1. Which coordinate gives us information about east-west position - latitude or longitude?

2. What is the name of the reference or zero line for latitude? for longitude?
3. Is the Earth's equatorial plane perpendicular to its axis of rotation or does it mark the plane of the
Earth's orbit around the Sun?
4. Which geographic reference is defined by people rather than the position and motion of the Earth
in space?
5. Being able to determine one of the two coordinates - latitude or longitude, was not readily doable
until about the time of American Independence. Which? And what key technology made it
possible?
6. Latitude and longitude are the two coordinates that determine a specific point on the Earth's
surface. How does knowing the location of a point help us make maps?
7. What are the uses and purposes of maps? What do they show?
8. How is a point on Mars defined?
9. Do different map projections use the same coordinate information for features on the Earth's
surfaces or require a different set?

http://earthguide.ucsd.edu/earthguide/diagrams/latitude_longitude/

https://www.timeanddate.com/time/map/

Latlon activity paper answer key

A--0�, 0�
B--60�N, 0�
C--0�, 120�E
D--30�N, 90�W
E--30�S, 30�E
F--60�N, 180�
G--30�S, 150�E
H--60�N, 120�E
I--45�N, 105�W
J--15�N, 15�W
K--45�N, 30�E
L--15�S, 60�W

Finding Latitude and Longitude

Finding Latitude

Finding Latitude
One can easily determine latitude. Any star will consistently reach the same highest point in the
sky. Its height in the sky changes with the observer's latitude. This concept was known and
widely used from very ancient times. In the Northern Hemisphere the North Star was/is a
popular choice. The angle the North Star makes with the horizon (α in the figure to the right) is
the same as the observer's latitude.

Finding Longitude
Because one day is 24 hours long one can easily use time to calculate longitude. One hour of
time difference corresponds to 15° of longitude (360°/24 hours = 15°/hour). Suppose an
observer sets his accurate watch to 12:00 at noon in Greenwich, England and then travels a
great distance. The observer then notices that the sun is highest in the sky at 4:00 according his
watch. The observer then knows he is at longitude 60° W (4 hours ×15°/hour = 60°).
It should be pointed out that “noon” in general does not mean 12:00 PM. Rather, it is the time
when the sun is highest in the sky. Because of the use of time zones the sun will be highest in
the sky from about 11:30 AM to 12:30 PM local time for an observer. If longitude is known, the
time of astronomical noon can be calculated.
For example, Lincoln, NE, is at longitude 96.7° W. 96.7°/(15°/hour)=6.4467 hours. Being Central
Time, Lincoln is 6 hours away from Greenwich Mean Time, so astronomical noon is (6.4467 - 6)
hours = 0.4467 hours = 26.8 minutes from 12:00. Thus “noon” in Lincoln, Nebraska is about
12:27 PM (1:27 PM during daylight savings).

Trilateration

Global Positioning System (optional)

The finding of latitude and longitude by astronomical and temporal means has been somewhat
replaced by the public availability of the Global Positioning System (GPS). The GPS works by
means of trilateration. The system has several satellites (24) in geosynchronous orbit around
the earth.
Distance to a satellite is determined by radio signals. The satellites constantly emit a string of
code that essentially indicates time. When a GPS receiver receives the signal it merely needs to
calculate how long it took the signal to arrive. The time difference multiplied by the speed of light
gives the distance to the satellite.
If only the distance to one satellite is known, the observer can be anywhere on a circle (in three
dimensional space it is a sphere, but we are constrained to be on the surface of the earth). If the
distance to two satellites is known, the position reduces to two points. If the position to three
satellites is known, the position relative to all three satellites is known precisely. This is the
method of trilateration.
Because light travels extremely fast, the time difference between emission and reception of the
GPS signals are of order nanoseconds. To maintain that level of accuracy requires atomic
clocks. Each of the satellites has one. GPS receivers tend not to carry expensive atomic clocks.
If a GPS receiver's internal time is a little off the three distances will still reduce to single point,
but coordinates can still be wrong. However, if the distance to a fourth satellite is calculated, the
distances won't intersect at a point unless the time is correct. Thus GPS receivers tend to need
signals from four satellites – where the fourth signal serves to constantly readjust the receivers
internal clock. If the time is correct (and the receiver is constantly readjusting its time) then the
fourth satellite also allows altitude calculations.
Having the distances to four satellites gives a precise location relative to the four satellites. The
receiver carries an internal data table of all the satellites locations. Being in geosynchronous
orbits their positions are precise and stable. Even so, they are closely monitored and
perturbations from their positions are also transmitted. Once the locations of the satellites with
respect to the earth is known, the GPS receiver, knowing its position with respect to the
satellites knows exactly where it is on earth.

Principles
The earth is one of the nine planets that make up our solar system. Though it isn't one
of the largest planets, it still has a circumference of 25,000 miles or 40,000 kilometers.
It is also constantly in motion. In fact, the Earth orbits the sun at an incredible 66,500
rate of miles an hour. Yet, it still takes the earth 365 days, or one year, to complete a
full rotation around the sun. Moreover, as the Earth orbits the sun, it spins on its axis
in a counter-clock wise motion. It too rotates at a fast pace of 1042 miles per hour. On
its axis, the earth rotates 360 degrees every 24 hours. Or you can look at it as it takes
one day to complete a full circle. Divided up into an hourly rate, the earth rotates 15
degrees every hour (360/24). This number plays an important role in determining time
zones.

An important factor in determining time zones is the lines of latitude and longitude.
Imaginary lines known as latitude and longitude divide the Earth. Latitude lines are
"drawn" east and west and they measure north and south. The lines start at the equator
and measure distance from 0 degrees to 90 degrees north and also 0 degrees to 90
degrees south. They also become shorter the farther away they get from the equator.
On the other hand, longitude lines are "drawn" north and south and they measure east
and west. They start at the Prime Meridian (or 0 degrees) and measure from 0 degrees
to 180 degrees east and 180 degrees west. Unlike lines of latitude, these lines are
fairly equal in length. The origin of this spherical coordinate system is at 0 deg.
latitude and 0 deg. longitude. This spot can be found in the Atlantic Ocean just south
and west of Africa. Also, the two lines connect at 180 degrees or at the International
Date Line. This too helps to determine the different time zones throughout the world.

Together all of the above information can be used to calculate the difference of time
between two locations. First, you need to know what longitude the two places are
located in. Next, you would need to find the differences in longitude (in degrees)
between the two places. If both places are located on the same side of the Prime
Meridian, then the numbers are just simply subtracted to find the difference. If they
are on the opposite side of the Prime Meridian then the two numbers should be added
together to find the difference. Second you need to divide the difference (measured in
degrees) by 15 since there are 15 degrees in every hour. This will give you the
difference in time between the two locations. So if you know what time it is in one
location, and the longitude of another location, then just simple addition or subtraction
problem will give you the time in a different time zone. Let's look at another way we
may have to calculate the difference between time of two locations.

Another calculation you may have to make is over the International Date Line. This
line is strategically placed in the Pacific Ocean so that no two neighboring cities are
one day apart in time. It can be difficult to calculate though the International Date
Line when trying to determine the amount of time difference between locations on
either side. This calculation is very similar to the situation with the Prime Meridian.
You must start by finding the difference in longitude (or degrees) of the two places.
You do this by adding the two numbers. Then, divide by the 15 degrees that occurs in
one hour and this will give you the time difference between two locations through the
International Date Line. And again, just add or subtract that difference from the time
that you already know to come up with the new time in the new time zone.

Example Calculations
To review, to find the difference between the two longitudes and divide by 15, this
gives you the difference in hours between the two locations. Second, add or subtract
the number of hours from the time of day that was already known, you will need to
add the numbers if you are going east, and subtract if you are going west. Here are
some examples of how we may need to calculate the difference of time zones.

If you are in London at 12:00, and want to know what time it is in Japan, you would
need to first figure out that London is 0 degrees (right on the prime meridian), and
Japan is 135 degrees East. So the difference is 135 degrees (135-0), divided by 15
which equals 9. Which means there is a 9-hour difference between London and Japan.
Since Japan is further east than London is, you would add 9 hours to 12:00. The
answer is at 12:00 noon London time, it is 9:00pm in Japan.

Now how about going through the International Date Line. Pretend you are in Japan,
which is 135 degrees east and you wanted to know what time it is in Hawaii, which is
150 West. Well, there is 45 (180-135) degrees difference between Japan and the IDL.
Also there is 30 (180-150) degrees difference between the IDL and Hawaii. Therefore
the difference in time is (45+30/15=5) 5 hours. Now the tricky part is that Japan and
Hawaii are on different days. It is one day ahead on the left side of the IDL compared
to the right side. If it is 3:00pm in Japan on Thursday that means it is 3:00+ 5 hours =
8:00pm in Hawaii. However notice that when crossing the IDL we subtract a day
going east. So, in Hawaii it is 8:00pm on Wednesday.

Review Questions
1. Lines of latitude: A. begin with the prime meridian; B. are designated by being East
or West from an origin; C. are of equal length; D. become shorter away from the
equator; E. none of the above.

2. All of the following are true statements about longitude, except: A. has its origin at
the prime meridian; B. extend east and west to 180 degrees longitude; C. are relatively
equal in length; D. could be determined by sailors using a device called the sextant; E.
could not be determined by sailors until the introduction of the chronometer.

3. You are told that the earth rotates on its axis at a speed of about 1042 miles per
hour. Given that the rotation occurs in 24 hours, what is the circumference of the
earth? A. 40,000 miles; B. 25,000 miles; C. 2400 miles; D. 76,000 miles; E. none of
the above.

4. How many degrees of a full circle can you travel eastward or westward from the
zero (prime) meridian before heading back toward the Prime Meridian? A. 60deg. B.
90deg. C. 360deg. D. 180deg. E. none of the above.

5. 0deg. longitude and 0deg. latitude is located: A. over central Australia; B. in Brazil;
C. in the Atlantic south and west of Africa; D. at the South Pole; E. none of the above.

6. To find longitude, a sailor needs to know: A. the elevation of the sun above the
horizon; B. the latitude at the prime meridian; C. local time and the time at another
line of longitude; D. the relative space; E. none of the above.

7. Latitude and longitude is a spherical coordinate system with its origin at

0deg.latitude and 0deg. longitude. This point is in the Atlantic Ocean just below the
African country of the Ivory Coast. Locations are measured in degrees away from this
origin in north, south, east, and west directions. 23.34deg. S and 46.38deg.W is
probably located in: A. Russia; B. Canada; C. South Africa; D. South America; E.
none of the above.

8. The circumference of the earth at the equator or along any line of longitude is
approximately: A. 25,000 KM; B. 40,000KM; C. 36,000 KM; D. 46,000 KM.

9. It is 1:00 PM on Friday at 90deg. W. what time is it at 90deg. E? A. 7:00 PM

Friday; B. 7:00 AM Friday; C. 7:00 AM Saturday; D. 1:00 AM Saturday; E. 1:00 PM
Saturday.

10. It is 12 noon, Monday at 90deg. W. what time and day should it be at 75 degrees
east longitude? A. 11PM, Monday; B. 11 AM; Tuesday; C. 1 AM; Monday; D. 11
PM; Tuesday; E. 6 AM; Monday.

Answer Key: 1. D 2. D 3. B 4. D 5. C 6. C 7. D 8. B 9. D 10. A

What Is a Time Zone?

The term time zone can be used to describe several different things.
Our Time Zone Map is interactive.
timeanddate.com

A time zone is a region where the same standard time is

used.

Time Difference from UTC

The local time within a time zone is defined by its offset (difference)
from Coordinated Universal Time (UTC), the world's time standard.
This offset is expressed as either UTC- or UTC+ and the number of hours and minutes.
Time Zone Map – Always up-to-date

More Than 24 Time Zones

If each time zone were one hour apart, there would be 24 in the world.
However, the International Date Line, creates 3 more. In addtion, several time zones are
only 30 and 45 minutes apart. This makes the total number of time zones worldwide much higher.

Not the Same as Local Time

The term time zone is often used instead of local time. For instance, during Daylight Saving
Time(DST), it is common to say "California and Arizona are now in same time zone". However, the
correct thing to say would be: "California and Arizona now have the same local time".
The reason is that California's local time during DST is UTC-7, but the standard time in California is
minus one more hour: UTC-8. However, Arizona's local time is always UTC-7,
because there's no DST in Arizona, and they remain on standard time all year.

Local Time Zone Names

Mountain Standard Time or Mountain Daylight Time or just Mountain Time?PunchStock.com

To confuse matters more, each time zone has many different local time zone names, usually linked to
the geographical name of the country or region.
The time zone names in the US are consistent: On the mainland, there are 5 time zones with 5 names
and corresponding DST names. However, in other places in the world, the time zone names may be
completely different, even though the UTC offset is the same.
For instance in Miami, Florida, the standard time zone is UTC-5, and it's called Eastern Standard
Time – EST. In Havana, Cuba, the standard time zone is also UTC-5, but it's
called Cuba Standard Time – CST.

Military Time Zones

There are also 25 military time zones which follow the 15 degrees longitudes. These are named
according to the NATO phonetic alphabet: Alpha, Bravo, Charlie, etc. and are used in aviation, at
sea, and in telecommunications.

Identical Abbreviations
Another point that can cause confusion, is that some time zone's names in totally different places
have exactly the same abbreviation. For example:
 IST – India Standard Time – UTC+5:30
 IST – Israel Standard Time – UTC+2:00
In many parts of the world, especially in countries with only 1 time zone, time zone names are not
commonly used at all.
Time Zone Names and Codes Worldwide

Hot Political Potato

In most countries, the political decision to make adjustments regarding time zones or DST is made
for practical reasons, like saving energy, facilitating trade with neighboring areas, or boost tourism.
In some cases, time zone borders and Daylight Saving Time can be a political tool, most recently
in Russia, Ukraine, and North Korea.

When and Why?

The expansion of the railway, and other transport and communications, as well as trade globalization,
during the 19th century, created a need for a more unified time-keeping system,
and time zones were introduced.

How Many Time Zones Are There?

If each time zone were one hour apart, there would be 24 in the world. But several
time zones have only 30 and 45 minutes offsets, making the total number worldwide
much higher.
Our Time Zone Map shows all 39 time zones.
©timeanddate.com

More Than 24 Time Zones

If each time zone were one hour apart, there would be 24 in the world.
However, the International Date Line creates 3 more. In addition, several time zones are
only 30 or 45 minutes apart, increasing the total number of standard time zones even further.

Not the Same as Local Time

The term time zone is often used instead of local time. For instance, during Daylight Saving
Time (DST), it is common to say "California and Arizona are now in same time zone". However, the
correct thing to say would be: "California and Arizona now have the same local time".
The reason is that California's local time during DST is UTC-7, but the standard time in California is
minus one more hour: UTC-8. However, Arizona's local time is always UTC-7,
because there's no DST in Arizona, and they remain on standard time all year.
So, how many local times are there today? Find out!

Borders Vary
Theoretically, each full-hour time zone is 15 degrees wide, corresponding to a one-hour difference
in mean solar time.
The actual borders on a time zone map have been drawn to match up with both internal and
international borders, and rarely match up with the 15-degree time zone borders.
The time changes one hour forward and backward respectively for every 15 degrees east or west of
the Greenwich Meridian. The Greenwich Meridian is a north-south line selected as the zero-reference
line for astronomical observations.
GMT and UTC: Same Difference?
The Meridian in Greenwich, London, UK represents the world’s prime meridian – longitude zero
degrees. Every place on Earth is measured in terms of its distance east or west from this line.
One would need to divide the longitude, in degrees, by 15 to find the appropriate time zone, in hours.
For example:

 At 150 degrees west (or 150° W) longitude, the time should be 150 degrees divided by 15
degrees = 10 hours behind Coordinated Universal Time (UTC), or UTC-10.
 At 75 degrees east (or 75° E) longitude, the time would be 75 degrees divided by 15 degrees = 5
hours ahead of UTC/GMT, or UTC+5.
In addition, some geographically large (wide) countries, like India and China, use only one time
zone, while it would have been natural to expect several, like in the USA or Australia.
Below are all the different local times currently in use worldwide:

Currently 39 Different Local Times in Use

Sorted by time Sorted by type

UTC Offset Locations Example Name Example Location

UTC +14 Samoa and Christmas Island/Kiribati LINT Kiritimati

UTC +13:45 Chatham Islands/New Zealand CHADT Chatham Islands

UTC +13 New Zealand with exceptions and 4 more NZDT Auckland

UTC +12 Fiji, small region of Russia and 6 more ANAT Anadyr

UTC +11 much of Australia and 8 more AEDT Melbourne

UTC +10:30 small region of Australia ACDT Adelaide

UTC +10 Queensland/Australia and 6 more AEST Brisbane

UTC +9:30 Northern Territory/Australia ACST Darwin

UTC Offset Locations Example Name Example Location

UTC +9 Japan, South Korea and 4 more JST Tokyo

UTC +8:45 Western Australia/Australia ACWST Eucla

UTC +8:30 North Korea PYT Pyongyang

UTC +8 China, Philippines and 10 more CST Beijing

UTC +7 much of Indonesia and 8 more WIB Jakarta

UTC +6:30 Myanmar and Cocos Islands MMT Yangon

UTC +6 Bangladesh and 6 more BST Dhaka

UTC +5:45 Nepal NPT Kathmandu

UTC +5:30 India and Sri Lanka IST New Delhi

UTC +5 Pakistan and 8 more UZT Tashkent

UTC +4:30 Afghanistan AFT Kabul

UTC +4 Azerbaijan and 8 more GST Dubai

UTC +3:30 Iran IRST Tehran

UTC +3 Moscow/Russia and 24 more MSK Moscow

UTC +2 Greece and 30 more EET Cairo

UTC +1 Germany and 43 more CET Brussels

UTC +0 United Kingdom and 26 more GMT London

UTC -1 Cabo Verde and 2 more CVT Praia

UTC -2 regions of Brazil and South Georgia/Sandwich Is. BRST Rio de Janeiro

UTC -3 regions of Brazil, Argentina and 9 more ART Buenos Aires

 UTC Offset Locations Example Name Example Location

UTC -3:30 Newfoundland and Labrador/Canada NST St. John's

UTC -4 some regions of Canada and 29 more VET Caracas

UTC -5 regions of USA and 13 more EST New York

UTC -6 regions of USA and 9 more CST Mexico City

UTC -7 some regions of USA and 2 more MST Calgary

UTC -8 regions of USA and 4 more PST Los Angeles

UTC -9 Alaska/USA and regions of French Polynesia AKST Anchorage

UTC -9:30 Marquesas Islands/French Polynesia MART Taiohae

UTC -10 small region of USA and 2 more HAST Honolulu

UTC -11 American Samoa and 2 more NUT Alofi

UTC -12 much of US Minor Outlying Islands AoE Baker Island

Half Hour and 45-Minute Time Zones

While most time zones differ from Coordinated Universal Time (UTC) by a number of
full hours, there are also a few time zones with both 30-minute and 45-minute offsets.
Green and yellow areas have offsets of 30 or 45 minutes.
The local time in a time zone is defined by its difference from Coordinated Universal Time (UTC),
the world's time standard.
Interactive Time Zone Map

Not Only Whole Hours

A country in the UTC+3 time zone is 3 hours ahead of UTC; in the UTC-8 time zone, clocks are 8
hours behind UTC.
However, some countries and territories use uneven UTC offsets that differ by 30 or 45 minutes from
the usual one-hour time zone interval.
Time Zone Names Worldwide

Daylight Saving Time

Regions that use Daylight Saving Time (DST) change the time zone during the DST period. The
words "daylight" or "summer" are then usually included in the time zone name.
The areas that don't use DST, remain on their standard time zone all year.
DST Worldwide

India: UTC +5:30

 Standard time: India Standard Time (IST)
 Daylight time: None
 Example city: New Delhi
Time Zone Info: India

Australian States:
Australia has multiple time zones. Some of them are half-hour and quarter-hour time zones. Not all
states and territories use DST.
Time Zone Info: Australia

Northern Territory: UTC +9:30

 Standard time: Australian Central Standard Time (ACST)
 Daylight time: None
 Example city: Darwin

Eucla: UTC +8:45

 Standard time: Australian Central Western Standard Time (ACWST)
 Daylight time: None
 Example city: Eucla

South Australia: UTC +9:30 / +10:30

 Standard time: Australian Central Standard Time (ACST)
 Daylight time: Australian Central Daylight Time (ACDT)
 Example city: Adelaide

Broken Hill: UTC +9:30 / +10:30

 Standard time: Australian Central Standard Time (ACST)
 Daylight time: Australian Central Daylight Time (ACDT)
 Example city: Broken Hill

Lord Howe Island: UTC +10:30 / +11:00

 Standard time: Lord Howe Standard Time (LHST)
 Daylight time: Lord Howe Daylight Time (LHDT)
 Example city: Lord Howe Island

Cocos (Keeling) Islands: UTC +06:30

 Standard time: Cocos Islands Time (CCT)
 Daylight time: None
 Example city: Bantam

New Zealand, Chatham Islands: UTC +12:45 /

+13:45
 Standard time: Chatham Island Standard Time (CHAST)
 Daylight time: Chatham Island Daylight Time (CHADT)
 Example: Chatham Islands
Time Zone Info: New Zealand

Canada, Newfoundland: UTC -3:30 / -2:30

 Standard time: Newfoundland Standard Time (NST)
 Daylight time: Newfoundland Daylight Time (NDT).
 Example: St. John's
 NOTE: Only the island of Newfoundland and a few regions of Labrador follow NST/NDT. The rest
follow Atlantic Time (AST) and Atlantic Daylight Time (ADT).
Time Zone Info: Canada

Sri Lanka: UTC +5:30

 Standard time: India Standard Time (IST)
 Daylight time: None
 Example city: Colombo
Time Zone Info: Sri Lanka

Afghanistan: UTC +4:30

 Standard time: Afghanistan Time (AFT)
 Daylight time: None
 Example city: Kabul
Time Zone Info: Afghanistan

Iran: UTC +3:30 / +4:30

 Standard time: Iran Standard Time (IRST)
 Daylight time: Iran Daylight Time (IRDT).
 Example city: Tehran
Time Zone Info: Iran

North Korea: UTC +8:30

 Standard time: Pyongyang Time (PYT)
 Daylight time: None
 Example city: Pyongyang
Time Zone Info: North Korea

Myanmar: UTC +6:30

 Standard time: Myanmar Time (MMT)
 Daylight time: None
 Example city: Naypyidaw
Time Zone Info: Myanmar

Nepal: UTC +5:45

 Standard time: Nepal Time (NPT)
 Daylight time: None
 Example city: Kathmandu
Time Zone Info: Nepal

French Polynesia, Marquesas Islands: UTC -

9:30
 Standard time: Marquesas Time (MART)
 Daylight time: None
 Example city: Taiohae
Time Zone Info: France
Historic Half-Hour Offset
Venezuela changed its time zone by 30 minutes on May 1, 2016. The UTC-offset of Venezuelan
Standard Time (VET) went from UTC -4:30 to UTC -4:00.

Standard Time, Winter Time, Normal

Time
Standard time is also known as winter time or normal time in some countries.

Standard time is also known as winter time.

©iStockphoto.com/JaysonPhotography
The standard time of a region is the local time used outside of the Daylight Saving Time (DST)
period.
More than 60% of the countries in the world use standard time all year.
The remaining countries use DST during the summer months, generally setting clocks forward one
hour from standard time.
DST Changes – Dates and Local Times

Winter Time and Normal Time

Standard time is sometimes referred to as winter time or normal time, while DST may also be
called summer time, especially in the UK.

DST Period Longest

The periods of standard time and DST are not equally long. The standard time period is often shorter
than the DST period. In the US and Canada, for instance, the standard time period is only around four
and a half months. European countries are on standard time five months of the year and spend seven
months on DST.
What is a Time Zone?

Time Zone Name Changes

During DST, the local time zone name and abbreviation changes, usually to include either "daylight"
or "summer" in the time zone name.
For example, New York's standard time is Eastern Standard Time (EST), which is five hours behind
UTC (UTC–5). However, during DST, the city is on Eastern Daylight Time (EDT), which is four
hours behind UTC (UTC–4).
In day-to-day speak, however, the time zone is often referred to as just Eastern Time, regardless of
whether it's standard or Daylight Saving Time.

Time Difference From UTC

The local time within a time zone is defined by its offset (difference) from Coordinated Universal
Time (UTC), the world's time standard.
This offset is expressed as either UTC– or UTC+ and the number of hours and/or minutes.
A few countries' and territories' standard time have UTC offsets of 30 or 45 minutes instead of the
usual hour.
Interactive Time Zone Map

Countries Switch Time Zones

From time to time, countries decide to change their standard time. Some changes are for political
reasons, like in Russia and North Korea.
More common reasons are practical, like facilitating trade with neighboring areas, or to boost
tourism.

Two-Hour Change Every Year

Instead of DST, some locations change their standard time back and forth every year. For example:
the Antarctic research station Troll switches from Greenwich Mean Time (GMT) to Central
European Time (CET), and then later, it goes on to Central European Summer Time(CEST) in order
to adjust to the local time in its mother country, Norway. Troll time then changes back again to CET
and finally to GMT.

What and Where is the International

Date Line (IDL)?
The International Date Line (IDL) is an imaginary line of longitude on the Earth’s
surface located at about 180 degrees east (or west) of the Greenwich Meridian.

The International Date Line on the map

The date line is shown as an uneven black vertical line in the Time Zone Map above and marks the
divide where the date changes by one day. It makes some deviations from the 180-degree meridian to
avoid dividing countries in two, especially in the Polynesia region.
The time difference between either side of the International Date Line is not always exactly 24 hours
because of local time zone variations.
Interactive Time Zone Map
If you travel around the world, changing standard time by one hour each time you enter a new time
zone, then a complete circuit would mean that you adjusted your clock or watch time by 24 hours.
This would lead to a difference of one day between the date on your clock and the real calendar date.
To avoid this, countries are on either side of the International Date Line which runs down the middle
of the Pacific Ocean. If you cross the date line moving east, you subtract a day, whereas if you are
moving west you add a day.

GMT vs. UTC

The Greenwich Meridian is a north-south line selected as the zero-reference line for astronomical
observations. The line in Greenwich in London, UK represents the world’s prime meridian –
longitude zero degrees. Every place on Earth is measured in terms of its distance east or west from
this line.
The United Kingdom observes GMT only in the winter.
The line divides the Earth’s eastern and western hemispheres just as the equator divides the northern
and southern hemispheres.
The Earth’s crust moves very slightly on an ongoing basis so the prime meridian’s exact position is
also moving very slightly. However, the prime meridian’s original reference remains to be the Airy
Transit Circle in the Royal Observatory in Greenwich, even if the exact location of the line may
move to either side of the transit circle’s meridian.
The Royal Observatory in Greenwich is the home to the time zone named Greenwich Mean Time
– GMT. This time zone was first adopted as the world’s time standard at the Washington Meridian
Conference in 1884. However, GMT is now loosely interchanged with UTC to refer to time kept on
the Greenwich meridian (longitude zero).

Why Do We Have Time Zones?

The expansion of transport and communication during the 19th century created a
need for a unified time-keeping system.
Time zones have a long history.
©thinkstockphoto.com

What is a time zone?

Before clocks were invented, people kept time using different instruments to observe the Sun’s zenith
at noon. The earliest time measuring devices we know of, are sundials and water clocks.
UTC is the World's Time Standard
The pendulum clock was developed during the 17th century. However, these clocks were not
sufficiently accurate to be used at sea to determine longitude and for scientific time measurement in
the 18th century.

Chronometers Changed the Game

In 1764, the chronometer was invented. Chronometers measured time accurately in spite of motion or
varying conditions, and became popular instruments among many merchant mariners during the 19th
century.
Still, even after developments regarding longitude, many towns and cities set clocks based on sunsets
and sunrises. Dawn and dusk occur at different times but time differences between distant locations
were barely noticeable before the 19th century because of long travel times and the (lack of) long-
distance communications. The use of local solar time became increasingly awkward as railways and
telecommunications improved. Time zones were, therefore, a compromise, relaxing the complex
geographic dependence while still allowing local time to be approximate with mean solar time.

19th Century Challenges

American railroads maintained many different time zones during the late 1800s. Each train station set
its own clock making it difficult to coordinate train schedules and confusing passengers. Time
calculation became a serious problem for people traveling by train (sometimes hundreds of miles in a
day), according to the Library of Congress. Every city in the United States used a different time
standard, so there were more than 300 local sun-times to choose from. Railroad managers tried to
address the problem by establishing 100 railroad time zones, but this was only a partial solution to
the problem.
Operators of the new railroad lines needed a new time plan that would offer a uniform train schedule
for departures and arrivals. Four standard time zones for the continental United States were
introduced on November 18, 1883. Britain, which already adopted its own standard time system for
England, Scotland, and Wales, helped gather international consensus for global time zones in 1884.

One Prime Meridian

Various meridians were used for longitudinal references among different countries before the late
1800s. However, the Greenwich Meridian was the most popular of these. The Greenwich
Observatory's reputation for the reliability and accuracy in publications of its navigational data was
one factor that contributed to the Greenwich Meridian’s popularity. Moreover, the shipping industry
would benefit from having just one prime meridian. Many people informally recognized the
Greenwich Meridian as the prime meridian before the International Meridian Conference in 1884.
Sir Sandford Fleming was one of the key players in developing a satisfactory worldwide system of
keeping time, according to sources such as the Canadian Encyclopaedia. He advocated the adoption
of a standard or mean time and hourly variations from that following established time zones. He also
helped convene the International Meridian Conference in 1884, where the international standard time
system was adopted.

The International Meridian Conference

The International Meridian Conference in Washington DC, USA, adopted a proposal in October
1884. The proposal stated that the prime meridian for longitude and timekeeping should be one that
passes through the center of the transit instrument at the Greenwich Observatory in the United
Kingdom (UK). The conference established the Greenwich Meridian as the prime meridian
and Greenwich Mean Time (GMT) as the world’s time standard. The international 24-hour time-
zone system grew from this, in which all zones referred back to GMT on the prime meridian.
The main factors that favored Greenwich as the site of the prime meridian were:
 Britain had more shipping and ships using the Greenwich Meridian than the rest of the world
put together (at the time). The British Nautical Almanac started these charts in 1767.
 The Greenwich Observatory produced data of the highest quality for a long time.
(Sheen, cited in Brannel Astronomy, n.d.).

Time Zones in the 20th Century

Interestingly, many French maps showed zero degrees in Paris for many years despite the
International Meridian Conference’s outcomes in 1884. GMT was the universal reference standard –
all other times being stated as so many hours ahead or behind it – but the French continued to treat
Paris as the prime meridian until 1911. Even so, the French defined legal time as Paris Mean Time
minus nine minutes and 21 seconds. In other words, this was the same time as GMT. France did not
formally use to Coordinated Universal Time (UTC) as a reference to its standard time zone (UTC+1)
until August in 1978 (Sheen, cited in Brannel Astronomy, n.d.).
Standard time, in terms of time zones, was not established in United States law until the Act of
March 19, 1918, sometimes called the Standard Time Act. The act also established daylight saving
time in the nation. Daylight saving time was repealed in 1919, but standard time in time zones
remained in law, with the Interstate Commerce Commission (ICC) having the authority over time
zone boundaries.
Many countries started using hourly time zones by the late 1920s. Many nations today use standard
time zones, but some places use half-hour deviations from standard time or use quarter-hour
deviations. Moreover, countries such as China use a single time zone even though their territory
extends beyond the 15 degrees of longitude.
Non-standard and Half Hour Time Zones
Note: timeanddate.com wishes to acknowledge sources such as Brannel Astronomy, the Library of
Congress (USA), the National Institute of Standards and Technology (NIST), the UK’s National
Maritime Museum, and The Canadian Encyclopaedia, for historical information made available
about time zones. It is also important to note that any mention of the United Kingdom (U.K.) or the
United States (U.S.A.) in this article refers to what is now considered to be the UK or the US.

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