Basics of Geography

TOPICS COVERED 

• Origin of universe
• Stars and constellations
• Solar system
  • Asteroids
  • Meteor, meteoroid and meteorite
  • Sun
  • Comet
  • Planets
  • Dwarf planets
  • Theories of planet formation

• Evolution of earth
• Formation of moon
• Geological time scale
• Latitude and Longitude
  • Important parallels of latitudes
  • Heat zones of the earth
  • Greenwich Meridian Time
  • Time Zone
• Motion of the earth
  • Rotation and revolution
  • Axial tilt
  • Seasons (Solstice and Equinox)
• Origin Of Universe

The Big Bang Theory: Understanding the Universe’s Origins

• Initially, there was a “Tiny Ball” (singular atom) with an unimaginably small volume, infinite temperature, and infinite density.
• Violent explosion of “Tiny Ball” (Big Bang took place 13.7 billion years before the present).
• Rapid expansion within fractions of a second after the bang then the expansion has slowed down.
• Scientists believe that though the space between the galaxies is increasing, observations do not support the expansion of galaxies.

Star Formation and Lifecycle

• The formation of stars is believed to have taken place some 5-6 billion years ago.
  • Growing nebula develops localised clumps of gas that continue to grow into even denser gaseous bodies, giving rise to formation of stars.
• A galaxy contains a large number of stars.
• Galaxies spread over vast distances that are measured in thousands of light-years.
  • Light year is a measure of distance and not of time. It is the distance that light travels in one Earth year. 1 light year is equal to 9.46 trillion km.

Constellations and Celestial Navigation

• Constellations are the various patterns formed by different groups of stars.
  • Example: Ursa Major or Big Bear
  • Small bear or Saptarishi is a group of seven stars that forms a part of the large Ursa Major Constellation
• In ancient times, people used to determine directions during the night with the help of stars.
• The North star
  • Indicates the north direction.
  • It is also called the Pole Star.
  • It always remains in the same position in the sky.
  • We can locate the position of the Pole Star with the help of the Saptarishi.

SOLAR SYSTEM 

Asteroids: Small Celestial Bodies in the Solar System

• Asteroids are a class of small rocky objects of the Solar System orbiting around the Sun.
• They have also been called planetoids, especially the larger ones.
• They are found between the orbits of Mars and Jupiter.
• The largest asteroid is the
• Scientists are of the view that asteroids are parts of a planet which exploded many years back.

Difference between Meteor, Meteoroid, and Meteorite

They’re all related to the flashes of light called “shooting stars” sometimes seen streaking across the sky. But we call the same object by different names, depending on where it is.

• Meteoroids are objects in space that range in size from dust grains to small asteroids. Think of them as “space rocks”.
• When meteoroids enter Earth’s atmosphere (or that of another planet, like Mars) at high speed and burn up, the fireballs or “shooting stars” are called meteors.
• When a meteoroid survives a trip through the atmosphere and hits the ground, it’s called a meteorite.

Solar Phenomena: Sun, Solar Flares, Sunspot Cycle, and Impact 

• The Sun is a star at the centre of the solar system.
• Primarily made up of hot gases.
• Important sources of energy for life on Earth are produced from nuclear fusion of hydrogen nuclei.
• Solar Winds: Ejections of plasma (extremely hot charged particles) that originate in the layer of the Sun known as the corona (outermost layer, hidden due to sun’s light, visible in solar eclipse).
• Solar/Stellar Flare
  • It is a dramatic increase in brightness of a star (Due to the magnetic energy stored in the star’s atmosphere).
  • Occur in active regions around sunspots.
  • Often accompanied by coronal mass ejection.
  • Solar flare ejects clouds of electrons, ions and atoms along with electromagnetic radiations.
  • Bombardment with such a huge amount of energy (as observed in Proxima centauri) can strip water from the atmosphere or Oceans and sterilise the ground.
  • Impact
  • When flare is ejected in the direction of the earth, the particles hitting the upper earth’s atmosphere may cause AURORA/Polar-light (Aurora-Borealis/Northern-light and Aurora-Australis/Southern-Light)
  • X-rays and UV rays may affect ionosphere and disrupt long range radio communication
  • Radiation risks posed by flares are one of the major hurdles in manned space missions

• Sun-spot Cycle
  • Amount of magnetic flux that rises up to the Sun’s surface varies with time in a cycle called the solar cycle.
  • This cycle lasts 11 years on average cycle and is sometime referred as the sunspot cycle
  • Sunspots are darker, magnetically strong, cooler areas on the surface of the sun (photosphere)
  • Not pesent all over the sun, present between 25°-30° latitude.
  • It will help in understanding of the long-term variations of the Sun and its impact on earth climate (Aditya L-1 Mission Objective).

Comets: Cosmic Snowballs Orbiting the Sun 

• Cosmic snowballs of frozen gases, rock and dust that orbit the Sun.
• When frozen, they are the size of a small town.
• When a comet’s orbit brings it close to the Sun, it heats up and spews dust and gases into a giant glowing head larger than most planets.
• The dust and gases form a tail that stretches away from the Sun for millions of miles.
• There are likely billions of comets orbiting our Sun in the Kuiper Belt and even more distant Oort cloud.

Planets: Criteria Established by the International Astronomical Union

According to the International Astronomical Union in 2006, a planet must do 3 things:

• It must orbit a star (in our cosmic neighborhood, the Sun).
• It must have sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly spherical) shape.
• It must be big enough that its gravity clears away any other objects of a similar size near its orbit around the Sun.

Planets

Mercury: The Small, Dense, and Swift Planet Closest to the Sun

• The smallest planet in our solar system and closest to the Sun.
• Second densest planet.
• No moons or rings.
• Second hottest planet (Venus hottest).
• Lack of seasons on its surface due to the smallest tilt than all other planets.
• Only slightly larger than Earth’s Moon.
• Fastest planet, zipping around the Sun every 88 Earth days.

Venus: Earth’s Sister Shrouded in Clouds and Extreme Heat

• Sister of Earth due to proximity, mass and size.
• Surface of Venus is hidden by an opaque layer of clouds which are formed from sulphuric acid.
• A thick atmosphere traps heat in a runaway greenhouse effect, making it the hottest planet in our solar system.
• Venus spins slowly in the opposite direction from most planets.
• Rotate clockwise (other planets – counter clockwise)
• Referred to as “morning star”, “evening star” (due to brightness).
• Named after a female figure.

Earth: Our Vibrant Blue Planet Teeming with Life

• Largest of all the terrestrial planets.
• Most dense planet in the solar system.
• Only place we know of so far that’s inhabited by living things.
• It’s also the only planet in our solar system with liquid water on the surface.
• Ozone Layer protects it from harmful solar radiation.
• The moon (no atmosphere, only one face is ever seen from earth and this condition is known as Tidal Locking).

Mars: The Red Desert World with Polar Ice Caps

• Mars is a dusty, cold, desert world with a very thin atmosphere.
• There is strong evidence Mars was—billions of years ago—wetter and warmer, with a thicker atmosphere.
• Mars is the only other planet besides Earth that has polar ice caps.
• Seasons like Earth, but they last twice as long.
• Red Planet without magnetic field.
• Olympus Mons: The tallest mountain known in the terrestrial planets system
• Notable Moons: Phobos and Deimos

Jupiter: Giant Planet with the Great Red Spot and Moons

• Jupiter is more than twice as massive as the other planets of our solar system combined.
• Giant planet’s Great Red spot is a centuries-old storm bigger than Earth. Shortest day and highest gravity among the eight planets .
• Atmosphere: 90% hydrogen and 10% helium, nearly the same as the Sun’s.
• Notable moons: Europa, Ganyemede (largest moon in the solar system), Callisto

Saturn: The Ringed Giant Planet with Spectacular Moons

• “The Ringed Planet’’: The other giant planets have rings, but none are as spectacular as Saturn’s.
• Second largest planet (diameter and mass).
• Gives off more energy than it receives from the Sun.
• Saturn appears pale yellow in colour because the upper atmosphere contains ammonia crystals.
• Least density in the solar system.
• Notable moons: Titan, Rhea, and Enceladus

Uranus: The Tilted Ice Giant of the Outer Solar System

• It rotates at a nearly 90-degree angle from the plane of its orbit. This unique tilt makes Uranus appear to spin on its side.
• Coldest planet in the solar system.
• Lightest of giants in the outer solar system.
• Uranium (discovered in 1789) was named after it.
• Voyager 2: only spacecraft to have flown by it
• Notable moons: Oberon, Titania, Miranda, Ariel, and Umbriel

Neptune: The Distant Blue Giant Discovered by Math

• Second largest gravity of any planet.
• It was the first planet located through mathematical calculations.

Dwarf Planets: Celestial Bodies Beyond Traditional Definitions

It is a celestial body that

• Is in orbit around the Sun.
• Has sufficient mass for itself-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape.
• Has not cleared the neighbourhood around its orbit.
• Is not a satellite.

Some prominent dwarf planets

• Pluto
• Ceres
• Makemake
• Haumea
• Eris

Comparing Terrestrial and Jovian Planets: Contrasting Worlds in Our Solar System

• While the terrestrial planets are made of solid surfaces, the Jovian planets are made of gaseous surfaces.
  • The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.
  • As terrestrial planets were closer to the sun, the intense solar winds blew off lots of gas and dust. Solar winds were not intense/strong enough at the location of Jovian planets to cause similar removal of gases.
  • Terrestrial planets are smaller and their low gravity could not hold escaping gases.
• When comparing the size, the Jovian planets are much larger than the terrestrial planets.
• While the atmosphere of terrestrial planets is composed mainly of carbon dioxide and nitrogen, hydrogen and helium are found in abundance in the atmosphere of Jovian planets.
• The core of the Jovian planets is denser than the terrestrial planets.
• The terrestrial planets spin less, and are therefore less flattened at the poles.
• The Jovian planets have more moons when compared to terrestrial ones.

THEORIES OF FORMATION OF PLANETS

The Nebular Hypothesis: Explaining the Formation of Our Solar System

• Given by Immanuel Kant and re-examined by Pierre-Simon Laplace.
• Nebula (Accumulation of hydrogen gas in the form of a very large cloud)
• Pockets of dust and gas began to collect into denser regions due to the gravitational collapse at the center of the cloud (Result of a passing star or shock waves from a supernova).
• Denser regions pulled in more and more matter (Conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up).
• Most of the material ended up in a ball at the center while the rest of the matter flattened out into a disk that circled around it.
• While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc.
• Planets were formed out of a cloud of material associated with a youthful sun, which was slowly rotating.
• Accretion from this disc led to the formation of planets.
• Leftover debris that never became planets congregated in regions such as the Asteroid Belt, Kuiper Belt, and Oort cloud.

Binary Theories: Sun’s Hypothetical Companion and Planet Formation

• According to these theories, the sun had a companion.
• Chamberlain and Moulton considered that a wandering star approached the sun resulting in separation of a cigar-shaped extension of material from the solar surface.
• As the passing star moved away, the separated material of the solar surface continued to revolve around the sun and slowly condensed into planets.

From Barren Rock to Flourishing Biosphere: Evolution of Earth’s Surface and Interior

• Earth initially was a Barren, rocky and hot object with a thin atmosphere of hydrogen and helium.
• Evolution of life on the surface of the planet (between the 4,600 million years and the present).
• The earth has a layered structure with non-uniform materials from the outermost end of the atmosphere to the centre of the earth.
• The atmospheric matter has the least density.
• The earth’s interior has different zones from the surface to deeper depths. Each of these contains materials with different characteristics.

Formation of the Lithosphere: From Volatile State to Solidification

• Volatile state during primordial stage – Temperature inside has increased (Due to gradual increase in density) – Material inside started getting separated depending on densities (Heavier materials like iron sink towards the centre) – It cooled further and solidified and condensed with passage of time
• The gases and water vapor were released from the interior as the earth cooled down.
  • The process through which the gases were outpoured from the interior is called
  • Continuous volcanic eruptions contribute water vapor and gases to the atmosphere.

Hydrosphere Formation: From Condensation to Ocean Formation

• As the earth cooled, the released water vapor started getting condensed.
• The carbon dioxide in the atmosphere got dissolved in rainwater. This further decreased temperature causing more condensation and more rain.
• The rainwater falling onto the surface got collected in the depressions to give rise to oceans.
• The earth’s oceans were formed within 500 million years from the formation of the earth.

Atmospheric Evolution: From Primordial Gases to Oxygen-Rich Atmosphere

• The early atmosphere, with hydrogen and helium, is supposed to have been stripped off as a result of the solar winds.
• The present composition of earth’s atmosphere is chiefly contributed by nitrogen and oxygen.
• There are three stages in the evolution of the present atmosphere.
  1. Loss of primordial atmosphere.
  2. Evolution of the atmosphere by the hot interior of the earth.
  3. Modification of atmospheric composition by the living world through the process of photosynthesis.

FORMATION OF MOON

Moon Formation: The Fission Theory Explained

• Single rapidly rotating body possessing the Earth and the Moon à Whole mass became a dumbbell-shaped body à Separation
• Material forming the moon was separated from what we have at present the depression occupied by the Pacific Ocean

The Giant Impact Hypothesis: How the Moon was Formed

• Earth struck by Theia (Mars size object) à Huge amount of debris à Debris coalesces during orbiting the Earth à Moon created Geological Time Scale

Eons: Understanding Earth’s Geological Time

• Broadest category of geological time.
• Earth’s history is characterized by four eons; the Hadeon, Archean, Proterozoic, and Phanerozoic (in order from oldest to youngest).
• Collectively, the Hadean, Archean, and Proterozoic are sometimes informally referred to as the “Precambrian.”
• We live during the Phanerozoic, which means “visible life.”

Eras: Divisions of Earth’s Geological Time

• Eons of geologic time are subdivided into eras, which are the second-longest units of geological time.
• The Phanerozoic eon is divided into three eras: the Paleozoic, Mesozoic, and Cenozoic.

Periods: Subdivisions of Geological Eras

• Just as eons are subdivided into eras, eras are subdivided into units of time called periods.
• The most well-known of all geological periods is the Jurassic period of the Mesozoic era.

Epochs and Ages: Further Divisions of Geological Periods

• Periods of geological time are subdivided into epochs.
• In turn, epochs are divided into even narrower units of time called ages.

Decreasing order

Supereon > Eon > Era > Period > Epoch

mya = Million years Ago 

EONS 

The Hadean Eon: Earth’s Formative Years Before Life

• Indicates the time before reliable (fossil) evidence of life.
• Extremely hot temperature.
  • Much of the Earth was molten (extreme volcanism) leading to formation of crust after cooling.
• Volcanic outgassing probably created the primordial atmosphere (No oxygen) and then the ocean.
• Heavy CO2 atmosphere with water vapor and hydrogen.

The Archean Eon: Emergence of Life and Continental Formation

• Beginning of life on Earth (Evidence of cyanobacteria dates to 3500 mya).
• Life was limited to Prokaryota (simple single-celled organisms lacking nuclei).
• No oxygen in the atmosphere.
• Formation of continents due to the cooling of Earth’s crust.
• Higher volcanic activity than today with multiple lava eruptions.

The Proterozoic Eon: Emergence of Oxygen and Tectonic Activity 

• Last eon of the Precambrian “supereon”.
• Oxygen production started by Bacteria leading to the sudden rise of life forms.
• Eukaryotes (have a nucleus) emerged.
• The early and late phases of this eon may have undergone Snowball Earth periods (the planet went through extensive glaciation resulting in a drop in sea levels).
• Very tectonically active

The Phanerozoic Eon: Emergence of Complex Life and Continental Drift

• Complex multicellular life arose.
• Plant life on land emerged in the early Phanerozoic eon.
• Pangaea forms and is later disassociated into Laurasia and Gondwana.

This Eon is divided into 3 eras:

1. Palaeozoic = An era of ancient life (arthropods, amphibians, fishes)
2. Mesozoic = Age of reptiles and gymnosperms (climatic extinction of the non-avian dinosaurs)
3. Cenozoic = Age of mammals and angiosperm

Latitude and Longitude

Latitudes: Mapping Earth’s North-South Coordinates

• Latitude is the angular distance of a place north or south of the earth’s equator.
• Latitude tells you where you are between the North Pole and the South Pole.
• These are measured in degrees.
• Parallels or latitude lines
  • Lines that run across the Earth from east to west at a constant latitude.
  • Everywhere on a parallel must have the same latitude.
  • An example is the equator, which is at zero degrees of latitude.
• A circle of latitude is an imaginary ring linking all points sharing a

Longitudes: Mapping Earth’s East-West Coordinates

• Longitude is measured by imaginary lines that run around the Earth vertically (up and down) and meet at the North and South Poles.
• These lines are known as
• The line which runs through Greenwich in London is called the Greenwich Meridian or Prime Meridian. The Prime Meridian is 0° longitude.
• The Earth is then divided into 180° east and 180° west.
• Longitude is the measurement east or west of the prime meridian.
• Half of the world (the Eastern Hemisphere) is measured in degrees east of the prime meridian. The other half (the Western Hemisphere) is degrees west of the prime meridian.
• The anti-meridian is halfway around the world, at 180°. It is the basis for the International Date Line.
• Unlike parallels of latitude, all meridians are of equal length.

Key Parallels of Latitude: Equator, Tropics, Circles, and Poles 

Equator (0° latitude)

• Imaginary circular line running on the globe and divides it into two equal parts.
• These are measured in degrees.
  • Important reference point to locate places on the earth.
• All parallel circles from the equator up to the poles are called parallels of latitudes.
• All parallels north/south of the equator are called ‘north latitudes’/’south latitudes’. It is indicated by the letter ‘N’ or ‘S’.
• The size of the parallels of latitude decreases as we move away from the equator.

Tropic of Cancer (23½° N) in the Northern Hemisphere

• It is the farthest northern latitude at which the sun can appear directly overhead.
• It occurs on the June-solstice (marked between June 20 and June 22).
• North of this line is the subtropics and Northern Temperate Zone.

Tropic of Capricorn (23½° S) in the Southern Hemisphere

• It is the farthest southern latitude at which the sun can appear directly overhead.
• It occurs on the December-solstice (marked between December 20 and December 23).
• South of this line is the subtropics and Southern Temperate Zone.

Arctic/Antarctic Circle at 66½° north/south of the equator

• Parallel of latitude around the Earth at approximately 66.5° N/S
• Because of the Earth’s inclination of about 23 1⁄2° to the vertical, It marks the southern/northern limit of the area within which, for one day or more each year, the Sun does not set (about June 21) or rise (about December 21).

North Pole (90°N) and South Pole (90° S)

• The North/South Pole is the northernmost/southernmost point on Earth.
• It is the precise point of the intersection of the Earth’s rotational axis and the Earth’s surface.
• Polaris (the current North Star) sits almost motionless in the sky above the North Pole, making it an excellent fixed point to use in celestial navigation in the Northern Hemisphere.
• All lines of longitude meet at the North and the South Pole.

Geographical Coordinates of India: Extent and Position in the Northern Hemisphere

• India is a vast country and lies entirely in the Northern hemisphere.
• The mainland extends between latitudes 8°4’N and 37°6’N and longitudes 68°7’E and 97°25’E.
• The Tropic of Cancer (23°30’N) divides the country into almost two equal parts.

Heat Zones of the Earth: Torrid, Frigid, and Temperate Regions Defined by Latitudes 

Torrid Zone

• Areas lying between the Tropic of Cancer and the Tropic of Capricorn.
• This area receives the maximum heat and is called the 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.
• The tropics are known for their lush green vegetation and moist climate.
• Average temperatures range from warm to hot year round.
• Many places in the tropics experience rainy seasons which range from one to several months of consistent rainfall.

Frigid Zone

• 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 called Frigid Zones.
• 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.
• Area is very cold because here the sun does not rise much above the horizon. Therefore, its rays are always slanting and provide less heat.

Temperate Zones

• The areas lie between the Torrid Zone and Frigid Zone and have moderate temperatures.

Greenwich Meridian Time (GMT)

• Places east of Greenwich see the sun earlier and gain time (East-Gain-Add).
• Places west of Greenwich see the sun later and lose time (West-Lose-Subtract).

Countries through which Prime Meridian passes 

Time Zones and Daylight Saving Time 

• A time zone is a region of the Earth that has adopted the same standard time, usually referred to as the local time.
• Most adjacent time zones are exactly one hour apart, and by convention compute their local time as an offset from Greenwich Mean Time (GMT).
• Standard time zones can be defined by geometrically subdividing the Earth’s spheroid into 24 lunes (wedge-shaped sections), bordered by meridians each 15° of longitude apart. The local time in neighboring zones is then exactly one hour different.
  • However, political and geographical practicalities can result in irregularly-shaped zones that follow political boundaries or that change their time seasonally (as with daylight saving time), as well as being subject to occasional redefinition as political conditions change.

The earth rotates 360° in about 24 hours, which means 15° an hour or 1° in four minutes.

• The earth has been divided into twenty-four time zones of one hour each (24 hours x 15° rotation = 360° rotation in a day)

Indian Standard Time (IST)

• Time along the Standard Meridian of India (82°30’E) passing through Mirzapur (in Uttar Pradesh) is taken as the standard time for the whole country.
• IST at 82°30’E is 5 hours and 30 minutes ahead of GMT.
  • Indicates 05:30 hours earlier sunrise in India than the countries taking GMT as a standard time.
• It goes through following states
  • Uttar Pradesh
  • Madhya Pradesh
  • Chhattisgarh
  • Orissa
  • Andhra Pradesh

International Date Line

• It serves as the “line of demarcation” between two consecutive calendar dates.
• It passes through the mid-Pacific Ocean and roughly follows a 180° (located halfway round the world from the prime meridian).
• It bends and goes zig zag at the Bering Strait between Siberia and Alaska, Fiji, Tonga and in some other islands.
• It follows a zig-zag pattern to avoid the confusion of having different dates in the same country.
• The date changes by exactly one day while crossing it.
  • A traveller crossing the date line from east to west loses a day and while crossing the dateline from west to east gains a day.

Daylight saving time (DST)

• Daylight saving time (DST) or summer time is the practice of advancing clocks during summer months by one hour.
• It is done so that evening daylight lasts an hour longer i.e. fully utilizing the surplus sunlight in summers while compensating the short day length in winters.
• Typically, regions with summer time adjust clocks forward one hour close to the start of spring (terms “spring forward”) and adjust them backward in the autumn to standard time (terms “fall back”).
• Daylight saving time practice is prevalent in many temperate countries
  • To conserve energy by utilizing the day light and reduce evening use of incandescent lighting.
  • To compensate for variation in day length experienced from season to season.

Motions Of The Earth

Earth’s Motions: Rotation and Revolution

2 types of motion

1. Rotation

• Movement of the earth on its axis.
• As the Earth rotates, each area of its surface gets a turn to face and be warmed by the sun. This is important to all life on Earth.
• If the Earth did not rotate, one half of the Earth would always be hot and bright, and the other part would be frozen and dark. Life would not have been possible in such extreme conditions.

2. Revolution

• Movement of the earth around the sun in a fixed path or orbit.
• It takes 365¼ days (one year) to revolve around the sun. 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. Such a year with 366 days is called a leap year.
• The earth is going around the sun in an elliptical orbit.

Axial Tilt and Precession: Understanding Earth’s Orbital Dynamics

• Axial tilt is the angle between the planet’s rotational axis and its orbital axis.
• A planet’s orbital axis is perpendicular to the ecliptic or orbital plane, the thin disk surrounding the sun and extending to the edge of the solar system.
• Earth’s axis is not perpendicular. It has an axial tilt or obliquity.
  • The axis of the earth, which is an imaginary line, makes an angle of 66½° with its orbital plane.
• Some planets, such as Mercury, Venus, and Jupiter, have axes that are almost completely perpendicular, or straight up-and-down.
• Uranus has the largest axial tilt in the solar system. Its axis is tilted about 98 degrees, so its north pole is nearly on its equator.

Axial Precession 

• Earth’s axis appears stable but it actually wobbles very slowly like a spinning top.
• This wobble is called axial precession.
• Earth’s axis helps determine the North Star.
  • Currently, Earth’s axis points toward a star called Polaris (current North Star due to its position almost directly above the North Pole).
• Polaris will not always be the North Star, however. The Earth’s axis is slowly wobbling away from Polaris. In another 13,000 years, it will point toward the new North Star, a star called Vega.

Circle of illumination

• Due to the spherical shape of the earth, only half of it gets light from the sun at a time and experiences day.
• The circle that divides the day from night on the globe is called the circle of illumination.
• This circle does not coincide with the axis

Solstices: Earth’s Seasonal Extremes 

• Solstice means that either the Northern or Southern Hemisphere is tilted toward the sun and receives the maximum intensity of the sun’s rays throughout the year.
• Solstices and shifting solar declination are a result of Earth’s 23.5° axial tilt as it orbits the sun.
• From the North Pole, the sun is always above the horizon in the summer and below the horizon in the winter.
  • This means the region experiences up to 24 hours of sunlight in the summer and 24 hours of darkness in the winter.

Balanced Days: Understanding Equinoxes

• There are only two times of the year when the Earth’s axis is tilted neither toward nor away from the sun.
• On 21st March and September 23rd, direct rays of the sun fall on the equator. This results in a “nearly” equal amount of daylight and darkness at all latitudes. It is known as equinox.
• 23rd September = Autumn season (season after summer and before the beginning of winter) in the northern hemisphere and spring season (season after winter and before the beginning of summer) in the southern hemisphere.
• 21st March = Spring in the northern hemisphere and autumn in the southern hemisphere.

Earth’s Orbital Extremes: Perihelion and Aphelion

• Perihelion
  • It is the point when Earth is closest to the Sun.
  • It occurs around 3rd January.
  • The distance is 147.5 million kms.
• Aphelion
  • It is the point when Earth is farthest from the Sun.
  • It occurs on July 4.
  • The distance is 152.5 million kms.
• Speed of Earth is fastest at Perihelion and slowest at Aphelion (Kepler’s Second Law).

Uttarayan: The Northern Migration of the Sun

• For 6 months of the year, the Sun appears to be moving north.
• The Northward migration of Sun appears to begin after December 22 and is completed on June 21, when the Sun is directly overhead 23.½°
• In India, we call this apparent migration Uttarayan.

Dakshinayan: The Southern Migration of the Sun

• After June 21, Sun appears to be moving South for the next 6 months.
• This southward migration appears to get finished when Sun is directly overhead the 23.½°
• In India, we call this apparent migration Dakshinayan.

Effects of Earth’s Motion: Day, Night, and Seasons 

Reasons for day and night as well as seasons

• Days and Nights à Rotation
• Seasons à Revolution

The schematic representation of seasonal variation is given below. The diagram depicts the four different seasons on the earth according to its position in space at that time.

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