Longitude: lines of longitude, definition, examples of longitude

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What is longitude?

Longitude can be used to determine the time of that place. Western and the eastern position of any location from the Prime Meridian can be determined by the use of longitude. The degree of longitude of a part of it is termed as longitude. The most important use of longitude according to great scholars of geography is to calculate the local time of a place.

What is latitude and longitude?

Latitude and longitude are some imaginary lines running from east to west and from north to south respectively drawn on a map of the world, this lines can be used to determine the location of any place. These lines are known as the parallels of latitude and the meridians of longitude respectively.

Definition of longitude

Longitude is defined an imaginary line drawn on the earth (globe) running from North to South, it is an angular distance measured in degrees east and west of the Green Meridian. The longitude passing through the Green Meridian is known as Prime Meridian and it is termed longitude.

The equator can be numbered in degrees, minutes and seconds and imaginary lines can be drawn through these points from north to south pole and these are known as the meridians of longitude. The longitudes are also known as meridians. The meridian which passes through the Greenwich can be taken as the Prime Meridian and the angular distance of the meridians can be measured from the prime meridian.

Important lines of longitude

All longitudes are numbered in degrees East to West of longitude 00. places on the same longitude have the same time. The time along 0is called Greenwich Time (GTM). It is a standard time from which other local time are calculated. Lines of longitude are called meridians, the usefulness of these lines is that they are used for calculating local time of places. They are also used to locate places on the map.

Lines of longitude
Lines of longitude


Similarities between lines of latitude and lines of longitude

  • Both are angular distances measured in degrees and minutes
  • Both lines are used in locating places on the map
  • They are imaginary lines drawn on the globe
  • Both latitude and longitude contain great circle

Differences between lines of latitude and lines of longitude

Differences between Lines of Latitude and Longitude
Differences between Lines of Latitude and Longitude


Prime Meridian

The prime meridian is defined as an imaginary line that runs through the Royal Observatory in Greenwich, England, the surrounding of London. The prime meridian cuts across 00 longitude and divides the earth into two hemisphere (the Eastern and Western Hemispheres). The Eastern Hemisphere includes the continents of Asia, Australia and Europe, on the other hand, the Western Hemisphere includes North and South America. All meridians (lines of longitude) east of the prime meridian (0 and 180) are graduated from 1 to 1800E; the lines west of the prime meridian (0 and 180) are graduated from 1 to 1800W. There are no letters attached to the 0 and 180 lines as in the case of latitude. The value of this line has been determined as 0.

Other meridians of longitudes can be drawn through the angular distance from the Prime Meridian. For example, the meridians that lie at 450 East of the prime meridian is known as 450 East Longitude or east meridian and all the places lying on this longitude has the same longitudinal value of 450 East Longitude. In this case, we can say that the longitude of any given point on the globe is measured as an angular distance eastward or westward from the prime meridian. We also know that the longitude of the prime meridian is 0. The value of the angles created by the circumference at centre of the earth is 3600. So, if 3600 meridians are drawn from one pole to another at equal interval of 10, then they will lie at 10 of longitude apart. Hence, the total number of 360 meridians are equally divided into two parts of 1800 each in the western and the eastern side of the prime meridian. As the earth is a sphere, so the 1800 east and west longitude is virtually the same line. Like in the case of latitudes, the longitudes can also be divided into minutes and seconds. One minute of longitude is 1/60th part of a degree of longitude. The value of both the latitude and longitude will be 0 when the prime meridian crosses the equator vertically. And this point has been recognised as a place located somewhere in the Gulf of Guinea.

How to determine the longitude of a place

The longitude of a place is determine using these two methods:

  • By the difference of local time
  • By Greenwich mean time

By the difference of local time– we understand that the earth is a sphere; it has been moving constantly round its own axis from west to east. However, different places of the earth have been facing the sun at different times. Whenever, the meridian of any place comes in front of the sun i.e. when the sun appears just overhead, then it is considered as noon and the local time is taken as 12 noon. Ultimately other times of the place are determined on the basis of local time.

However when calculating the local time of a place it is important to have these in mind:

  • That the earth rotates through an angle of 10 in 4 minutes
  • That the earth rotates through an angle of 1in 1 hour
  • That the earth makes one complete rotation through an angle of 360in 24 hours.

For example: If the time of Yaound (Cameroun) 300E is 3.00pm, what time will it be in London 00?

To solve this, you have to know that London is located west of Yaound by 300. Since the earth rotates through an angle of 10 in 4 minutes,

Therefore: 1= 4 minutes

300 = 30 x1x4 minutes = 120 minutes

If 60 minutes = 1hour

Then 120/60 x 1 hour = 2 hours

In this case, London is 2 hours behind Yaound (300E) and so we have to subtract the calculated hours from the initial time (GTM) that was given.

Time in London = 3.00pm 2 hours

i.e 15 hours -2 hours = 13 hours

The time in London will then be 1.00 pm.

By Greenwich Mean Time : We have now understood that the longitude of Greenwich is 0. To determine the longitude of a place by sextant, one will have to record the highest altitude that the sun reaches at a particular location at noon and that position will have to be regarded as 12.00 noon. To calculating the local time of places east of the prime Meridian, a student should understand that we are to add the initial time given (GTM) to the calculated hour obtained. As for all places existing west of prime meridian, we subtract the calculated hour from the initial time given (GTM). The reason behind this is that the earth rotates from west to east thereby exposing places east of the Greenwich first to the suns rays before the places in the west. It therefore shows that when it is midday or noon in some places in the east, it might just be dawn in some other places in the west.

If the GTM is 8.00a.m in London and it is noon in Kinshasa. What is the longitude of Kinshasa?

Solution: The difference in the time between GTM London and Kinshasa is 12 noon

Therefore: 12 noon 8.00am =4hours.

Since 1 hour =150, 4 hours= 4/1x 150 = 600

From the question above, Kinshasa is ahead of GTM, therefore, Kinshasa is East of the Greenwich Meridian which means Kinshasa is located on longitude 600E

Great circle

Great circle is any circle that divides the globe into two halves or hemisphere. The equator is a great circle and all lines of longitude including the Greenwich Meridian line are great circle. There are lots of great circle existing and they extend in any direction, East to west, north to south and so on. Great circles are equal in length. The shortest distance between two points on a spherical surface lies along a great circle. Hence, aircrafts and ships often make use of great circle route ways on a long distance.

Examples of Great Circles

  • The equator (0latitude)
  • The Greenwich meridian (0longitude)
  • Longitude 180W and 1800 E
  • Longitude 90E and 900 W
  • Longitude 1500E and 300W

International date Line

This is an imaginary line running from North to South through the Pacific Ocean. The lines is zigzag in some places , this is to avoid some island groups from counting two different local times in two different parts of the same island, thereby removing the confusion of dividing one island into different dates. The meridian at 180 degrees is called the International Date Line. The International Date Line (180 degrees longitude) is opposite the prime meridian and this indicates the start of each day (Monday, Tuesday, etc.). When the international line is crossed a whole day is either gained or lost. Crossing international date line to the East means a day is gained while crossing it to the West means a day is lost. To the travellers going west of the international date line on Monday 16th, crossing the international date line the date and day becomes Tuesday 17th, while those crossing to the East on Monday the 16th, the date and day becomes 15th.

Officially, each day starts at 12:01 a.m., at the International Date Line. It is important to understand the International Date Line and not to confuse it with the prime meridian which is the 00 longitude. The actual International Date Line does not follow the 1800 meridian exactly. A number of changes have been made to the International Date Line to accommodate political agreements to include an island or country on one side of the line or another. The International Date Line is known to be 1800 east and west longitude. This line crosses through the Aleutian on the north-eastern part of Siberia, Fiji and Chatham islands. To prevent the problems of adjusting local time, the International Date Line has been set so that it will pass only over the water bodies turning 120 towards east just near the Bering Strait, 70 towards west near the Aleutian islands and 110 towards east near Fiji and Chatham islands. If wasnt drawn in this way, a lot of people would have to count two different local times in two different parts of the same island.

Standard time

This time is generally adopted by a country which is usually taken from the Central Meridian (called standard time) of that country. The importance of standard time is to eliminate differences in local time between one town and the others within the same country.