The term Geography has evolved from Greek words 'geo' (the earth) and 'graphein' (to write).

Geography is the study of man, his environment and the interaction between the two.

Geology is the science that studies the origin, composition and the history of the earth.

The central theme of geographical study is a spatial approach or space relations and spatial organization of various processes, patterns and phenomena. It studies spatial distribution and organization of phenomena and provides and explanation for the same. Everything that occurs in space (area), thus, constitutes the subject matter of Geography.

The zone that can support life is called the biosphere.

Analysis and explanation of spatial patterns is the basic theme of geographical studies. The spatial dimension sets this discipline fro other sciences and social sciences.

Eratosthenes was the first person to use the word "geography" and invented the discipline of geography as we understand it. He invented a system of latitude and longitude.

The credit of defining the scope of geography and separating it from history goes to Immanuel Kant. Immanuel Kant defined geography as a chorological science and history as a chronological science.

Physical Geography and Human Geography are the most fundamental divisions of geography.

Geomorphology can be defined as the branch of physical geography that studies configuration of the earth surface. It can be defined as the study of landforms. It is also concerned with the materials constituting the earth's crust and the processes and forces in the interior of the earth. Geomorphology is closest to geology.

Biogeography is the study of relationships of organisms with their environment.

Kant proposed the theory of "Gaseous Hypothesis" for the origin of earth.

Laplace proposed the theory of "Nebular Hypothesis" for the origin of earth.

Jeans and Jeffreys proposed the theory of "Tidal Hypothesis".

The denser matter forms the central part of the earth and the surface rocks are made up of the lighter materials.

The rocks forming the surface layer of the earth which are richer in lighter minerals such as aluminium are call sial (silica and aluminium).

The denser rocks forming the inner layers of the earth are called sima (silica and magnesium) and nife (nickel and iron).

The solid crust of the earth came into existence about 4.6 billion years ago and it is taken as the beginning of the history of the earth.

The most popular method of estimating the age of rocks forming the crust is carbon dating.

The whole history of the earth is divided into three Eras – Palaeozoic, Mesozoic and Cenozoic.

The period before the Palaeozoic Era is called Pre-Cambrian time.

The Palaeozoic Era is sub-divided into six geologic periods (from oldest to youngest): the Cambrian, Ordovician, Silurian, Devonian, Carboniferous, and Permian. It spans roughly from 542 to 251 million years ago

Bacteria originated about 3.5 billion years ago in the Cambrian period.

Corals originated during the Devonian period (app. 435 million years ago).

Fish originated during the Lower Carboniferous period (410 million years ago).

Amphibians originated during the Upper Carboniferous period (360 million yrs).

Reptiles originated during the Carboniferous period (330 million years ago).

The Mesozoic Era consists of three periods and they are Triassic period, Jurassic period, and Cretaceous period.

Mesozoic Era is an interval of geological time from about 250 million years ago to about 65 million years ago. It is often referred to as the Age of Reptiles because reptiles, namely dinosaurs, were the dominant terrestrial and marine vertebrates of the time.

Dinosaurs appeared during the Triassic period of the Mesozoic Era, approximately 240 million years ago.

Dinosaurs became extinct by the end of the Cretaceous period of the Mesozoic Era.

The Cenozoic Era is the latest of the three geologic eras and contains Quaternary period (Holocene Epoch, Pleistocene Epoch) and Tertiary period (Pliocene Epoch, Miocene Epoch, Oligocene Epoch, Eocene Epoch and Palaeocene Epoch).

Small mammals originated during the Palaeocene Epoch (app. 63 million years ago) during the Tertiary period of the Cenozoic Era.

Modern human beings developed during the Pleistocene Epoch (2 million years ago) of the Quaternary period of the Cenozoic Era.

Currently, we are in the Holocene Epoch of the Quaternary period of the Cenozoic Era.

The earth is a member of a group of celestial bodies called the solar system.

The solar system comprises the Sun, eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune) revolving around the Sun.

The Sun is the largest body of the solar system and it is also the centre of the solar system.

The international Astronomical Union, in its conference in August 2006 evolved a more precise definition of a planet. According to this new definition a planet is a celestial body that a) is in orbit around the sun, b) has sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and c) has cleared the neighbourhood around its orbit.

According to this new definition of a planet, the Astronomical Union has excluded Pluto from the category of planets and has named it a dwarf planet.

All planets except Venus and Uranus rotate upon their axis from west to east.

Venus and Uranus rotate upon their axis from east to west.

The direction of the planets in their revolution around the Sun is from west to east.

Jupiter has 66 satellites (moons) and 4 Galilean moons (Io, Europa, Ganymede and Callisto.) discovered by Galileo Galilei.

Ganymede the satellite (moon) of Jupiter is the largest satellite in our system.

Earth has one satellite which is moon.

Mercury and Venus have no satellites.

Saturn has 62 known moons. Titan is the largest satellite of Saturn and the second largest satellite after Ganymede a satellite of Jupiter.

Titan is the only satellite in the solar system that has atmosphere.

Copernicus proposed "heliocentric" view, which states Sun as the centre of the universe.

Ptolemy proposed "geocentric" view, which states earth as the centre of the universe.

The average distance of the Sun from the Earth is about 15,00,00,000 kms.

Earth is the third nearest planet to the Sun after Mercury and Venus.

The planet nearest to Earth is Venus.

Mercury and Venus which are closer to the Sun than the Earth are called Inferior Planets.

Mars, Jupiter, Saturn, Uranus and Neptune which are at a greater distance from the Sun than the earth are called Superior Planets.

Mercury, Venus, Earth and Mars are called as the Inner Planets. Inner planets are also called as Terrestrial Planets.

A terrestrial planet, telluric planet or rocky planet is a planet that is composed primarily of silicate rocks or metals.

Jupiter, Saturn, Uranus and Neptune are called the Outer Planets. These are also called as Jovian planets (Jupiter-like).

Earth is the largest terrestrial planet; Mercury is the smallest terrestrial planet.

Earth is the fifth largest planet of the solar system.

Earth is the most densest planet; Saturn is the least densest planet

Jupiter is the largest Jovian planet; Neptune is the smallest Jovian planet.

Venus is the brightest among the planets and also the hottest planet in the solar system. It is named after the Roman goddess of love and beauty.

Venus is the planet that has the slowest rotational velocity.

Jupiter is the planet that has the fastest rotational velocity.

Mercury is the smallest planet while Jupiter is the largest planet.

Mars is called as the Red Planet.

Earth is also called as the "Living planet" or the "Blue planet"

Neptune has the longest period of revolution around the Sun, which is 165 years.

Mercury has the shortest period of revolution around the Sun, which is 88 days.

Venus has the longest period of rotation which is 243 days.

Jupiter has the shortest period of rotation which is 9 hours and 50 minutes.

The shape of the earth is not a perfect sphere but it as an oblate spheroid or an oblate ellipsoid. The earth is compressed along the poles but slightly bulging along the equator.

The equatorial diameter of the earth is about 12,757 km (7,927 miles).

The polar diameter of the earth is about 12,715 km (7,901 miles).

The equatorial circumference of the earth is 40,077 km.

The polar circumference of the earth is 40,005 km.

Earth has a spinning rotational motion and completes rotation once every 23 hours 56 minutes and 4.09 seconds.

Earth completes the revolution around the Sun, once every 365 days, 6 hours, 9 minutes and 9.54 seconds.

Highest point on land – Mt.Everest (8,848 metres above sea level.)

Lowest area on land – Shore of the Dead Sea (399 metres below the sea level).

Deepest area – Mariana Trench in Pacific ocean, southwest of Guam (11,033 metres below sea surface)

Chemical make up of the Earth's Crust (per cent by weight): Oxygen 46.6; Silicon 27.7; Aluminium 8.1; Iron 5.0; Calcium 3.6; Sodium 2.8; Potassium 2.6; Magnesium 2.0; and other elements 1.6.

Mean surface temperature of Earth is 14 deg. C.

Highest temperature recorded on Earth is 58 deg. C at Al Aziziyah, Libya.

Lowest temperature recorded on Earth is – 89.6 deg. C at Vostok station in Antarctica.

Earth's inclination of the Polar axis to the Orbital Plane: 66 deg.33 min.1 second.

Earth's orbital speed about the Sun is 29.8 km/second.

Earth's mean distance from the Sun is 14, 95, 98,500 km (one astronomical unit).

The maximum distance between the Earth and the Sun is at the time of aphelion between July 2 and 5 and is about 152 million km.

The minimum distance between the Earth and the Sun is at the time of perihelion between January 2 and 5 and is about 147 million km.

The latitude of a place is defined as its distance north or south of the equator, measured as an angle.

Latitudes are imaginary lines drawn parallel to the equator and run from east to west.

The lines of latitudes are called parallels.

The longitudes show the distance of a point east or west of the prime meridian which is at 0 and passes through Greenwich village near London in England.

There are 360 longitudes and all the longitudinal lines converge at the poles.

The longitude of a place can be defined as its distance east or west of the meridian of Greenwich, measured as an angle.

Lines joining all places situated at the same longitude are called meridians.

The parallel (or latitude) of 0 degrees is called the Equator.

The parallel (or latitude) 23 1/2 degrees N is called the Tropic of Cancer.

The parallel (or latitude) 23 1/2 degrees S is called the Tropic of Capricorn.

The parallel (or latitude) 66 1/2 degrees N is called the Arctic Circle.

The parallel (or latitude) 66 1/2 degrees S is called the Antarctic Circle.

The equator not only divides the earth into two hemispheres, the northern and southern, but it is also the line of reference for all latitudes.

The sun is exactly overhead on the tropics only once a year – Tropic of Cancer at the time of summer solstice and the Tropic of Capricorn at the time of winter solstice.

The Arctic and the Antarctic circles enclose the polar zones where one can experience a continuous day or night for or more than 24 hours. Outside these polar zones the day or night can never be as long as 24 hours or more.

Among the longitudes, the 0 degrees or the meridian of Greenwich is known are the prime meridian and it is the reference line for measuring longitude of various places.

The plane of prime meridian divided the earth into two hemispheres, the eastern and the western hemispheres. Area extending to its east up to 180 degrees E is called the Eastern Hemisphere while that extending to its west up to 180 degrees W is called the Western Hemisphere.

The line just opposite the prime meridian on the globe, i.e., the 180 degrees longitude is called the International Date Line.

The International Date Line is the basis of keeping date and the date changes as one crosses this line. The date in the Eastern Hemisphere is ahead of that in the Western Hemisphere.

A great circle is a circle on the surface of the earth the plane of which passes through the centre of the earth, i.e., it divides the earth into two equal parts of hemispheres.

A great circle is the largest circle that can be drawn on the globe.

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.

Among the meridians of longitude, all are great circles.

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 minimize the distance of their journeys.

The earth has two motions, namely daily rotation and annual revolution. Rotation is the spinning movement of the earth on its axis and the earth completes one rotation with respect to the sun (so that a given meridian passes in front of the sun for the second time) in about 24 hours. This motion of the earth is called daily motion.

The rotation of the earth causes the formation of day and night.

The earth revolves around the sun and completes one revolution in about 365.25 days. This movement of the earth is called annual motion too.

The annual revolution is responsible for the change of seasons.

The earth is nearest to the sun around January 3, when the earth is said to be in perihelion (the distance being 14,70,00,000 km)

The earth is farthest to the sun around July 4, when the earth is said to be in aphelion (the distance being 15,20,00,000 km)

Due to the changing distance between the earth and the sun, the earth as a whole receives more energy from the sun during January than during July.

The earth's axis of rotation (the polar axis) is not vertical but inclined and it makes an angle of 66 1/2 degrees with the plane of the ecliptic (the plane of orbit of the earth around the sun) and is tilted 23 1/2 degrees from a line perpendicular to the plane of ecliptic.

When the angle of incidence of sunrays at a place is vertical or nearly vertical, more energy reaches the earth surface and the place concerned experiences summer season.

When the sunrays at a place are slanting, it receives lesser energy from the sun and experiences winter.

The change of season on the earth occurs due to the combined effect of the revolution of the earth around the sun and the axial tilt (23 1/2 degrees ) of the earth.

Had the axis of the earth been perfectly perpendicular to the plane of the ecliptic, the sunrays would have been vertical over the equator all the year around and the amount of energy received at any place would have remained constant throughout the year, hence would have been no change of season. Also in that situation, the days and nights all over the earth would have been exactly equal throughout the year as it happens at the time of equinoxes.

Due to the axial tilt, the earth attains four critical positions in respect of the sun during its revolution around the sun. These are the equinoxes and solstices.

On June 21, the earth is so located in its orbit that the north polar end of its axis leans at the maximum angle of 23 1/2 degrees toward the sun, i.e., the Tropic of Cancer receives the vertical rays of the sun. This condition is known as summer solstice (the longest day in the northern hemisphere).

At the time of summer solstice, the North Pole experiences a continuous day and the South Pole experiences a continuous night.

On December 22, the Tropic of Capricorn receives the vertical rays of the sun. This condition is the winter solstice.

At the time of winter solstice, the North Pole experiences a continuous night and the South Pole experiences a continuous day.

At the time of the winter solstice the sun is not visible to a person on the North Pole and during the summer solstice, the sun is not visible to a person on the South Pole.

Midways between the two solstices occur the equinoxes when the earth's axis makes a 90 degrees angle with a line drawn to the sun, i.e., the equator receives the vertical rays of the sun and day and night are equal all over the world.

The vernal or the spring equinox occurs on March 21.

The autumnal equinox occurs on September 23.

The longer-than 24-hour days are experienced only in the Arctic and the Antarctic circles. The maximum length of the day as well as the night is observed at the two poles.

At the equator the days and nights are exactly equal all the year around but elsewhere it is possible only at the time of equinoxes.

The time on the earth's surface is reckoned with respect to the overhead sun. When the sun is overhead at a given meridian, it is 12:00 noon for all places situated at that meridian. This is called the local time and it varies from one place to another.

The earth completes one rotation of 360 degrees in 24 hours. Therefore, the time required for a one-degree longitude rotation of the earth is four minutes.

Since the earth rotates on its axis from west to east, a place that is situated westwards of a given place will experience the overhead sun later and the time of this place will be behind the time of the place situated eastwards of it.

The time differs from one place to the other at the rate of four minutes per degree of longitude.

Standard time refers to an area, e.g., a country. It is the time of a centrally located meridian and the same time is conventionally assigned to the whole area or the country. This the local time of the central meridian is treated as the standard time of the country or the region concerned. The region concerned is called a time zone.

Since the time varies from one time zone to the other, for the purpose of comparison the time of Greenwich (0 degrees longitude) is taken as the reference time and it is called the Greenwich Mean Time (GMT). The time of different places or countries of the world is stated as ahead or behind the GMT.

The central meridian for India is the longitude of 82 degrees 30' E. This longitude passes near Allahabad and the Indian Standard Time (IST) is the time of this meridian. Indian Standard Time is 5 hours 30 minutes ahead of GMT.

The concept of International Date Line has been adopted to avoid the confusion of the difference of one day that people would face while travelling across the world.

Counting from the Prime Meridian, the date to the east of it is one day ahead than to its west.

A person travelling westwards of the Prime Meridian up to 180 degrees W will put his watch back by 12 hours and the other person travelling eastwards up to 180 degrees E will have put his watch 12 hours forward. Thus the watches of these two persons meeting at the International Date Line will have the same time but different dates.

A person crossing International Date Line from its west to east gains a day, i.e., he puts the date in his watch one day behind. This is because the area on the eastern side of the International Date Line lies on the western hemisphere and the time and date in this are is behind. Thus he gains a day as he moves from west to east across the International Date Line.

A person crossing International Date Line from its east to west (moving from Western hemisphere to the Eastern Hemisphere) will have to put his watch one day ahead and he is said to lose a day.

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.

The time required for a 360 degrees 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.

The sidereal day is the time taken by the earth to complete a 360 degrees rotation on its axis and this time period (23 hours 56 minutes) is slightly shorter than the solar day.

Time taken by the earth to complete on revolution in its orbit is called one year. This time can be measured in a number of ways, e.g., the solar year, measured with respect to the sun i.e., the time taken by the sun to appear successively at the point of Aries.

Sidereal year is the period of one revolution of the earth with respect to a fixed star.

A calendar year is regulated using leap years so that it is equal to the solar year. To achieve this, one day is added to the usual year of 365 days to the month of February every fourth year, making it a leap year. This correction of one day every fourth year is too large as the exact solar year has 365.2419 days and the correction considers it having 365.25 days. Therefore, the leap year is omitted in the century year unless the year is divisible by 400 (a leap century).

The months were originally calculated by the revolution of the moon around the earth but now the lunar month of 29.5 days has been made slightly longer to ensure the occurrence of the months and seasons at the same time.

Julius Caesar added 90 days to the year 46 B.C. as the calendar by that year run 90 days behind the season. He declared that each year after this will have 365 days and every fourth year will be a leap year. The final touch to the calendar was given by Pope Gregory XIII by making the last year of the century a year of 365 days unless the year was divisible by 400. The Gregorian calendar is now universally used.

The moon, being a satellite of the earth, revolves around it. At the same time moon also rotates on its axis and the axis of the moon is more or less parallel to the axis of the earth (the axis of the moon is inclined at an angle of about 58 degrees 43' with respect to the plane of ecliptic).

The moon has a diameter of about 3,480 km and has a mass about 1/81 that of the earth. Like that of earth, the moon's orbit is also elliptical.

At its nearest point to the earth, the moon is said to be in perigee, and its farthest, in apogee.

Distance between earth and the moon varies from about 3,56,000 km at perigee to about 4,07,000 km at apogee.

As the moon completes one revolution around the earth every month, the perigee and apogee are monthly phenomena unlike the perihelion and aphelion that are annual phenomena.

The moon's period of revolution with reference to the sun is about 29.53 days (29 days, 12 hours 44 minutes and 2.8 seconds). This period is called synodic month

In terms of sidereal time, it takes about 271/2 days for the moon to be re-located in exactly the same meridional position with respect to the stars. This period of about 271/2 days is called sidereal month and is the time required by the moon to complete one revolution about the earth with respect to the stars.

The time taken by the moon in completing one revolution around the earth (synodic month) is longer than the sidereal month because of the fact that while the moon revolves around the earth, the earth also revolves around the sun and thus moon has to travel a longer distance to be relocated exactly in the same position with respect to the earth and the sun.

The time taken by the moon to complete one rotation on its own axis is also exactly one sidereal month.

We always see the same face of the moon from the earth. Also, we can never see from the earth the complete surface of the moon. Only 59 per cent of the total surface of the moon is visible from the earth. The remaining 41 pr cent is never seen.

An eclipse is a complete or partial obscuration of light from a celestial body as it passes through the shadow of another celestial body.

On the earth, we are concerned mainly with two eclipses, i.e., of the moon and the sun. The former is called a lunar eclipse and the latter a solar eclipse.

The eclipses are caused when a heavenly body casts its shadow over another heavenly body. The inner cone shaped part of the shadow is called umbra and the outer lighter part is called penumbra.

When the sun, moon and the earth are in a straight line they are said to be in syzygy.

Due to its revolution around the earth, the moon is bound to come in between the sun and the earth thus obscuring a part or whole of the sun. This is called solar eclipse. The solar eclipse occurs when the sun and the moon are in conjunction, i.e., on the same side of the earth. At such a time one cannot see the moon from the earth as the illuminated part of the moon faces away from the earth. It is thus the period of the "new moon".

The solar eclipse may be total, annular or partial. An eclipse is total when the apparent diameter of the moon is equal to or greater than the apparent diameter of the sun.

If the apparent diameter of the moon is smaller than that of the sun, the moon is not able to obscure the sun completely and the sun is visible as a ring around the edge of the moon. This is called an annular eclipse.

Due to the rotation of the earth and the revolution of the moon the umbral shadow passes across the surface of the earth. This path is called the "zone of totality". This zone is only about 260 km wide. An observer in this zone will see a total solar eclipse.

When a solar eclipse is observed from either side of the "zone of totality" a part of the sun remains visible and the eclipse will be partial.

If the line up of the sun, moon and the earth is not exact, there may be only a partial eclipse without any zone of totality.

A total solar eclipse lasts only a few minutes – generally not more than 7.5 mts.

As the earth goes around the sun and the moon around the earth, the earth is also bound to come between the moon and the sun once every lunar month. It causes an eclipse of the moon or the lunar eclipse.

A lunar eclipse occurs when the sun and the moon are in opposition, i.e., they are located on the opposite sides of the earth. At such times the moon is obscured as the shadow of the earth is cast on it.

The lunar eclipses occur on the days of full moon, whereas the solar eclipses occur on the days of new moon.

The reason eclipses do not occur on every full moon or new moon days as the line up of the sun, moon and the earth is not perfect on each of these days.

For a lunar eclipse to occur it is necessary that not only the sun, the moon and the earth are in a straight line but they should also be located in the same plane and this does not occur on every full moon day.

The lunar eclipses can be total, partial or penumbral. A total lunar eclipse occurs when the entire moon passes through the umbra of earth's shadow. During a partial lunar eclipse, moon does not completely enter the umbra and a part of it remains visible from the earth. During a penumbral eclipse, the moon passes only through the penumbra of the shadow of the earth.

There can only be a maximum of five solar eclipses and three lunar eclipses in a year.

Eclipses of the same type repeat after an interval of just over 18 years, known as the saros period.

Phases of the Moon: The synodic month begins when the sun and the moon are in conjunction. The moon at this time is dark and it is called the new moon. It is followed by the crescent moon when the moon is visible as a thin crescent. This phase comes about 3.75 days after the conjunction. After about 7.5 days of the conjunction phase is called the first quarter when moon appears as a half circle. After about 11.25 days three quarters of the moon appears illuminated and this phase is called the gibbous moon. When the entire illuminated part of the moon is visible from the earth it is called the full moon. This phase occurs about 14.75 days after the conjunction or the new moon. During the remaining part of the synodic month the same phases of the moon are repeated in a reverse sequence and they are known and gibbous moon, the last quarter and the crescent moon. The synodic month will end with the moon again being completely invisible – the new moon.