Travel geography
Time Zones 6.1
Introduction:
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Before clocks were invented and time read, people
watched the sun to determine day and night. Primitive
man planned his activities according to the different
position of the sun.
Because everyone wants to measure their day with the
sun being at its highest point at midday, scientists came
up with the idea of dividing the earth into different
time zones.
The time is the same everywhere within one time zone,
but is different to all the other time zones.
Travel geography
Time Zones 6.1
Introduction:
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There are 24 hours in a day, so there are 24 time zones.
There are 360 degrees of longitude on the earth.
Scientists have divided these by 24, that means that
there are 15 degrees of latitude in every time zone.
When you move from one time zone to the next one,
you change your watch by one hour.
If you are travelling in an easterly direction you move
your watch one hour forwards, and if you are moving in
westerly direction you move it one hour backward.
Travel geography
Time Zones 6.2
Greenwich Mean Line:
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The planet is divided into 360⁰ of imaginary lines which
run vertically from pole to pole. These lines are called
Meridians, at 0⁰, passes through Greenwich, England.
The Greenwich Meridian Line, Longitude 0⁰ is the
center of world time.
Time zones are determined by how many degrees east
or west of the Prime Meridian one is , up to 180⁰ ,
which is on the opposite side of the planet, and the
location of the International Date Line.
Cont…..
Travel geography
Time Zones 6.2
Greenwich Mean Line:
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The prime meridian was defined by the position of the
large “Transit Circle” telescope in the Observatory’s
Meridian Building in 1884.
The Greenwich Meridian separates east from west in
the same way that the Equator separates north from
the south.
But its position is marked in hundreds of other places
too. The meridian passes through England, France,
Spain, Algeria, Mali, Burkina Faso, Togo, Ghana and
Antarctica.
Travel geography
Time Zones 6.3
Greenwich Mean Time (GMT):
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GMT is a time zone at the longitude 0 degree 0 minutes
the Prime Meridian at Greenwich, in the united
Kingdom, has served as the reference line for GMT since
the last 19th century.
Time zones are determined by how many degrees east
or west of the Prime meridian one is, up to 180⁰, which
is on the opposite side of the planet, and the location of the
International Date Line. This is where time changes a whole
day.
GMT was adopted as the world’s time standard at
Washington Meridian Conference in 1884, also established
24 hour time-zone system.
Travel geography
Time Zones 6.4
Universal Time:
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GMT is a term that is commonly referred to as
Coordinated Universal Time (UTC). GMT was at first
calculated by the 24 hour clock starting at noon.
However, since 1925, the day of GMT starts at
midnight. GMT was later renamed to Universal Time or
UT. It is also known as UTO.
UTC is the primary standard by which the world
regulates clocks and time. For most common purposes,
UTC is synonymous with GMT, but GMT is no longer
precisely defined by the scientific community. Cont…..
Travel geography
Time Zones 6.4
Universal Time:
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UTC is based on International Atomic Time(TAI), a time
standard calculated using a weighted average of signals
from atomic clocks located in nearly 70 national
laboratories around the world.
The only difference between the two is that UTC is
occasionally adjusted by adding a leap second in order
to keep it within one second of UT1, which is defined
by the earth’s rotation.
In the 40 years up to and including 2011, a total of 25
leap seconds have been added, the most recent was
added on 30 June 2012.
Travel geography
Time Zones 6.5
Time Zones:
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Militaries, large international companies and banking
systems all use GMT as a reference when dealing with
other parts of the world.
Before 1972, all time zones were specified as an offset
from GMT. Since 1972 all official time services have
broadcast radio time signals synchronized to UTC, a
form of atomic time that includes leap seconds to keep
it within 0.9 seconds of GMT, now called UT1.
Many countries now legally define their standard time
relative to UTC, although some still legally refer to GMT,
Including UK. UTC is also called as Zulu Time By
astronomers.
Cont…..
Travel geography
Time Zones 6.6
International date Line:
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It is an imaginary line of longitude on the earth’s
surface located at about 180 degrees east (or west) of
the Greenwich Meridian, in the middle of the pacific
Ocean and makes two consecutive calendar days.
To avoid crossing nations internally the line deviates
around the far east of Russia and then various island
groups in the pacific. These various deviations generally
accommodate the political and /or economic
affiliations of the affected areas.
The time difference between either side of the IDL is
not always exactly 24 hours because of local time zone
variations.
Travel geography
Time Zones 6.7
Daylight Saving Time:
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The relationship between GMT and other time zones
may change, but only because those other time zones
are adjusted for Daylight Saving Time. When DST is not
observed, it is called standard time, normal time or
winter time.
It is also called “Summer time”. Many countries, and
sometimes just certain regions of countries, adopt
daylight saving time during part of the year.
This involves advancing clocks by an hour near the start
of spring and adjusting back in autumn. Purpose is to
make better use of daylight.
Travel geography
Time Zones 6.8
Calculating Time:
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One way to calculate time is to use world time. The
twenty-four time zones correspond to meridians of
longitude. Each zone is spaced 15 degrees apart and
differs from its adjacent time zone by one hour.
Universal Time or UT was designed to provide a single
time keeping method, a single time zone that everyone
would understand and which would avoid the
confusion of local time zones.
When converting times from one time zone we must
take into account the time zone difference and also the
effects of daylight savings.
Cont…..
Travel geography
Time Zones 6.8
Calculating Time:
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Some basic principles would apply namely that if you
take the time at the Prime Meridian and you add time
for locations to the west of it and subtract time for
locations to its east.
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The number of time zones is equal to the number of
hours to add or subtract to the coordinated universal
time at the Prime meridian.
Travel geography
Time Zones 6.9
India Standard Time (IST):
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India’s Time Zones were established in 1884. Originally
there were two Time Zones, The Bombay Time and
Calcutta Time.
Calcutta time was 5 hours,30 minutes and 21 seconds
in advance of GMT, while Bombay Time was 4 hours
and 51 minutes ahead of GMT. Bombay continued to
have a different time (39 minutes behind IST) until
1955.
Indian Standard Time came into existence in 1905.
Travel geography
Time Zones 6.9
India Standard Time (IST):
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Indian Standard Time is 5:30 hours ahead of Greenwich
Mean Time. Indian Standard Time is the time observed
throughout India.
India does not observe daylight saving Time or other
seasonal adjustments.
Indian Standard Time is calculated on the basis of
82.5⁰ E longitude, from a clock tower in Mirzapur
(25.15⁰ N 82.58⁰ E) near Allahabad in the state of utterPradesh which is nearly on the corresponding longitude
reference line.