Module 4: The Earth


  • Earth is a planet in the solar systems lying between the Venus and Mars.
  • It is the third planet from sun and ranks fifth in size.
  • The Earth is the largest inner planet and the densest planet
  • Moon is the only natural satellite of earth. Moon is about 1/4th size of the earth. The star nearest to the earth is sun.
  • Earth is approximately 4600 million years old.
  • It revolves around the sun in an elliptical orbit and rotates its own axis.
  • The plane passing through the earth’s orbit is called orbital plane.
  • The Earth’s axis is inclined and marks an angle 66½0 with the orbital plane.
  • Geodesy is the science that studies the shape and size of the earth.
  • The earth is spherical in shape with compression at the poles and bulge at the equator. Due to this particular shape, the shape of the earth is generally referred to as oblate spheroid or Geoid which literally means Earth shaped.
  • The bulging at the equator is due to its rotation and causes the diameter at the equator to be 43 km larger than the poles.
  • Equator divides the earth into two hemispheres ie, the Northern Hemisphere and the Southern Hemisphere. The former is dominated by land masses and the latter by the oceans and seas.
  • The earth is the only known planet with developed life in the universe.
  • The earth appears blue from outer space because its two-thirds surface is covered by water. It is, therefore, called blue planet. It is the only planet in solar system with liquid water.
  • Earth is the only planet in our solar system not named after a Greek or Roman deity.
  • Earth has a magnetic field generated by the molten outer core of the Earth. It extends thousands of kilometers out from the surface of the Earth – forming a region called the magnetosphere.
  • Earth Day is celebrated every year on April 22. It was first celebrated in 1970. Earth Day celebrations are coordinated globally by the Earth Day Network.


  • Shape ⇒ Oblate Spheroid or Geoid
  • Age ⇒ 4.6 billion years
  • Radius ⇒ 6371 km
  • Mean Diameter ⇒ 12,734 km
  • Diameter at the equator ⇒ 12,755 km
  • Diameter at the poles⇒  12,712 km
  • Circumference at the equator ⇒ 40,075 km
  • Circumference at the poles ⇒ 40,024 km
  • Total surface area ⇒ 510 million sq.m
  • Land area ⇒ 29.08%
  • Water surface ⇒ 70.92%
  • Mean distance from the sun ⇒ 149,407,000km
  • Mean distance to the moon ⇒ 384,403.0 km
  • Time of rotation ⇒ 23 hours 56 minutes 4.09 seconds
  • Period of revolution ⇒ 365 days 5 hrs 48 min 45.51 sec
  • Total Mass ⇒ 5.972 x 10∧24 kg
  • Total volume ⇒ 1.083 x 10∧24 litres.
  • Mean Density ⇒ 5.517 gm/cm 3
  • Mean surface temperature ⇒ 140C
  • Highest temperature recorded ⇒ 57.70C (Al Aziziyah, Libya, Sep 13, 1922)
  • Lowest temperature recorded ⇒ -89.20C (Vostok, Antartica, July 21, 1983)
  • Warmest month ⇒ July
  • Coldest month ⇒January
  • Highest point on Earth’s surface ⇒ Mt. Everest (8848 m)
  • Lowest point on the land ⇒ Shore of Dead Sea (396 m below sea level)
  • Deepest point in the ocean ⇒ Challenger Deep (Mariana Trench, Pacific Ocean, 10430m below sea level)
  • Tallest mountain on earth from base to summit ⇒ Mauna Kea, Hawaii 10204 m
  • Orbital speed ⇒ 29.8 km/s
  • Farthest point from center of earth ⇒ Mt. Chimborazo in Ecuador
  • Escape Velocity of Earth ⇒ 11.2 Km/sec
  • Largest rock on Earth ⇒ Mount Augustus(Australia)
  • largest living structure on Earth ⇒ The Great Barrier Reef (Australia)



  • The spinning of earth around its axis is known as rotation. The axis of the earth which is an imaginary line, makes an angle of 66½° with its orbital plane.
  • The earth takes about 24 hours to complete one rotation around its axis. The period of rotation is known as the earthday. This is the daily motion of the earth.
  • Earth rotates from west to east (counterclockwise). That is why the Sun, Moon, planets, and stars all rise in the east.
  • Earth has maximum rotation speed at the equator (1680 km/hr) and minimum and decrease towards poles, where it is zero.
  • Rotation causes day and night, formation of tides, change in direction of winds and ocean currents.
  • The circle that divides the day from night on the globe is called the circle of illumination.
  • The Foucault pendulum is a device conceived as an experiment to demonstrate the rotation of the earth. It was introduced by French Physicist Leon Foucault in 1851.


  • Revolution is the movement of earth around the sun on an elliptical path called ‘Orbit’.
  • It takes 365 days 5 hrs 48 minutes and 45.5 1 seconds to revolve around the sun. We consider a year as consisting of 365 days only and ignore six hours for the sake of convenience.
  • Six hours saved every year are added to make one day (24 hours) over a span of four years. This surplus day is added to the month of February. Thus every fourth year, February is of 29 days instead of 28 days. Such a year with 366 days is called a leap year.
  • The seasons are caused by revolution of earth and its tilted axis.



  • The time of the year when the difference between the length of days and night is largest is referred to as solstice.
  • There are two solstice days each year corresponding to the longest day (the summer solstice) and shortest day (the winter solstice).

Summer solstice

  • On 21st June, the northern hemisphere tilts towards the sun and the sun shines directly over the Tropic of Cancer. This is called summer solstice.
  • It marks the summer season in northern hemisphere
  • June 21st has the longest day and shortest night in Northern hemisphere.
  • It marks longest night and shortest day in southern hemisphere.

Winter solstice

  • On 22 December, the southern hemisphere tilts towards the sun and the sun shines directly over the Tropic of Capricorn. This is called winter solstice.
  • December 22 is the longest day and shortest night in southern hemisphere.


Southern Hemisphere Northern Hemisphere
Summer Solstice 21 December 21 June
Winter Solstice 21 June 21 December


  • An equinox is one of the two days in the year when day and night are of equal length. It happens when the sun passes directly over the equator.
  • During these days sun’s rays fall vertically over the equator, since neither of the poles is tilted towards the sun.
  • Vernal equinox (spring equinox) is March 21. Autumnal equinox is September 23.
  • Apogee: It is the point in the orbit of moon when it is closest to earth.
  • Perigee: It is the point in the orbit of moon when it is farthest from earth.
  • Aphelion: It is the point in the orbit of a planet, asteroid, or comet at which it is furthest from the sun
  • Perihelion: the point in the orbit of a planet, asteroid, or comet at which it is closest to the sun


  • Eclipses occur when the sun or the moon is hidden from our view for a short period
  • This happens only when the sun, moon and the earth are a straight line.

Solar Eclipse

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  • The sun gets hidden from view when the moon passes between the earth and the sun. This is called Solar Eclipse.
  • Solar Eclipse occurs on new moon days. But it does not occur every new moon day because of inclination of moon’s orbital plane.

During an eclipse, two shadows are cast. The first is called the umbra. It is the region of complete shadow resulting from the total obstruction of light by moon or earth. This shadow gets smaller as it goes away from the sun. The second shadow is called the penumbra. It is the partially shaded outer region of the shadow cast by the penumbra gets larger as it goes away from the sun.

Lunar Eclipse


  • Lunar eclipse occurs when the shadow of the earth falls on the moon. This happens when the earth passes between the sun and the moon.
  • Lunar eclipse occurs on full moon days. But it does not occur on every full moon day because moon is so small and the plane of its orbit is tilted about 5 degree with respect to earth’ orbital plane.
  • One can see both the setting sun and a rising moon on a full moon day.
  • A Blue moon refers to the second full moon in a single month is called Blue Moon.
  • A super moon is a phenomenon whereby the moon looks bigger and brighter than average full moon, when it reaches its closest point to earth.
  • A Blood moon occurs during a lunar eclipse. When the moon gets shadowed by the Earth, sunlight passing through the Earth’s atmosphere break down in its constituent colors and the red color gets scattered by the atmosphere and falls on the Moon’s surface, thereby making it look like reddish or blackish-red. It is also known as copper moon.
  •  A Super Moon, Blue Moon and blood moon were visible in the sky on January 31, 2018.
  • The syzygy refers to the scenario in which three or more celestial bodies align themselves roughly in a straight line.
  • On March 10, 1982, all planets in the solar system and earth’s moon were on the same side of the sun at the same time.



  • Latitude is a geographic coordinate specifies the north or south position of a point on the earth surface. It helps to pin-points any place on the earth’s surface.
  • The equator represents 00 latitude, and is the longest line of latitude. It lies midway between the poles.
  • Equator is also called ‘Great circle’. It divides earth in two equal hemispheres: Northern and Southern.
  • The length of the equator is equal to the circumference of the earth.
  • All lines of latitudes are circles parallel to the equator. Therefore, lines of latitudes are called parallels of latitude.
  • All parallels north of the equator are called ‘north latitudes.’ Similarly all parallels south of the equator are called ‘south latitudes.’
  • Untitled9.png The important line of latitudes other than equator are Tropic of cancer (23 ½0 N), the Tropic of Capricorn (23 ½0 S), the Arctic circle (66 ½0 N) and Antarctic Circle (66 ½ 0 S)
  • 90 degrees north latitude marks the North Pole and 90 degrees south latitude marks the South Pole.
  • The total number of latitudes is 90+90+1(Equator) = 181. The total number of parallels of latitude is 181–2(the poles are points) = 179.
  • The distance between two successive lines of latitude remains constant (111 km) because latitudes are parallel to each other.
  • Equator passes through 13 countries: Ecuador, Columbia, Brazil, Gabon, Democratic Republic of Congo, Republic of Congo, Uganda, Kenya, Somalia, Indonesia, Maldives, Kiribati and Sao Tome & Principe
  • Tropic of Capricorn (23.50 S) passes through Argentina, Brazil, Chile Paraguay, Namibia, Botswana, South Africa, Mozambique, Madagascar.
  • Tropic of Cancer (23.50 N) passes through Mexico, Bahamas (Archipelago), Niger, Algeria, Mauritania, Egypt, Libya, Mali, Western Sahara, Myanmar, Omen, Bangladesh, India, Saudi Arabia, China, United Arab Emirates, Taiwan.
  • Equator, Tropic of Cancer and the Tropic of Capricorn pass through the African continent.
  • River Congo (also known as Zaire) crosses the equator twice
  • River Limpopo crosses Tropic of Capricorn twice
  • Brazil is the only country through which both equator and Tropic of Capricorn passes.
  • If the stars are seen to rise perpendicular to the horizon by an observer, he is located on the equator
  • Tropic of Cancer passes through 8 Indian states: Gujarat, Rajasthan, Madhya Pradesh, Chhattisgarh, Jharkhand, West Bengal, Tripura and Mizoram.
  • The capital city in India nearest to equator is Thiruvananthapuram. The metropolitan city nearest to equator is Chennai.

The International Solar Alliance (ISA) is a common platform of more than 121 countries for cooperation among solar-resource rich countries lying fully or partially between the Tropics of Cancer and Capricorn.  The ISA is the first international body that will have a secretariat in India.


  • Longitude is a geographic coordinate that specifies the east-west position of a point of the earth surface.
  • There are 360 longitudes in a globe and are also called Meridians.
  • The meridian which passes through the Royal Astronomical observatory at Greenwich, near London is universally accepted as 00 longitude or Prime Meridian or Greenwich Meridian. The Prime Meridian divides the earth into two equal halves, the Eastern Hemisphere and the Western Hemisphere.gr14.jpg
  • Time is calculated globally based on Greenwich Mean Time, which is also known as World Time.
  • As the earth rotates from west to east, those places east of Greenwich will be ahead of Greenwich Time and those to the west will be behind it.
  • Since the earth makes one complete revolution of 3600 in one day (24 hours), it passes through 150 in one hour (360÷24) or 10 in every 4 minutes.

Local Time

  • Local time is determined in relation to GMT.
  • Local time varies from GMT at the rate of 4 minutes per one degree of longitude.
  • The earth rotates from west to east, so every 150 we go eastwards, local time is advanced by 1 hour. If we go westwards, local time is retarded by 1 hour

Standard Time

  • Standard time is the uniform local time fixed by each country.
  • India has accepted 82½0 E meridian for the standard time. The local time at this meridian is taken as the standard time for the whole country. It is known as the Indian Standard Time (IST).
  • IST is 5.30 hrs ahead of Greenwich Mean Time (GMT + 5.30).

How to determine the time difference with GMT?

  • Suppose a country follows a standard meridian of X0 E to determine local time, then, the time difference between Greenwich Mean Time and the country’s local time will be: X / 15.
  • If the country lies east of Greenwich line, then time = GMT + ( X/15)
  • If the country lies west of Greenwich line, then time = GMT – ( X/15)

For e.g., Meridian adopted by India = 82.50 East

Time difference from GMT = 82.5/15 = 5.5 (5.30 hours)

IST (Indian Standard Time) = GMT + 5.30


  • 82½0 E passes through the Indian states of Uttar Pradesh, Madhya Pradesh, Chhattisgarh, Orissa and Andhra Pradesh.
  • National Physical Laboratory (NPL) in New Delhi is the official timekeeper of the nation
  • India government seeks to legalize the IST which means all service providers across the country will have to take time from the National Physical Laboratory.
  • Sri Lanka also follow the Indian standard time.
  • The whole world has been divided into 24 Standard Time Zones, each of which differs from the next by 150 in longitude or one hour in time.
  • Some countries have more than one time zones.
  • The country with the most time zones is France. It has 12 time zones.
  • Russia and USA have 9 time zones each.

International Date Line

  • The International Date Line is an imaginary line along the 1800 meridian (exactly opposite to Prime Meridian).
  • When a person crosses the International Date Line from east to west, he loss one day and when a person crosses from west to east, he gains one day.
  • International Date Line is not a straight line, but a zig-zag line. This was to avoid inconvenience it causes to the land masses through which it passes by dividing these places into two with each having different dates on the same day.
  • The 180° meridian was selected as the International Date Line because it mostly runs through the least populated Central Pacific Ocean. It was decided at the International Meridian Conference in 1884 in Washington, D.C. where 26 countries attended.


International Date Line


  • From Pole to Pole, the Prime Meridian covers a distance of 20,000 km.
  • In the Northern Hemisphere, the Prime Meridian passes through the UK, France and Spain in Europe and Algeria, Mali, Burkina Faso, Tongo and Ghana in Africa.
  • The only landmass crossed by the Prime Meridian in the Southern Hemisphere is Antarctica.
  • The largest country in Africa through which Greenwich line passes is Algeria
  • The meridian accepted by countries to determine the local time will be always the multiples of 7.50 (since 7.50 represent the time difference of half an hour.
  • The Earth has been divided into 24 time zones of one hour each. Each zone covers 15º of longitude.
  • International Date Line passes through Arctic Oceans, Bering Strait, Pacific Ocean, Antarctica, Fiji etc.
  • Daylight Saving Time (DST), also called the “summer time”, is the practice of advancing clocks forward to gain additional daylight during the early evening. This is not followed in India.
  • Equator and the Prime Meridian intersect at Gulf of Guinea (south of Ghana)

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