Earth: The Blue Planet – Geography

The Earth is also called Blue Planet. It is the densest of all planets. Circumference: 40,232 km. Area: 510 million sq. km. Average distance from sun: 149 million-km.

Perihelion: Nearest position of earth to sun. The earth reaches its perihelion on January 3 every year at a distance of about 147 million-km.

Aphelion: Farthest position of earth from sun. The earth reaches its aphelion on July 4, when the earth is at a distance of 152 million km.

The shape of the earth is oblate spheroid or oblate ellipsoid (i.e. almost spherical, flattened a little at the poles with a slight bulge at the centre).

EARTH: IMPORTANT FACTS ABOUT THE EARTH

EARTH MOVEMENTS

Earth has 2 types of movements:

1. Rotation or daily movement.
2. Revolution or annual movement.

1. ROTATION

Spins on its imaginary axis from west to east in 23 hrs, 56 min and 40.91 sec.

Rotational velocity at equator is 1667 km/h and it decreases towards the poles, where it is zero.

Earth’s rotation results in-

• causation of days and nights
• a difference of one hour between two meridians which are 15° apart;
• change in the direction of wind and Ocean currents
• rise and fall of tides every-day.

The longest day in North Hemisphere is June 21, while shortest day is on 22 Dec (Vice-versa in South Hemisphere).

Days and nights are almost equal at the equator.

2. REVOLUTION

Itis earth’s motion in elliptical orbit around the sun. Earth’s average orbital velocity is 29.79 km/s.

Takes 365 days, 5 hrs., 48 min and 45.51 sec. It results in one extra day every fourth year.

Revolution of the earth results in-

• change of seasons;
• variation in the lengths of days and nights at different times of the year;
• shifting of wind belts;
• determination of latitudes.

Inclined Axis

The axis is an imaginary line running from north to south and passing through the center of the earth. It always remains inclined at an angle of 66¹⁄₂° to the plane of the earth’s orbit, and is tilted 23¹⁄₂° from a line perpendicular to this plane.

The two facts, i.e., a fixed angle of the earth’s axis to the plane of the orbit and the axis always pointing in the same direction when combined with the earth’s movements, results in varying lengths of days and nights, seasonality and changes in the altitude of sun at different times of the year.

Seasons

Seasons are periods into which the year can be divided as a result of the climatic conditions, largely due to the changes in the duration and intensity of solar radiation.

The 4 seasons are:

1. Spring:

On March 21, the sun is directly overhead the equator. This is the season of spring in the northern hemisphere.

2. Summer

On June 21, the sun is directly overhead the Tropic of Cancer. Thus, the northern hemisphere experiences summer.

3. Autumn

On September 23, the sun returns to the equator, and the northern hemisphere experiences autumn.

4. Winter

On December 22, the sun is at the Tropic of Capricorn, and the northern hemisphere experiences winter.

EQUINOXES

These are dates when days and nights are equal. During these days the sun shines directly over the equator.

1. March 21 – Vernal equinox
2. September 23 – Autumnal equinox

SOLSTICE

The time of the year when the difference between the length of days and the length of nights is the largest. During these days the sun shines directly over the tropics.

1. June 21 – Summer Solstice
2. Dec 22 – Winter Solstice

MIDNIGHT SUN

It is a phenomenon, observable in the latitudes 66¹⁄₂° North and South (or the Arctic and Antarctic Circles) where the sun does not sink below the horizon during summer.

This result due to the tilt of the earth’s axis, each hemisphere being inclined towards the sun during its summer. The duration of the phenomenon increases towards the poles, where it may be observed for six months of each year.

1. North Pole experiences day from 21st March to 23rd September.
2. South Pole experiences day from 23rd September to 21st March.

LATITUDE

Imaginary lines drawn parallel to the equator. Measured as an angle whose apex is at the center of the earth. The equator represents 0° latitude, while the North Pole is 90°N and the South Pole 90°S.

1. 23¹⁄₂°N represents Tropic of Cancer.
2. 23¹⁄₂°S represents Tropic of Capricorn.
3. 66¹⁄₂°N represents Arctic Circle.
4. 66¹⁄₂°S represents Antarctic Circle.

There are total 181 latitudes including the equator. Each parallel of latitude is a circle, but they are not equal. The circle becomes smaller towards the poles. Equator is the Greatest Circle’ that can be drawn on the earth’s surface.

The distance between any two parallels of latitude is always equal.

One-degree latitude = Approx. 111 km.

LONGITUDE

It is the angular distance measured from the centre of the earth. On the globe the lines of longitude are drawn as a series of semicircles that extend from the North Pole to the South Pole through the equator. They are also called meridians.

The distance between any two meridians is not equal. At the equator, 1 degree = 111 km. At 30°N or S, it is 96.5 km. It goes on decreasing this way until it is zero at the poles.

There are 360 meridians of longitude. The prime meridian is a longitude of 00, passing through the Royal Observatory at Greenwich near London. This meridian is taken by geographers to divide the earth into the eastern and the western hemispheres.

Each meridian of longitude is a semi-circle. 180° meridian (international Date Line) lies exactly opposite to 0° meridian. Such points are called ANTIPODAL POINTS.

The earth is divided into 24 longitudinal zones, each being 15 or 1 hour apart in time (4 minutes = 1 degree).

Longitude and Time

Places that are on the same meridian have the same local (sun) time. Since the earth makes one complete revolution of 360° in 24 hours, it passes through 15 in one hour or 1° in 4 minutes.

The earth rotates from west to east, hence places east of Greenwich see the sun earlier and gain time whereas places west of Greenwich see the sun later and lose time.

A suitable memory acronym can be: East- Gain-Add (E.G.A.) and West- Lose- Subtract (W.L.S.). So, if it is noon in London (near 0°), 15° east (15×4=60min.) will be one hour ahead of London or 1 p.m. and Chennai of 80°E will be 5 hours 20 minutes ahead.

To avoid confusion about having many local times within one country, a particular meridian is chosen for the whole country whose time is known as ‘standard time’.

Generally, the standard meridians are chosen to differ from the Greenwich meridian by the multiples of fifteen degree or seven and a half degree, i.e., by exact number of hours or half hours.

The world is thus divided into a number of time zones. Larger countries like Russia, Canada, USA etc., have greater east-west extension, so they adopt several time zones. Russia has 11 time zones while USA and Canada have 5 time zones.

India, whose longitudinal extent is approx. 30°, has adopted only one time zone, selecting the 82.5°E for the standard time which is 5 hours and 30 minutes ahead of GMT (Greenwich Mean Time).

INTERNATIONAL DATE LINE

It is the 180° meridian running over the Pacific Ocean, deviating at Aleutian Islands, Fiji, Samoa and Gilbert Islands.

Travelers crossing the Date Line from west to east (i.e., from Japan to USA) repeat a day and travelers crossing it from east to west (i.e., from USA to Japan) lose a day.

ECLIPSES

LUNAR ECLIPSE

When earth comes between sun and moon. Occurs only on a full moon day. However, it does not occur on every full moon day because the moon is so small and the plane of its orbit is tilted about 5° with respect to the plane of the earth’s orbit. It is for this reason that eclipses do not occur every month.

Can last up to 1 hour 40 minutes. The moon does not become completely dark during most lunar eclipses. In many cases, it becomes reddish. The earth’s atmosphere bends part of the sun’s light around the earth and towards the moon. This light is red because the atmosphere scatters the other colours present in sunlight in greater amounts than it does red.

SOLAR ECLIPSE

When moon comes between sun and earth. Solar eclipse can be partial or total. Occurs only on a new moon day when the moon is in line with the sun. However, due to the inclination of the moon’s orbit, a solar eclipse doesn’t occur on every new moon day.

TIDES

Refer to the phenomenon of regular rise and fall of the sea water. Though both sun and moon exert gravitational force on earth, resulting in the production of tides, the moon, by nature of its closeness to the earth, has greater control over the timings of the tidal rises and falls.

The interval between two tides is 12 hrs and 26 minutes.

SPRING TIDE

When the sun, moon and the earth are in a straight line, the gravitational force is at its greatest because tide producing forces of both sun and moon complement each other and they pull together. This produces tides of unusually great range, called the spring tide.

These occur about twice a month: at new moon when the sun and the moon are in conjugation and at full moon when they are in opposition.

NEAP TIDE

Lowest magnitude as the tide producing forces of sun and moon act opposite to each other, as they form a triangle. This happens during phases of first and third quarter, i.e., at half moon, the sun’s tide producing force tends to balance the tide producing force of the moon., resulting in tides of unusually small range known as neap tides.

INTERNAL STRUCTURE OF EARTH

THE CRUST

It is the outermost and the thinnest layer of the earth’s surface, about 8 to 40 km thick. The crust varies greatly in thickness and composition- as small as 5 km thick in some places beneath the oceans, while under some mountain ranges it extends up to 70 km in depth.

The crust is made up of two layers– an upper lighter layer called the Sial (Silicate + Aluminium) and a lower density layer called Sima (Silicate + Magnesium).

The average density of this layer is 3 gm/cc.

THE MANTLE

This layer extends up to a depth of 2900 km. Mantle is made up of 2 parts: Upper Mantle or Asthenosphere (up to about 500 km) and Lower Mantle.

Asthenosphere is in a semi-molten plastic state, and it is thought that this enables the lithosphere to move about it. Within the asthenosphere, the velocity of seismic waves is considerably reduced (Called ‘Low Velocity Zone’).

The line of separation between the mantle and the crust is known as Mohoviricic Discontinuity.

THE CORE

Beyond a depth of 2900 km dies the core of the earth. The outer core is 2100 km thick and is in molten form due to excessive heat out there.

Inner core is 1370 km thick and is in plastic form due to the combined factors of excessive heat and pressure. It is made up of iron and nickel (Nife) and is responsible for earth’s magnetism. This layer has the maximum specific gravity.

The temperatures in the earth’s core lie between 2200°c and 2750°c. The line of separation between Mantle and Core is called Gutenberg-Wiechert Discontinuity.

Temperature Inside the Earth: In the first 100 km, 12° increase per km. In the next 300 km, 2° increase per km. After that it is 1° increase per km.

COMPOSITION OF EARTH

Made up of over 100 elements. The following 8 are important:

CONTINENTAL DRIFT THEORY

This theory was given by Alfred Wegener, in 1915. It holds that portions of the original continent which comprised the entire land mass of the world underwent a series of horizontal displacement before the present continents were formed.

According to this theory, about 280 million years ago, the entire landmass formed one super continent, called Pangea. The entire water mass has been given the name Panthalasia.

First of all, it split into 2 – Northern Laurasia & Southern Gondwanaland, divided by a shallow sea called Tethys. Afterwards, further division of continents happened.

The sequence of continents area-wise is Asia, Africa, North America, South America Antarctica, Europe and Australia.

PLATE TECTONICS

Plate tectonics theory deals with the movement of rock structures which are in the form of plates and it is not only the continents which are in motion but the oceans as well.

These plates have an average thickness of 100 km and float on the Asthenosphere, carrying the continents and oceans on their back.

Due to convection currents originating in the interior of the earth, the plates move in various directions. This movement of plates has been responsible for the present distribution of continents and oceans.

The zones where the neighboring plates are going away from each other are called the divergent or constructive margins. Such margins are found in the zones of mid-oceanic ridges where along the fracture zone between the edges of the two plates raises the molten rock from the interior and the new crust is formed.

Margins where plates moving from opposite directions converge against each other are called the convergent or the destructive margins. Here the plates sink into the asthenosphere or get folded to produce features like marine trenches and fold mountains.

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