each.
6. It is though that the moon was used to calculate time
as early as 10,000-28,000 BC. Lunar calendars were among the first to appear,
either 12 or 13 lunar months (either 346 or 364 days). Lunisolar calendars
often have a thirteenth month added to some years to make up for the difference
between a full year (now known to be about 365.24 days) and a year of just
twelve lunar months. The numbers twelve and thirteen came to feature
prominently in many cultures, at least partly due to this relationship of
months to years.
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