NCERT / CBSE NOTES : Chapter Summary
Globe: Latitudes and Longitudes
In olden days, people believed that the earth was flat. The earth is flattened at the North and South Poles, and is bulging at the centre. It is also called a spheroid.
A globe is a miniature model of the earth. Countries, continents and oceans on a globe are shown to scale. The ancient Greeks knew the world was spherical. Crates, a Greek, was credited with making the first globe in about 150 BC.
The first of the modern-day globes were made in the late 15th century by Martin Behaim of Nuremberg, and the great inventor, Leonardo da Vinci. The earth is constantly rotating so; a globe can be rotated too. The axis of the earth, Latitudes and Longitudes are some imaginary lines on the globe that provide us with reference points. One of these references is the earth’s axis. The globe spins on a needle on which it rotates. This is called the axis. We need some points of reference to describe the location of a point on a sphere like the earth. The earth rotates on an imaginary axis. The earth’s axis is tilted at an angle of 23 ½ ° to the vertical.
The two points through which the needle passes represent the two poles – North Pole at the top and South Pole at the bottom. Thus, the points through which the earth’s imaginary axis passes are the North Pole and the South Pole.
Globes can be:
A terrestrial globe:
- Depicts the areas on earth keeping their shapes, directions or relative sizes intact.
- Ignores the flattening of the earth at the poles and its slight bulge below the equator.
A celestial globe is designed to show the positions of the stars in the sky.
Sea-farers, including Magellan and Columbus, used their knowledge of latitudes to navigate the globe.
Latitudes are imaginary lines that run from west to east, ranging from zero to 90 degrees. Another imaginary line on the globe that divides it into two equal parts at zero degree latitude is called the equator. The equator divides the earth into the Northern Hemisphere and the Southern Hemisphere. The North and South Poles are at 90 degrees from the equator.
The distance from the equator to the poles is 1/4th of the circle around the earth. Each latitude is followed by the letter N or S, for North and South respectively. The letter N or S with a latitude indicates whether it is located in the Northern Hemisphere or the Southern Hemisphere. Places at different locations may have the same latitude, but may be north or south of the equator.
Apart from the equator, other important parallels of latitude are:
Tropic of Cancer, which is located 23 ½ degrees North,
Tropic of Capricorn, which is located 23 ½ degrees South,
Arctic Circle, which is located 66 ½ degrees North and
Antarctic Circle, which is located 66 ½ degrees, South.
Features of Latitudes:
The lines of Latitudes are parallel to each other and are called Parallels of Latitudes.
These lines become shorter towards the Poles and end at two points, the North and the South Pole.
Parallels of Latitudes also determine the climatic zones of the world. The temperature decreases gradually from the equator towards the poles.
The angle of the sun’s rays on the surface of the earth increases as we go away from the equator. This is the reason the intensity of the sun’s rays decreases with an increase in latitude.
The areas between the Tropic of Cancer and the Tropic of Capricorn receive the maximum heat. The belt between the Tropic of Cancer and the Tropic of Capricorn is called the Torrid Zone.
Between the Tropic of Cancer and the Arctic Circle in the Northern Hemisphere and between the Tropic of Capricorn and the Antarctic Circle in the Southern Hemisphere, one will find temperate weather, i.e. moderate temperatures. These areas with temperate weather are called Temperate Zones.
In the areas between the Arctic Circle and the North Pole and between the Antarctic Circle and the South Pole, the sun’s rays have a huge slant. This makes these areas freezing cold and hence they are called the Frigid Zones.
It is relatively easier to determine the latitude at a particular place by using celestial bodies and their movements as reference points. In the Northern Hemisphere, the altitude of the Pole Star is always the same as the latitude of a place.
The imaginary lines running from the North Pole to the South Pole on the globe are called longitudes. An English clockmaker, John Harrison, devised the concept of longitudes. Knowing the latitude and the longitude of a place, we can determine the location of that place on the globe accurately.
If you plot the lines of latitude and longitude on a map, you get a grid pattern. This is called the Earth grid, or Graticule. Longitudes are also known as meridians.
The word ‘meridian’ means mid-day. All the places situated on a particular longitude have their noon at the same time. Therefore, they are called meridians of longitude.
Characteristics of longitudes:
- Distances between longitudes are measured in ‘degrees of longitude.’
- Each degree is further divided into minutes, and minutes into seconds.
- Longitudes are semi-circular in shape.
- The distance between longitudes reduces as they move towards the poles.
At the poles, the distance between the longitudes is zero. It is shortest at the poles where all the meridians meet. The distance at the poles is zero km.
- All longitudes are of equal length
- The 0° longitude passes through Greenwich and is called the Prime Meridian
- The Prime Meridian divides the earth into the eastern hemisphere and the western hemisphere
- The longitude of a place is followed by the letter E or W. This letter denotes whether the longitude is east or west of the Prime Meridian
- 180°East meridian and 180° West meridian are the same.
If you cross the 180˚ longitude, the calendar date changes. The imaginary line on the Earth that separates two consecutive calendar days is called the International Date Line.
To avoid having two different dates in the same country, the International Date Line has been made to bend at several places. A great circle bisects the earth into two equal hemispheres. A great circle is the shortest distance between any two points on the globe.
Differences between longitudes and latitudes:
- Latitudes are horizontal lines on the globe that run west to east, whereas longitudes are vertical lines that run north to south.
- Latitude lines run parallel to the equator and to each other and measure the distance north or south of the equator while Longitude lines, are neither parallel to each other nor to the Prime Meridian and measure the distance east and west of the Prime Meridian.
- Values of latitude range from 0° at the equator to 90° N or 90°S at the poles while values of longitude range from zero degrees at the Prime Meridian to 180°E or 180°W.
- The lines of Latitudes get shorter as we move towards the poles and latitudes become points at the poles while Longitudes, are all of equal length and converge to meet at the poles.
Longitudes and Time
Time at any location in the world is calculated on the basis of its distance from the Prime Meridian. The local time at Greenwich, is considered as the accurate time for the Prime Meridian, and is called Greenwich Mean Time or GMT.
Since the earth rotates from west to east, the places east of Greenwich are ahead of Greenwich Mean Time and those to the west are behind it. All the places located on a particular meridian or a particular longitude have the same local time.
How would you calculate the time at a specific longitude?
- The earth rotates 360° in about 24 hours, means it rotates: 15° an hour or 1° in four minutes
Areas on the east of the Prime Meridian are ahead of areas on the west. Each place crosses the sun at different times. So, the time of sunrise also differs from place to place. Sunrise in places east of the Prime Meridian occurs earlier than that in places to the west of the Prime Meridian.
Japan is called the Land of Rising Sun because that is where the Sun rises first.
Uniformity in time needs to be maintained throughout the country and is done by determining the standard time for the country. Most countries adopt the local time of some central meridian as the standard time for the country.
The Indian Standard Time is based on the time on longitude 82.5 degrees east that passes through Mirzapur. This means that the difference between the standard meridian of India and that of Greenwich is 82 ½ degrees. India lies on the east of the Prime Meridian so IST is 5 hours 30 minutes ahead of GMT.
As you move east from the Prime Meridian, you lose time, and as you move west, you gain time.
There are countries like the USA and Russia that have multiple time zones. Multiple time zones exist in these countries because they are spread across a large number of longitudes. Russia has 11 time zones hence it is difficult to choose a single longitude as the standard time.
Canada spreads across about 90 degrees of longitude and has six time zones and each zone has its own standard time. The earth has a total of 24 time zones of one hour each.
To compete for a round-the-world speed record, a flight must cover a distance equal to at least the length of the Tropic of Cancer. It should also cross all meridians and end at the same place from where it took off.