Back to Main Page

Web Notes

SHAPE & SIZE OF THE EARTH

Geography Shape and Motions of Earth

Shape and Size of the Earth

The earth when viewed from space looks like a sphere. However, it has been found that the earth is not a perfect sphere as earth's Equatorial Diameter is more than the Polar Diameter. The difference between the Equatorial Diameter and the Polar Diameter is about 43,524 meters or 43.5 kilometers. Hence, the Earth could not be called a perfect sphere and is instead called a 'Geoid'.

The bulge at equator and flattening at the poles is caused by the 'Centrifugal Force' produced by the rotation of the Earth along its axis. The flattening and bulging causes the equatorial and polar diameters to differ.

Evidences of earth’s sphericity

Earlier it was believed that earth is a flat and if one ventures too far one will fall down from its edge. But now we know that earth is not flat but instead spherical in shape. How do we know that earth is spherical?

Circumnavigation of Earth – Magellan in the 16th century circumnavigated the earth and it was established that earth is not flat but spherical.
Horizon appears circular - Horizon is the place where sky and ground appears to meet and it looks circular when viewed from a distance. Also as we go higher the circular horizon widens which is possible only at a spherical shape.

Heavenly Bodies - All heavenly bodies whether the sun, stars, moon, other planets are spherical and earth cannot be an exception.
Lunar Eclipse - It’s when the Earth passes in between the Sun and our moon and casts a shadow over the lunar surface. If you look closely, you can pick out a slight curvature which indicates that earth is round.

Sun rises at different times at different places – Had the earth been flat, sun would rise at all places simultaneously and we will not be having different time zones.
Aerial Photographs – Aerial photographs taken from our satellites in space show that earth is spherical.

latitude, longitude and time zones

The earth being spherical does not have corners which could be taken as reference points to determine location of a place. Hence, we divide the earth into a grid using a series of vertical and horizontal imaginary lines and these are used to determine the location of a point on earth’s surface.

Latitudes and Longitudes are imaginary lines used to determine the location of a place on earth.
The shape of the earth is ‘Geoid’. Hence, the location of a place on the earth can be mentioned in terms of latitudes and longitudes.

Latitude

It is the angular distance of a point on the earth’s surface that is measured in degrees from the centre of the earth.
Parallels of latitudes- It is parallel to a line, the equator which lies midway between the poles. These lines are therefore called Parallels of latitudes.

Equator- It is an imaginary line on the surface, equidistant from the North and South Poles, dividing the Earth into two equal Northern and Southern Hemispheres

Important lines of latitude

Equator(0°)
Tropic of cancer (23.5° north),
Tropic of Capricorn (23.5° south)
Arctic Circle (66.5° north)
Antarctic Circle (66.5° south)

As the earth is slightly flattened at the poles, the linear distance of a degree of latitude at the pole is a little longer than that at the equator.
For example at the equator (0°) it is 68.704 miles, at 45° it is 69.054 miles and at the poles it is 69.407 miles. The average is taken as 69 miles (111km).

Latitudinal Heat zones of the earth

Torrid Zone- The mid-day sun is exactly overhead at least once a year on all latitudes in between the Tropic of Cancer and the Tropic of Capricorn. This area, therefore, receives the maximum heat and is called the Torrid Zone.
Temperate Zone- The mid-day sun never shines overhead on any latitude beyond the Tropic of Cancer and the Tropic of Capricorn. The angle of the sun’s rays goes on decreasing towards the poles. As such, the areas bounded by the Tropic of Cancer and the Arctic circle in the northern hemisphere, and the Tropic of Capricorn and the Antarctic circle in the southern hemisphere, have moderate temperatures. These are, therefore, called temperate zones.
Frigid zones- Areas lying between the Arctic circle and the north pole in the northern hemisphere and the Antarctic circle and the south pole in the southern hemisphere, are very cold. It is because here the sun does not raise much above the horizon. Therefore, its rays are always slanting. These are, therefore, called frigid zones.

Longitude

It is an angular distance, measured in degrees along the equator east or west of the Prime (or First) Meridian.
On the globe it is shown as a series of semi-circles that run from pole to pole passing through the equator.
It was decided in 1884, by international agreement, to choose as the zero meridian the one which passes through the Royal Astronomical Observatory at Greenwich, near London.
This is the Prime Meridian (0°) from which all other meridians radiate eastwards and westwards up to 180°.
As the parallels of latitude become shorter pole-ward, so the meridians of longitude, which converge at the poles, enclose a narrower space.
Function- They determine local time in relation to G.M.T. (Greenwich Mean Time), which is sometimes referred to as World Time.

Longitude and Time

Since the earth makes one complete revolution of 360° in one day or 24 hours, it passes through 15° in one hour or 1° in 4 minutes.
The earth rotates from west to east, so every 15° we go eastwards, local time is advanced by 1 hour. Conversely, if we go westwards, local time is retarded by 1 hour.
Hence, places east of Greenwich see the sun earlier and gain time, whereas places west of Greenwich see the sun later and lose time.

If we know G.M.T., to find local time, we merely have to add or subtract the difference in the number of hours from the given longitude.

Why is it 5.30 PM in India when it is 12.00 noon in London?

India and the United Kingdom lie on different longitudes. Each degree of longitudes corresponds to a difference of four minutes. This is because the earth rotates 3600 in 24 hours, 10 in 4 minutes.
The standard meridian of India is 82° 30’ E, and that of London is 0°. This means a difference of 4 minutes x 82.5 = 330 minutes = 5.5 hours. So when it is 12 noon in London, it is 5.30 p.m. in India.

Standard Time and Time Zones

Travelers going from one end of the country to the other would have to keep changing their watches if they wanted to keep their appointment that is impractical and very inconvenient.
To avoid all these difficulties, a system of standard time is observed by all countries.
Most countries adopt their standard time from the central meridian of their countries.
In larger countries such as Canada, U.S.A., China, and U.S.S.R, it would be inconvenient to have single time zone. So these countries have multiple time zones.
Both Canada and U.S.A. have five time zones—the Atlantic, Eastern, Central, Mountain and Pacific Time Zones. The difference between the local time of the Atlantic and Pacific coasts is nearly five hours.
U.S.S.R had eleven time zones before its disintegration. Russia now has nine time zones.

Why do we have standard time?

The local time of places which are on different meridians is bound to differ.
For example, it will be difficult to prepare a time-table for trains which cross several longitudes. In India, for instance, there will be a difference of about 1 hour arid 45 minutes in the local times of Dwarka in Gujarat and Dibrugarh in Assam.
It is, therefore, necessary to adopt the local time of sonic central meridian of a country as the standard time for the country.
In India, the longitude of 82½ E (82 30E) is treated as the standard meridian. The local time at this meridian is taken as the standard time for the whole country. It is known as the Indian Standard Time (1ST).
Some countries have a great longitudinal extent and so they have adopted more than one standard time. For example, in Russia. There are as many as eleven standard times. The earth has been divided into twenty-four time zones of one hour each. Each zone thus covers 15° of longitude.

India’s Standard Time

Chaibagaan Time (India)

150 years ago British colonialists introduced “chaibagaan time” or “bagaan time”, a time schedule observed by tea planters, which was one hour ahead of IST.
This was done to improve productivity by optimizing the usage of daytime.
After Independence, Assam, along with the rest of India, has been following IST for the past 66 years
The administration of the Indian state of Assam now wants to change it’s time zone back to Chaibagaan time to conserve energy and improve productivity.
Indian government didn’t accept to such a proposal.

Solar and Sidereal Time

The system in which we measure time is based on the concept of the solar day. It can be defined as the average time period required for the successive passages of the sun over a given meridian, and it is exactly 24 hours. This is known as the mean solar day. On the other hand, the time required for a 360 rotation of the earth, causing a given star in the sky to return to the same position in relation to the earth is known as the Sidereal day, and this period is about four minutes less than the mean solar day.

meridian of 82-1\2o E passing through Allahabad, which is situated roughly in the middle part of the country.

INTERNATIONAL DATE LINE

What Is the International Date Line (IDL)?

The International Date Line (IDL) is an imaginary line on Earth's surface defining the boundary between one day and the next.

The International Date Line (IDL) on the map.

The International Date Line is located halfway around the world from the prime meridian (0° longitude) or about 180° east (or west) of Greenwich, London, UK, the reference point of time zones. It is also known as the line of demarcation.

The Dateline Is Not Straight

The dateline runs from the North Pole to the South Pole and marks the divide between the Western and Eastern Hemisphere. It is not straight but zigzags to avoid political and country borders and to not cut some countries in half. On timeanddate.com's Time Zone Map, the IDL is shown as a black line (see image).

What Happens When You Cross the Dateline?

When you cross the International Date Line from west to east, you subtract a day, and if you cross the line from east to west, you add a day.

Depending on which time zone the country follows, the time difference on either side of the line is not always 24 hours. For example, if you travel the 1061 kilometers (659 miles) across the dateline from Baker Island to Tokelau you have to add 25 hours, or 1 day and 1 hour.

Three Dates at the Same Time

Every day between 10:00 and 11:59 UTC, three different dates on the calendar are in use at the same time on Earth. For example, our Time Zone Converter shows:

At 10:30 UTC on May 2, it is
23:30 (11.30 pm) on May 1 in American Samoa (UTC−11),
06:30 (6:30 am) on May 2 in New York (UTC-4), and
00:30 (0:30 am) on May 3 in Kiritimati (UTC+14).

Changing Sides of the Dateline

The dateline is not defined by international law. Countries are free to choose the date and time zone that they want to observe.

For example, when the Republic of Kiribati gained independence from being a British colony in 1979 some of the islands were on one side of the dateline, and the rest were on the other. They corrected the anomaly in the eastern half of Kiribati by skipping January 1, 1995 and ever since Kiribati has been the first country to enter the New Year.

In 2011, Samoa changed the time zone from UTC-11 to UTC+13 by shifting the dateline to the west and removing December 30, 2011 from the calendar. They did this to facilitate trade with Australia and New Zealand, and Tokelau followed Samoa for the same reasons.

Drawn up in 1884

The 180° meridian was selected as the International Date Line because it mostly runs through the sparsely populated Central Pacific Ocean. It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.

Jet Lag

A state of exhaustion experienced by air travelers when they cross several time zones in a relatively short time as when one flies from India to U.S.A. The condition arises from disturbances in the body's habitual rhythms of 'internal clock' by suddenly requiring it to adapt to a strange new 24-hour time-scale. Time zones are imaginary lines that run from north to south on the map. These help standardize time throughout the world since it is only possible to cross them by travelling from east to west or visa versa, jet lag is, therefore, experienced only when long distances are travelled along the east-west axis. Jet lag generally includes a feeling of fatigue, hunger at unexpected times, irregular bowel and bladder movements. The only way to combat it is adequate rest at the end of the flight.

THE EARTH IN SPACE

Life did not come into being with the birth of the Earth. Scientists agree that the life of the Earth may range from 3 to 5 billion years. The Earth cooled from its gaseous state into liquid one. It later covered itself with a solid layer on further cooling. The formation of its atmosphere must have simultaneously taken place. Life began in the sea in the form of one cellular organism which belonged to both animal and vegetation kingdom. Later, the animals and vegetations both evolved into more complex life forms.

The age of the Earth.... 5,000,000,000 years (Approximately)

The dawn of life on Earth..... 500,000,000 years (Approximately)

Appearance of Man on Earth....500,000 years (Approximately)

Basic Planetary Data

The mean distance of the Earth from the Sun is approximately 1.5 X 108 kilometres. The planet orbits the Sun in a path that is presently more nearly a circle than are the orbits of most other planets. The direction of the Earth’s revolution—counterclockwise as viewed down from the north—is in the same sense (direction) as the rotation of the Sun; the Earth’s spin, or rotation about its axis, is also in the same “direction” sense i.e. west to east or anti-clockwise. The length of a day (23 hours, 56 minutes, and 4 seconds) is typical of other planetary objects; Jupiter and most asteroids have days less than half as long, while Mercury and Venus have days more nearly comparable with their orbital periods. The tilt (inclination) of the Earth’s axis to its orbit (23.5º), also typical, is responsible for the change of seasons.

Compared with the other eight planets of the solar system, the Earth is relatively small. Although it is the largest of the inner planets, it is considerably smaller than the gas giants of the outer solar system. The Earth has a single satellite, the Moon. The Moon is one of the bigger natural satellites in the solar system and is in fact relatively large compared with the Earth itself. Some people consider the Earth-Moon system a double planet, with some similarities to the Pluto-Charon system.

Some Important Facts about the Earth
1.	Age	5000 million years
2.	Total surface area	51 x 107 km2
3.	Land area	29.2% of the total surface area
4.	Water area	70.8% of the total surface area
5.	Highest land point	8848[1] metres (Mt. Everest)
6.	Lowest land point	397 metres below sea level (Dead Sea in Jordan)
7.	Mean ocean depth	3554 metres
8.	Mean land elevation	3800 metres
9.	Average density	5.5 gm/cm³
10.	Average temperature	15oC
11.	Lowest ocean point	11033 metres (Mariana Trench)
12.	Difference between polar & equatorial diameters	43 Kms
13.	Difference between longitudinal &equatorial circumference	69

The Earth’s gravitational field is manifested as the attractive force acting upon a free body at rest, causing it to accelerate in the general direction of the centre of the planets. Departures from the spherical shape and the effect of planetary rotation cause gravity to vary with latitude over the terrestrial surface. The average gravitational field at sea level is about 9.80 m/s2, although values range from about 9.78 m/s2 at the Equator to about 9.83 m/s2 at the poles. So, the gravitational pull is essentially higher at poles than at equator due to proximity to the core.

Gravity typically is not measured at sea level. So corrections must be made for its decrease in value with increasing elevation. Such height-related gravity anomalies may be corrected for by using free-air or Bouguer reductions. In the Bouguer reduction, the effect of the attraction of the additional mass located above sea level is taken into account, while in the free-air reduction this mass effect is ignored. The Bouguer anomaly can be used to indicate variations of density within the Earth by measuring the corresponding variation in gravity.

The Earth’s gravity keeps the Moon in its orbit around the planet and also generates tides in the body of the Moon. Such deformations are manifested in the form of slight bulges at the lunar surface, detectable only by sensitive instruments. The Moon, owing to its relatively large mass, exerts a gravitational force that likewise causes tides on the Earth. These are most readily observable as the daily rises and falls of the ocean water, although tidal deformations occur in the solid Earth as well as in its atmosphere.

Continents

Continent	Area Square Kilometer	Percentage of Earth’s area	Population Estimates (million)	Highest Point (from Sea-level) in meters	Lowest Point (from Sea-level) in meters
1	2	3	4	5 6	7 8
Asia Africa North America South America Europe Australia Antarctica	43,998,000 29,800,000 21,510,000 17,598,000 9,699,000 7,699,000 13,600,000	29.5 20.0 16.3 11.8 6.5 5.2 9.6	3538.5 758.4 301.7 327.1 729.2 18.3 ---	Everest 8848 Kilimanjaro 5894 Mckinley 6194 Aconcagua 6960 Elbrus 5663 Kosciusko 2228 Vinson Massif 5140	Dead Sea -396.8 Lake Assai -156.1 Death valley–85.9 Valdes Penin-39.9 Caspian Sea –28.0 Lake Eyre -15.8 - -
Australia with New Zealand, Tasmania, New Guinea and the Pacific Island, (Micronesian, Melanesian and Polynesian Islands) is called AUSTRALASIA by some geographers, while others call it OCEANIA.