Question & Answer:
Who came up
with the concept of time?
The early people probably began to wonder about
passing of time from observing natural phenomena
such as sunrise and sunset, the waxing and waning
of the moon, and cycle of birth, growth and death
of all living things. This led to the concept
that events may take place in a sequence, and
there were certain intervals between these events.
Archaeological findings dating back 60,000 years
showed that people then already had a concept
of past, present and future. As civilisations
progressed, more and more precise time measuring
systems were developed in various cultures to
mark the increasingly complex events in their
daily lives, as well as to explain the relationship
between man and the universe in a spiritual way.
How long is
An average person’s heart beats about 70
times a minute. So, the duration it takes from
one heartbeat to the next is slightly less than
one second. We, of course, have to measure the
second more precisely than that. Scientists used
to do this by measuring duration of a day, or
a year, through astronomical observations –
that is observing distant stars. The duration
is then divided by a large number – 86,400
if it’s a day, or about 31 million if it’s
a year – to give a second. Now, scientists
use what is known as atomic clocks to generate
the second. An atomic clock is based on the oscillation
of some cesium atoms. They are so accurate that
the time derived from them will stay within one
second in almost a million years.
on ‘seconds’, ‘minutes’
& ‘hours’ to measure time?
These units of time evolved through many generations
of ancient cultures such as the Egyptians, the
Babylonians and the Greeks, and were later adopted
throughout the world.
are 24 hours in a day and not more or less?
The ancient Egyptians used the rising and setting
of certain stars as markers in their calendar.
In doing so, they observed that during any night,
the rising of these stars occur at regular intervals
such that the night could be roughly divided into
twelve equal parts. Later, daylight was also divided
into twelve equal parts to follow the night, which
gave our day twenty-four hours.
Why must 60-second
make 1 minute and 60 minutes make 1 hour, not
the other way round?
Subdivisions of the hour probably have their
origin in ancient Babylon. The Babylonians developed
a number system called Sexagesimal around 4,000
years ago. Sexagesimal is based on 60. The system
that we are more familiar with is the decimal
system, which is based on 10. The Babylonians
found that, in astronomy, Sexagesimal was far
superior to the other systems, and it was later
adopted to subdivide the hour into 60 minutes,
and the minute into 60 seconds. Through the ancient
Greek Empire, this system gained acceptance in
the Mediterranean cultures and later spread throughout
the world. It is still in use today, not only
in time but also in angular measurements.
the “o’clock” come from?
The “o” is the abbreviation “of”,
and “o’clock” is to tell the
“number of clock”.
‘a.m.’ and ‘p.m.’?
“a.m.” came from Latin words “ante
meridian”, which means “before midday”
or “before noon”. It is the portion
of the day between midnight and the following
“p.m.” came from Latin words “post
meridian”, which means “after midday”
or “afternoon”. It is the portion
of the day between noon and the following midnight.
How is time
Any measurement of time is based on counting
the cycles of some phenomenon that repeats itself
regularly, and then accurately measuring fractions
of that cycle. One example is the rotation of
the earth on its own axis, which gives the cycles
of days and nights. Each cycle is then divided
into hours, minutes and seconds. Standard for
time is now provided by the natural frequencies
of oscillations of atoms. An atomic clock provides
the second by counting the cycles of oscillations
of cesium atoms.
What is a
global positioning system (GPS)?
Global Positioning System, or commonly known
as GPS, is a satellite based navigation system
operated by the US Department of Defense. GPS
satellites broadcast signals that contain information
on position and time. Anyone with a GPS receiver
can find out his/her position anywhere on the
surface of the earth quite accurately. For timekeepers,
GPS satellites provide useful means of comparing
time since they also carry on-board atomic clocks.
What is Singapore
Standard Time? How is SST determined? How is the
accuracy of SST assured?
SST is an abbreviation of Singapore Standard
Time. It is obtained by offsetting +8 hours from
UTC. The National Metrology Centre maintains the cesium atomic
clocks. They are regularly compared, through GPS
satellites, with some 260 atomic clocks maintained
in about forty countries around the world. An
international organisation BIPM (Bureau International
des Poids et Measures) collects data of these
atomic clocks and computes the world average time,
or international time. Singapore Standard Time
is then generated by aligning our cesium clock
to this international time and corrected for the
time zone of Singapore. Consequently, UTC of Singapore
is within the accuracy of 100 nanoseconds (one-billionth
of a second) from UTC at any time.
time zones are there in the world?
Our day has been divided into 24 hours. The world
is therefore divided into 12 time zones in the
Eastern Hemisphere and 12 in the Western Hemisphere.
How do they
decide what time it is anywhere in the world?
The international organisation BIPM continuously
calculates the world average time from the 260
atomic clocks in forty countries. The resulting
international time is called the Co-ordinated
Universal Time. Correcting the Co-ordinated Universal
Time for the time zone gives the time of that
location. For example, Singapore is in the eight-hour
time zone. So, adding eight hours to the Co-ordinated
Universal Time is the Singapore Standard Time.
Why is it
so important to get the right time?
In our everyday life, we need to have a common
time reference so that we can co-ordinate our
activities. For example, we start school or work
at a given time. Buses and MRT have to run on
schedule. Planes and ships have to arrive and
depart at the ports at specific time. Time is
also critical in transactions at the banks and
stock exchanges. Businesses are now done through
the Internet, which also requires correct time.
What is GMT?
GMT, representing the acronym of “Greenwich
Mean Time”, is the mean solar time at the
meridian of old observatory in Greenwich, England,
the time standard against which all other time
zones in the world are referenced. It is the same
all year round and not affected by Summer Time
or Daylight Savings Time. GMT was originally set-up
to aid naval navigation when the globe started
to open up with the discovery of the "New
World" (America) in the fifteenth Century.
Sometimes GMT is referred to as Z ("Zulu").
This arises from the military custom of writing
times as hours and minutes run together and suffixed
with a single letter designating the time zone:
2100Z = 21:00 UTC. The word "zulu" is
the phonetic word associated with the letter "z".
GMT is a historic term, which is in a strict sense
obsolete, though often used as a synonym for UTC,
e.g., BBC still uses this abbreviation for patriotic
reasons. However, GMT was not derived from atomic
time and had no leap seconds, it now has no clear
meaning and was replaced by Universal Time (UT)
in 1928. Today, GMT is still in use in some fields
like emails, cookies and US space shuttle. It
is still used as the UK's civil time. In fact,
no clocks read GMT. BBC and telephone time signals
in the UK and worldwide have broadcast UTC since
1972. The British set their clocks to UTC, not
What is UT?
UT (Universal time) indicates the mean solar
time on the Greenwich meridian beginning at midnight.
It is defined by the Earth's rotation and determined
from astronomical observations. Accordingly, the
day was divided into 86400 seconds. This establishes
a connection between second, the scientific unit
of time, and the Earth's rotation. This time scale
is slightly irregular. There are several different
definitions of UT, but the difference between
them is always less than about 0.03 s. Usually
one means UT2 when saying UT. UT2 is UT corrected
for pole wandering and seasonal variations in
the Earth's rotational speed.
What is UTC?
UTC is the acronym of “Coordinated Universal
Time”. As UT is affected by the motion of
the earth's rotation pole, UT1 corrected for this
effect was in use. However, UT1 is still not a
uniform time scale. The variation in UT1 is dominated
by seasonal oscillations due to the irregularity
of the earth's rotation. In an effort to derive
a more uniform time scale, scientists established
UT2 by applying an adopted formula deducting the
seasonal oscillations. Unfortunately, UT2 is also
not a uniform time scale due to other variations
including tides and winds and the exchange of
angular momentum. So rather than base time keeping
on the rotation of the earth, we now use Atomic
Time, time based on atomic measurements. Thus
time scale based on the atomic second but corrected
every now and again to keep it in approximate
synchronisation with the earth's rotation is called
UTC. The correction is known as the leap second.
The abbreviation UTC can be followed by an abbreviation
of the organisation that publishes this time reference
signal. For example, UTC(USNO) is the US reference
time published by the US Naval Observatory, UTC(PTB)
is the official German reference time signal published
by the Physikalisch Technische Bundesanstalt in
Braunschweig and UTC(BIPM) is the most official
time published by the Bureau International des
Poids et Mesures in Paris, however UTC(BIPM) is
only a filtered paper clock published each year
that is used by the other time maintainers to
resynchronise their clocks against each other.
UTC is an “average world time” calculated
by the BIPM from the readings of more than 200
atomic clocks located in metrology institutes
and observatories in more than 30 countries around
the world. Singapore is also part of this network
and UTC(SG) is the Singapore version of UTC. All
these UTC versions do not differ by more than
a few nanoseconds.
What is TAI?
TAI, (Temps Atomique International = International
Atomic Time), is a uniform and stable time scale
defined by the same worldwide network of atomic
clocks that defines UTC. The unit of Atomic Time
is the atomic second and was defined by the following
description in October 1967: the second is the
duration of 9192631770 periods of the radiation
corresponding to the transition between two hyperfine
levels of the ground state of casium-133 atom
without the influence from magnetic field. According
to the definition, any long time interval should
be the accumulative sum of the seconds. TAI is
formed not from a world clock in Paris, but from
the combined readings of more than 260 atomic
clocks worldwide. In contrast to UTC, TAI has
no leap seconds. UTC is identical with TAI except
that from time to time a leap second is added
to ensure that the Sun crosses the Greenwich meridian
at noon UTC to within 0.9 second when averaged
over a year.
What is a
UTC based on atomic time is a uniform time scale
and 86400 seconds define the length of a nominal
day. UTC is kept closely in line with the rotation
of the Earth. But because of the variations in
the earth's spin the length of the actual day
can be shorter or longer than the nominal day
of 86400 seconds, this causes UTC to depart by
more 0.9 seconds from UT1. Leap second is therefore
introduced and added to make the step correction
in order to bring UTC back into line. Leap seconds
are usually inserted after the 60th second of
the last minute of December 31 or June 30.
What is the
time delivered by a GPS receiver?
Navstar GPS (Global Positioning System) is a
satellite based navigation system operated by
the US Department of Defense. The signals broadcast
by GPS satellites, contain all information required
by a GPS receiver in order to determine both UTC
and TAI highly accurately. Commercial GPS receivers
can provide a time reference that is closer than
340 ns to UTC(USNO) in 90% of all measurements,
classified military versions are even better.
What is a
cesium atomic clock?
A cesium atomic clock is a device that uses as
a reference the exact frequency of the microwave
spectral line emitted by atoms of the metallic
element cesium, in particular its isotope of atomic
weight 133. It uses the exquisite reproducibility
of spinning atoms of the element cesium. The integral
of frequency is time, so this frequency, 9,192,631,770
cycles/second, provides the fundamental unit of
time, second. Cesium atomic clock is the world's
a difference between time referenced to UTC and
UTC is the modern successor of Greenwich Mean
Time. Or we can say UTC is atomic time while GMT
is the earth rotation time. Their difference is
less than 0.9 second.
If the difference
is insignificant, why is there a need to change?
UTC is the modern-day reference established and
adopted under international agreement. GMT is
an archaic system and is no longer used. As Singapore’s
economy becomes more global, there would be implications
for time-sensitive information exchange, especially
on the Internet. Potential legal disputes regarding
a difference in reference point could be avoided
if Singapore referenced itself to the international
norm. Furthermore, with the adoption of UTC, the
reference for Singapore Standard Time would now
reside in Singapore (as part of the international
network) instead of in Greenwich.
an authority on Singapore time? How is time in
Singapore set at present?
Currently time services in Singapore are available
through the Time Announcement Service of Singapore
Telecom (telephone number 1711) as well as through
television broadcasts provided by MediaCorp. These are widely
used by industry and public at large to synchronise
their clocks and watches. However these time signals
are not formally linked to a nationally or internationally
recognized time scale. Both SingTel and MediaCorp also
do not regard themselves as the national authority
for disseminating the correct time scale.
As the national measurement authority, NMC (National
Measurement Center) has established the national
time scale using an ensemble of cesium atomic
clocks. These clocks are continuously compared
with the international time scale through the
satellite system of the Global Positioning System
(GPS). Our time data are sent to the International
Bureau of Weights and Measures (BIPM) as part
of an international network which contributes
to the generation of the international time scale
¾ Coordinated Universal Time (UTC).
areas is Singapore time used?
The world is divided into a number of standard
time zones. Roughly speaking, there are 24 time
zones spaced at intervals of 15° in longitude.
(Practically, due to geographic and political
factors, the boundaries of time zones are more
circuitous.) The areas in the eighth time zone
of Eastern Hemisphere should in principle use
standard time UTC+8. Therefore SST is UTC(SG)
+ 8. But, there is no official international register
of national time zones. Any country is free to
choose and vary its time zone(s) at it pleases.
countries have standard time? How many in Asia?
About 60 time metrology laboratories from 30
countries around the world keep about 60 local
time scales UTC(k) which approximate UTC.
In Asia, the other following countries have their
own national time standard.
Australia, China, Chinese Taipei, India, Japan,
Korea, New Zealand, Malaysia, Thailand, Uruguay.