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The Earth

Study Material > Geography
  • Earth is the third planet from the Sun and the only object in the Universe known to harbor life. It is the densest planet in the Solar System and the largest of the four terrestrial planets.
  • Earthformed about 4.54 billion years ago. The four seasons are a result of Earth's axis of rotation being tilted 23.5 degrees from the plane of its orbit around the Sun.
  • The gravitational interaction between the Earth and Moon causes ocean tides.
  • About 71% of Earth's surface is covered with water, mostly by its oceans. The remaining 29% is land consisting of continents and islands that together have many lakes, rivers and other sources of water that contribute to the hydrosphere. 
  • The length of a year on Earth is 365 days, 6 hours, and 16 minutes. The length of a day on Earth is 23 hours and 56 minutes (Sideral day).

Shape of the earth

  • The shape of the Earth is called as Geoid. Geodesy is a branch of science deals with measuring the Earth’s size and shape.
  • The shape of Earth is approximately oblate spheroidal. Due to rotation, the Earth is flattened along the poles and bulging around the equator.
  • Because the Earth rotates, this sphere is distorted by the centrifugal force which causes objects to move outward away from the centre of gravity.
  • As the Earth rotates, due to centrifugal force which is greatest at the equator, causes a slight outward bulge there, giving that region a larger circumference and diameter.

Evidence of Earth oblateness

  • Variations in gravity along the Earth’s surface (stronger at the poles where sphere is flattened and weaker at the equator where sphere is bulging). Hence our weight will be less when we are at equator and when we are at poles our weight will be more.
  • Earth’s equatorial diameter is more than its pole to pole diameter
    1. Equatorial diameter:    12756
    2. Pole to pole diameter: 12714 Km
  • Hence earth’s polar perimeter is less then its equatorial perimeter
    1. Polar perimeter: 40008
    2. Equatorial perimeter: 40075

Rotation of Earth

  • Earth's rotation is the rotation of Planet Earth around its own axis. Earth rotates from west to east in anti-clock direction. Thus, the Sun, Moon, and Stars rise in the East and set in the West.
  • Rotation of the Earth is responsible for the cycle of day and night on it. Hence, it is also called Daily Motion.
  • Rotation decreases from equator to polewards. At equator the rotation speed is 1670 Km/h and at 450 latitudes about 1120 Km/h and at poles movement is very little.
  • Solar Day : Solar day is found when the duration of Earth’s revolution around the Sun is calculated in terms of days, while taking Sun as stationery. Duration of a solar day is exactly 24 hours.
  • Sideral Day: The duration between the passage of a particular star over a meridian twice is called Sideral Day. Its duration is of about 23 hours and 56 minutes and 4 seconds.

Note

Time taken by the Sun’s light to reach Earth – 8 min 18 sec.

Average distance from the Earth to the sun - 6 million km

Average distance from the Earth to the moon - 384,400 km

Highest elevation on Earth - Mt Everest (Asia- 29035 feet, 8850 m)

Tallest mountain on Earth from base to peak - Mauna Kea, Hawaii 33480 feet(rising to 13796 feet above sea level).

Point farthest from the centre of the Earth - The peak of the volcano Chimborazo in Ecuador at 20561 feet (6267 m) is farthest from the centre of the Earth due to its location near the equator and the oblateness of the Earth.

  • The solar day is measured using the passage of the mean Sun across the sky. It lasts 24 hours the average interval between two successive midnights.

Revolution

  • While rotating around its axis, the Earth also goes around the sun in a slightly elliptical Orbit called a Revolution. The elliptical path traced is called its Orbit. It takes one year for the Earth to make one Revolution at an average orbital velocity of about 18.5 miles per second (29.6 km per second).
  • Earth Sun relationships are described by a tropical year, which is defined as the period of time 365.25 Solar days from one vernal equinox to another to balance the tropical year with the calendar year a whole day(February 29) is added every fourth year (leap year) other minor adjustments necessary so as to balance the system. The average distance between the Earth and the Sun is approximately 93 million miles (150 million kilometers).
  • Since the earth's orbit is an ellipse rather than a circle the earth is closest to the sun on about January 3 a distance of 91.5 million miles (147 million km) this position in space is called perihelion which comes from the Greek peri meaning around or near and helios, meaning the sun.
  • Erath is farthest from the Sun on about July 4, at Aphelion, with a distance of 94.5 million miles (152 million kilometers).

 

Determination of Earth’s age

  • Several methods to determine the age of Earth
  1. Salinity of the oceans: Some geologists try to measure the age of the earth by the amount of salt in the ocean. First they need to find out how much salt is in the oceans. Then they find out how much salt rivers carry into the oceans each year. By comparing how much salt there is in the oceans now to how much is carried in by rivers each year, scientists can estimate the age of the oceans.
  2. Erosion: Scientific observation shows that erosion takes place at about 1 feet in every 5000 years. Knowing this method, we can estimate the age of rock formations like the Grand Canyon.
  3. Sedimentation: Another method used is to measure the rate at which sediment is deposited. Scientists think it takes between 500 and 10000 years to deposit 1 foot of sediment. This method is not as accurate as the erosion method. 
  4. Radioactive method: One of the methods used is to study the Uranium in igneous and metamorphic rocks. Uranium is a mildly radioactive substance that breaks down very slowly to form other elements and eventually lead. Uranium breaks down at a slow and steady pace which cannot be altered by temperature or pressure. Scientists calculate that it takes about 5 billion years for a half of a piece of Uranium to become lead. By looking at different rocks and comparing the amount of Uranium still in the rock to the amount of lead that has been formed, they can measure the age of the Earth. Using this method, scientists have determined that the Earth is 4.5 billion years old.
  5. Carbon-14 dating: All Organic life forms have carbon 14 in them when they die, no more carbon-14 is produced but instead the carbon 14 begins to slowly breakdown. Scientists think it takes 5600 years for half of a piece of carbon-14 to break down. The Fossil remains of creatures that had carbon 14 in them are found in sedimentary rock by comparing the amount of carbon 14 in a rock to the amount of other substances that have been formed as a result of the breaking down of the carbon -4, scientist can get the rock. Carbon-14 dating is used to find the age of rocks up to 15000 years old and the radioactive method is used for older rocks.

Seasons

  • The imaginary surface on which Earth orbits around the Sun is called as the plane of the ecliptic. The Earth's axis is inclined at 66.5 degrees to the plane of the ecliptic (or 23.5 degrees from the perpendicular to the plane of the ecliptic).
  • The earth maintains its orientation with respect to the stars. Thus, the north pole points in the same direction to the Polaris, the North star, as it revolves about the Sun. Consequently, the northern hemisphere tilts away from the sun during one half of Earth's orbit and towards the sun through the other half.
  • If the earth would not have been tilted on its axis. The days and nights would have been of the same duration always. Moreover, if the earth would not have been revolving around the Sun, one of the hemispheres would have long days and short Nights, whereas the other one would have short days and long Nights. Earth moves not only on its axis but also in its orbit around the Sun. Therefore, the earth changes its position continuously with respect to the sun.
  • There are four major positions of the earth, while it revolves around the Sun, and these positions bring different seasons on the earth. These are:
    1. Positions on 21st June: In this position the Sun shines perpendicularly over the Tropic of Cancer. This position is called the summer solstice in fact from 21st March, the sun starts shifting towards the north, gradually. It is the time of summer season in the Northern hemisphere, while in the southern hemisphere, it is Winter season. The day in the Northern hemisphere is longest on 21st june. From 21st june, the sun starts retreating towards the equator and hence the heat of Summer gradually decreases.
    2. Position of 22nd December: In this position the sun shines perpendicularly over the Tropic of Capricorn. This position is called Winter solstice. During this period days are long and nights are short in the southern hemisphere. The sun starts shifting towards south gradually from 23rd September. This position marks the summers in the southern hemisphere and winters in the Northern hemisphere.
    3. Positions on 21st March and 23rd September: In this two positions the Sun shines directly overhead the equator. Therefore half part of all latitudes receives the sunlight at this time. Hence, everywhere, the duration of day and night is equal dot seasons are also similar in both the hemispheres. These two positions are referred as a equinoxes, means equal days and nights.
    4. 21st March is called the spring of vernal Equinox where as 23rd September is called autumn Equinox.

Latitudes and longitudes

  • Latitudes and longitudes define the relative position of the point in terms of angular distances from centre of earth in north to south (Lnogitude/Meridian) and east to west (Latitude/Parallels).
  • Latitudes and longitudes are measured in degrees. For greater accuracy this degrees can be further subdivided into minutes and seconds: there are 60 minutes(‘) per degree and 60 minutes (“) per minutes.

Latitudes

  • The imaginary circular line running on the globe, which divides it into two horizontal equal parts, is known as the equator. The northern half of the Earth is known as the northern hemisphere and the southern half is known as the southern hemisphere.
  • Latitude is the point’s angular distance above or below the equator. It ranges from 900 north to 900 A line connecting all the points with the same latitude value is called a line of latitude.
  • All lines of latitude are parallel to the equator, and they are known as parallels. Parallels are equally spaced. The longest line of latitude is the equator, whose latitude is zero at the poles at latitudes 90 degree north and 90 degrees South the circles shrink to a point.
  • Tropic of Cancer is 23.5 degree north latitude runs through Mexico, Egypt, Saudi Arabia, India and Southern China.
  • The Tropic of Capricorn is at 23.5 degrees South latitude and it runs through Chile, Southern Brazil, South Africa and Australia. 66.5 degree north latitude in the Northern hemisphere is called the sub Arctic circle whereas 66.5 degrees South latitude in the southern hemisphere is called Sub Antarctic circle the latitudinal distance of 1 Degree is about 111 km. The axial inclination of 66.6 degree (or 23.5 degree from the perpendicular) explains the significance of certain parallels on the Earth. The noon sun shines directly overhead on the earth at varing latitudes on different days between 23.5 degrees South latitude and 23.5 degrees North latitude. During the winter and summer solstices, the area on the Earth between the Arctic Circle (at 66.5 degrees North latitude and the north pole has 24 hours of darkness and daylight, respectively. The same phenomena ocurrs for the area between the Antarctic circle (at 66.5 degrees South latitude) and the South Pole, except that the Seasons are reversed in the southern hemisphere. At the poles, the sun is below the horizon for 6 months of the year.
  • For those living outside the tropics (poleward of 23.5 degree north and South latitude), the noon Sun will never shine directly overhead. Hours of Daylight will also vary greatly during the year. E.g., daylight will range from approximately 9 Hours during the winter solstice to 15 hours during the summer solstice for persons living near 40 degree north latitude.

Longitudes

  • Lines of Longitude (meridians) are imaginary lines extending from pole to pole.
  • Since, the equator is a circle, it can be divided into 360 divisions. The longitude of a point is the marked value of the division where its Meridian meets the equator. Unlike latitudes, longitudes are semicircles and the distance between them decreases steadly polewards until it becomes zero at the poles, where all the meridians meet. Unlike latitude, where the equator is a natural origin, there is no start or end for the longitude circles. Prime Meridian is chosen as the reference line in whose respect the relative East/West position of point is defined. The Prime Meridian is at zero degree and is known as the Greenwich line as it passes through Greenwich, in London.
  • There are 180 Longitudes on both the sides (East and West) of the Prime Meridian, therefore there are 360 longitudes in total. The distance between two adjacent longitudes is less towards the poles and more at the equator. This is because longitudes Tapers towards the poles. Part of the Earth on the eastern side of the Prime Meridian is called Eastern hemisphere and that on the Western side of it is called Western hemisphere.

Great Circles and Small Circles

  • A great circle is a circle which divides the Earth, into two equal parts or hemispheres. A great circle is the largest circle that can be drawn on the globe. conversely, a circle on the surface of the earth, the plane of which does not pass through the centre of the earth and thus divides the earth into two unequal parts, is called a Small circle. Among the parallels of latitude only the equator is a great circle and all other parallels are small circles. However, among the meridians of longitude, all are great circles. Apparently an infinite number of great circles can be drawn on the surface of Earth. An arc of a great circle on the surface of the earth marks the shortest distance between two points on the earth. The navigators follow great circle routes to minimise the distance of their journey.

Time zone

  • Local time can be reckoned by the shadow cast by the sun, which is the shortest at noon and longest at sunrise and sunset.
  • Taking Greenwich Meridian as standard time (GMT Greenwich Mean Time) the whole world has been divided into 24 standard time zones, each of which differs from the next by 15 degree in longitude or 1 hour in time. Since,the earth rotates by 360 degree in 24 hours. The Earth takes 4 minutes to move by 1 Degree of longitude and 60 minutes to move by 15 degree of longitude.
  • Since, the Sun rises in the east and the earth moves from west to east, time on the eastern hemisphere is ahead of the Greenwich and that on the Western hemisphere is behind it.
  • For every 15 degree of longitudes there is a difference of 1 hour in time. In this way the time on the 180 degree east longitude is 12 hours ahead of the Greenwich and that on the 180 degree West longitude is 12 hours behind the time at Greenwich.
  • Hence, there is a difference of 24 hours in the time in East and West of the 180 degree longitude.
  • When the Sun is at highest point in the sky i.e., when the Sun is overhead a watch can be adjusted 12 0’ clock.
  • Since India extends from 680 W to 970 W, it is necessary to adopt the local time of some central meridian of a country as the standard time for the country.
  • The longitude of 820 30’ E 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 (IST).
  • Calculating Time: When it is 12 noon at greenwhich, the time at 15 degree East of Greenwich will be 15 X 4 = 60 minutes, i.e., 1 hour ahead of Greenwich time, which means 1 pm. But at 15 degree West of Greenwich , the time will be behind Greenwich time by one hour, i.e., it will be 11 am. Similarly, at 180 degree, it will be midnight when it is 12 noon at Greenwich.

Daylight Saving Time

  • Daylight saving time(abbreviated DST), also sometimes erroneously referred to as daylight savings time, is the practice of advancing clocks during summer months so that evening daylight lasts longer, while sacrificing normal sunrise times. 
  • Typically clocks are adjusted forward one hour near the start of spring and are adjusted backward in autumn.
  • Adding daylight to evenings benefits retailing, sports, and other activities that exploit Sunlight after working hours, but can cause problems for evening entertainment and other occupations tied to the Sun. Although, an early goal of Daylight Saving Time was to reduce evening usage of incandescent lighting, formerly a primary use of electricity, modern heating and cooling usage patterns differ greatly, and research about how Daylight Saving Time currently affects energy use is limited or contradictory.
  • Daylight saving time begins in the Northern hemisphere between March- April and ends between September-November. Daylight saving time begins in the southern hemisphere between September-November and ends between March-April.
  • International date line : International date line marks the place where each day officially begins. At the international date line, the west side of the line is always one day ahead of the east side, no matter what time of day it is when the line is crossed. The international date line has no astronomical or physical meaning. It is an imaginary line drawn at the 180 degree longitude, avoiding the continuous land parts. It is bent at 75 degree north latitude towards east to avoid division of Siberia and to separate Siberia and Alaska. Again, it is bent over the Berring Strait towards west. To maintain the continuity of land in Islands of Fiji and New Zealand, the International Date Line bends towards east in the southern Pacific Ocean. There is a difference of 24 hours or one day in East and West of it. Therefore, one day is either gained or lost while crossing this line. While crossing it from east to west one day is gained and while going from west to east one day is lost.

The Earth and the Moon

  • The fourth largest natural satellite in the solar system, Earth's moon has a diameter of 3476 km- less than one quarter the diameter of Earth. The Moon's mass is less than 180th that of Earth. The moon orbits Earth in an elliptical path.
  • When it is at perigee means when it is closest to the earth, it is 356410 km distant. When it is at apogee means farthest from earth, It is 406697 km distant.
  • The Moon completes one orbit around Earth every 27.3 days. Moon rotates at about at about the same rate that it orbits the Earth, observers on Earth only see one side of the Moon.
  • The changing angles between the Earth, the Sun and the Moon determine how much of the Moon's illuminated surface can be seen from the Earth and cause the Moon’s changing phases.
  • The complete illuminated phase of the moon is seen on the Earth once a month on the full moon. On the new Moon the Moon is not seen at all because on this night the completely dark part of the Moon faces the earth.
  • Mount Leibnitz (35000 feet) situated on the south pole of the Moon is the Highest point on the Moon’s surface.

Eclipse

  • An eclipse is a complete or partial obscuration of light from a celestial body as it passes through the shadow of another celestial body. One consequence of the Moon's Orbit about the Earth is that the moon can shadow the Sun's light as viewed from the Earth, or the moon can pass through the shadow cast by the earth. The former is called Solar Eclipse and the latter is called a Lunar eclipse.
  • When the Sun, the Earth and the Moon are aligned in a straight line, this position is called Syzygy. Where in conjunction (the sun and the moon in one side of the Earth) occurs in the case of Solar Eclipse while opposition ( The Earth between the Sun and the Moon) occurs in the case of Lunar eclipse.
  • Solar eclipse occurs near the New Moon position. When the Moon is between the Sun and the Earth thus obscuring a part or whole of the Sun.
  • When the Moon comes in between the Sun and the earth, the light of the sun is interrupted and Shadow of The Moon falls on the earth. Lunar eclipse occurs near the full moon position, when the Earth is between the Sun and the Moon. When the earth comes in between the sun and the moon, the light of the sun is not able to reach the moon, on which the shadow of the earth Falls. The inner shaped Part of the shadow is called umbra and the outer light part is called penumbra.
  • There would be maximum seven occasions for the Lunar and Solar eclipses in one year as Lunar and Solar eclipses do not take place on every full moon and New Moon respectively.
  • It is because of Moon’s inclination of 5 degree on its own axis.
  • Moreover, when the earth and the moon reach the same point of Revolutionary path, the Moon, due to its inclination, Moves a little bit ahead of the earth, as a result, eclipse does not occur on every new and full Moon.
  • A ring Solar clipse can be seen but a ring lunar eclipse is not possible, due to difference in the sizes of the sun, the Earth and the Moon.
  • There is a difference between Solar and Lunar Eclipse. Unlike a Solar Eclipse, which can only be viewed from a certain relatively small area of the world, a Lunar Eclipse may be viewed from anywhere on the night side of the Earth. A Lunar Eclipse lasts for a few hours, whereas a total Solar Eclipse lasts for only a few minutes at any given place, due to the smaller size of the Moon’s Shadow.

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