Solar System

Universe

• The Universe refers to the entirety of existence, including all matter, energy, planets, stars, galaxies, and the vast stretches of intergalactic space.
• It encompasses everything that exists in the known cosmos.

Cosmology

• Cosmology is the branch of science that studies the origin, evolution, structure, and future of the Universe.
• The word Cosmology is derived from:
  • Cosmos = Universe
  • Logos = Study or Science

Key Feature

• The Universe is considered boundless, with no confirmed physical boundary.

Galaxy

• A Galaxy is a massive system made up of billions of stars, along with gas (mainly hydrogen), dust, dark matter, and various celestial objects held together by gravity.

Number of Galaxies

• Scientists estimate that the observable universe contains nearly 100 billion (10¹¹) galaxies.

Number of Stars

• On average, every galaxy contains approximately 100 billion (10¹¹) stars.
• Consequently, the observable universe is believed to contain nearly 10²² stars.

Milky Way Galaxy

• The Milky Way is the galaxy that houses the Solar System and Earth.
• It is classified as a spiral galaxy with multiple spiral arms.

Formation

• The Milky Way is believed to have formed several billion years after the Big Bang, approximately 13.6–13.2 billion years ago according to modern estimates.

Smallest Known Galaxies

• Dwarf Galaxies are among the smallest and least luminous galaxies, often orbiting larger galaxies like the Milky Way.

Classification of the Universe

According to modern understanding, the Universe can broadly be divided into:

• Atmosphere (the gaseous envelope surrounding planets)
• Outer Space (the vast region beyond planetary atmospheres)

Origin of the Universe

• The most widely accepted explanation for the origin of the Universe is the Big Bang Theory, proposed by Georges Lemaître, a Belgian astronomer and physicist.

Nearest Major Galaxy

• The Andromeda Galaxy is the nearest major galaxy to the Milky Way and is expected to merge with it several billion years in the future.

The Big Bang Theory

Initial Stage

• The Big Bang Theory states that the Universe originated from an extremely hot, dense, and compact state, often referred to as a primordial singularity.

Expansion of the Universe

• Around 13.8 billion years ago, the Universe underwent a rapid expansion, giving rise to space, time, matter, and energy.
• This expansion eventually led to the formation of galaxies, stars, and other celestial bodies.

Present Scenario

• Observations indicate that galaxies continue to move away from one another, demonstrating that the Universe is still expanding.

Star

Formation

• Stars are formed when enormous clouds of gas and dust (nebulae) collapse under the influence of gravity, resulting in the birth of a new star.

Composition

• Stars are primarily composed of Hydrogen and Helium, where nuclear fusion continuously converts hydrogen into helium, releasing enormous amounts of energy.

Characteristics

• Stars are:
  • Self-luminous
  • Extremely hot
  • Massive celestial bodies
  • Major sources of light and heat.

Nearest Star

• The nearest star to Earth is the Sun.
• The nearest star beyond the Solar System is Proxima Centauri, located about 4.24 light-years away.

The Solar System

• The Solar System comprises the Sun, eight planets, their natural satellites, and numerous smaller celestial bodies such as asteroids, comets, meteoroids, and dwarf planets.
• The Sun occupies the centre of the Solar System, while all other celestial bodies revolve around it.

Gravitational Control

• The gravitational force of the Sun governs the movement of planets and other Solar System objects, maintaining them in their respective orbits.

Nature of Planetary Orbits

• The planets revolve around the Sun in elliptical orbits, in accordance with Kepler’s Laws of Planetary Motion.

Planetary Arrangement

• Nearest Planet to the Sun: Mercury
• Farthest Planet from the Sun: Neptune
• Pluto is classified as a Dwarf Planet and is no longer considered the ninth planet.

Extent of the Solar System

• The Solar System extends well beyond the planetary region, with its outer boundary (including the Oort Cloud) estimated to reach nearly 100,000 Astronomical Units (AU) from the Sun.

Mass Distribution

• The Sun contains approximately 99.86% of the total mass of the Solar System, making it the dominant gravitational body.

Importance of the Sun

• The Sun is the primary source of light and heat, providing the energy necessary to sustain life on Earth and drive planetary processes.

Classification of Planets

Terrestrial (Rocky) Planets

• Mercury
• Venus
• Earth
• Mars

Characteristics

• • Rocky composition
  • High density
  • Solid surfaces
  • Few or no natural satellites

Jovian (Gaseous) Planets

• Jupiter
• Saturn
• Uranus
• Neptune

Characteristics

• • Predominantly gaseous or icy composition
  • Large size
  • Lower average density
  • Numerous natural satellites
  • Ring systems are prominent around the outer planets, especially Saturn.

Members of Solar System

The Sun

Position in the Solar System

• The Sun is the central star of the Solar System, around which all planets and other celestial bodies revolve due to its gravitational force.
• It is also the nearest star to the Earth.

Size and Dimensions

• The Sun is approximately 109 times larger than the Earth in diameter and about 1.3 million Earths can fit inside it by volume.
• Its diameter is nearly 13.9 lakh km (1.39 million km).

Importance

• The Sun is the primary source of energy for the Earth.
• It provides light, heat, and drives essential natural processes such as the water cycle, photosynthesis, and climate system.

Chemical Composition

The Sun mainly consists of:

• • Hydrogen (≈73%)
  • Helium (≈25%)
  • Other heavier elements (≈2%), including oxygen, carbon, neon and iron.

Nuclear Fusion

• Inside the Sun, Hydrogen nuclei fuse to form Helium, releasing enormous amounts of heat and radiant energy.
• This process is known as Nuclear Fusion, which powers the Sun continuously.

Temperature

     Surface Temperature

• • The visible surface (Photosphere) has an average temperature of about 5,500°C (≈5778 K).

     Core Temperature

• • The core of the Sun reaches nearly 15 million°C, where nuclear fusion takes place.

Layers of the Sun

    Photosphere

• • The Photosphere is the visible outer surface of the Sun.
  • It emits most of the light and heat received on Earth.

    Corona

• • The Corona is the outermost atmospheric layer of the Sun.
  • It consists of extremely hot, ionised gases extending millions of kilometres into space.
  • The corona is normally invisible and can be observed during a Total Solar Eclipse or with a Coronagraph.

Additional Facts

• The Sun, along with the Solar System, revolves around the centre of the Milky Way Galaxy at nearly 828,000 km/h (≈230 km/s).
• The average distance between the Earth and the Sun is about 150 million km (1 AU).
• Sunlight takes approximately 8 minutes 20 seconds to reach the Earth.

The Planets

• Planets are non-luminous celestial bodies that revolve around the Sun in fixed elliptical orbits.
• They do not produce their own light but reflect the sunlight falling on them.

Number of Planets

• The Solar System contains eight recognised planets.
• The planets are arranged in the following order:

Mercury → Venus → Earth → Mars → Jupiter → Saturn → Uranus → Neptune

Size (Largest to Smallest)

The planets arranged by diameter are:

Jupiter → Saturn → Uranus → Neptune → Earth → Venus → Mars → Mercury

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

Classification of Planets

Planets are broadly classified on the basis of their composition, location, and physical characteristics.

Terrestrial (Rocky) Planets

These include:

• • Mercury
  • Venus
  • Earth
  • Mars

Major Features

• Located close to the Sun.
• Possess rocky and solid surfaces.
• Composed mainly of silicate rocks and metallic cores.
• Comparatively smaller in size but denser.
• Have few or no natural satellites.
• Ring systems are absent.

Jovian (Gaseous) Planets

These include:

• • Jupiter
  • Saturn
  • Uranus
  • Neptune

Major Features

• Located in the outer Solar System.
• Predominantly composed of Hydrogen, Helium, and other volatile gases.
• Extremely large in size with lower average density.
• Possess multiple natural satellites.
• All have planetary ring systems, with Saturn’s rings being the most prominent.

Inner Planets and Outer Planets

Inner Planets

• Mercury
• Venus
• Earth
• Mars

Characteristics

• Situated inside the Asteroid Belt.
• Located closer to the Sun.
• Made predominantly of rocks and metals.
• Have solid crusts and high density.
• Complete their revolution around the Sun in a shorter time.
• Possess thin atmospheres (except Venus, which has a dense atmosphere).
• Contain few or no natural satellites.
• No ring systems are present.

Outer Planets

• Jupiter
• Saturn
• Uranus
• Neptune

Characteristics

• Located beyond the Asteroid Belt.
• Positioned farther from the Sun.
• Mainly composed of Hydrogen, Helium, and ices.
• Possess thick gaseous envelopes with comparatively low density.
• Take a longer time to complete one revolution around the Sun.
• Have numerous moons.
• All possess ring systems, though Saturn’s rings are the most spectacular.
• They are commonly referred to as Gas Giants (Jupiter and Saturn) and Ice Giants (Uranus and Neptune).

Some Notable Facts About Various Planets and Satellites

Mercury

Position

• Mercury is the closest planet to the Sun and the smallest planet in the Solar System.

Surface Conditions

• It experiences extreme temperatures because it has almost no atmosphere to regulate heat.
• The daytime temperature can exceed 430°C, while the night temperature falls below −180°C.

Atmosphere

• Mercury possesses only a very thin exosphere, containing traces of Oxygen, Sodium, Hydrogen, Helium and Potassium.
• It lacks a dense protective atmosphere and has no ozone layer, exposing its surface to intense solar radiation.

Water and Life

• Mercury has no liquid water on its surface.
• Owing to the absence of a substantial atmosphere and extreme climatic conditions, it is considered unsuitable for life.

Other Features

• It has no natural satellites.
• A Mercurian year lasts only 88 Earth days, making it the shortest planetary year.

Venus

Position

• Venus is the second planet from the Sun and the nearest planetary neighbour of Earth.

Appearance

• Venus is the brightest planet visible in the night sky.
• Depending on the time of observation, it is popularly called the Morning Star or Evening Star.

Atmosphere

• Venus is covered by dense clouds of Carbon Dioxide with droplets of Sulphuric Acid, giving it the appearance of a Veiled Planet.

Similarity with Earth

• Venus closely resembles Earth in size, mass, and density, earning it the title of Earth’s Twin.

Rotation

• Venus rotates from east to west (retrograde rotation), opposite to the direction of most planets.
• It also has the longest rotational period among the planets.

Temperature

• Venus is the hottest planet in the Solar System, with an average surface temperature of nearly 465°C, primarily due to the runaway greenhouse effect.

Water and Life

• Venus has no liquid water and an oxygen-deficient atmosphere, making the planet inhospitable for life.

The Earth

Position

• Earth is the third planet from the Sun and the largest among the inner (terrestrial) planets.

Shape and Tilt

• The Earth’s rotational axis is inclined at 23½° to the perpendicular, or 66½° with respect to its orbital plane.
• This axial tilt is responsible for the change of seasons.

Rotation

• Earth completes one rotation on its axis in 23 hours 56 minutes 4 seconds (Sidereal Day).

Revolution

• Earth revolves around the Sun in 365 days 5 hours 48 minutes 46 seconds (approximately 365¼ days).

Special Features

• Earth is known as the Blue Planet because nearly 71% of its surface is covered with water.
• It is also called the Water Planet due to the abundance of liquid water.

Life Supporting Planet

• Earth is the only known planet capable of sustaining life because of:
  • Liquid water
  • Oxygen-rich atmosphere
  • Moderate temperature
  • Protective ozone layer
  • Suitable gravitational force

The Moon

• The Moon is Earth’s only natural satellite.

Size

• Diameter: 3,474.8 km
• Circumference: Approximately 10,921 km

Orbit

• The Moon revolves around Earth in an elliptical orbit.

Distance from Earth

• Apogee (Farthest Point): Approximately 405,500 km
• Perigee (Nearest Point): Approximately 363,300 km

Rotation and Revolution

• The Moon completes both rotation and revolution in about 27.3 days, resulting in the same side always facing Earth.

Sidereal Month

• Time taken by the Moon to complete one revolution relative to distant stars:
  • 27.3 days

Synodic Month

• Time between two successive Full Moons:
  • 29.53 days

Surface Features

• Only about 59% of the Moon’s surface is visible from Earth over time due to libration.
• The bright regions are known as Highlands, while the darker regions are called Maria (Lunar Seas).

Sea of Tranquility

• Sea of Tranquility (Mare Tranquillitatis) is the site where Apollo 11 made the first human landing on the Moon in 1969.

Highest Mountain

• One of the highest mountain ranges on the Moon is Mons Huygens, rising to approximately 5.5 km above the surrounding surface.

Atmosphere

• The Moon has no significant atmosphere, resulting in:
  • No weather
  • No sound transmission
  • No twilight
  • Extreme temperature variation

Temperature

• Day: Around 127°C
• Night: Around −173°C

Speed of Light

• Light reflected from the Moon reaches Earth in approximately 1.3 seconds.

Size and Gravity

• Diameter: About ¼ of Earth’s diameter.
• Surface Gravity: Nearly 1/6th that of Earth.

Composition

• The lunar surface contains minerals such as Iron, Magnesium, Titanium, Aluminium, and Silicon.

Study of the Moon

• The scientific study of the Moon is known as Selenology.

Other Name

• The Moon is often referred to as a Fossil Planet because of its geologically inactive nature.

Mars

Position

• Mars is the fourth planet from the Sun.

Appearance

• Due to the abundance of Iron Oxide (Rust) on its surface, Mars appears reddish, giving it the name Red Planet.

Natural Satellites

Mars has two natural satellites:

• • Phobos
  • Deimos

Jupiter

Position

• Jupiter is the fifth planet from the Sun.

Size

• It is the largest planet in the Solar System.

Composition

• Jupiter is a Gas Giant, composed mainly of Hydrogen and Helium.

Temperature

• Despite its enormous size, Jupiter has a very low average cloud-top temperature of about −145°C.

Largest Satellite

• Ganymede, a moon of Jupiter, is the largest natural satellite in the Solar System.

Saturn

Position

• Saturn is the sixth planet from the Sun.

Size

• It is the second-largest planet in the Solar System.

Rings

• Saturn possesses the most spectacular ring system, composed mainly of ice particles, rock fragments, and dust.

Largest Moon

• Titan is the largest satellite of Saturn and the second-largest moon in the Solar System.

Uranus

Size

• Uranus is approximately four times larger than Earth in diameter.

Appearance

• Its greenish-blue colour is due to the presence of Methane in its atmosphere.

Discovery

• Uranus was discovered in 1781 by Sir William Herschel.
• It was the first planet discovered using a telescope.

Position

• Uranus is the seventh planet from the Sun.

Temperature

• Uranus is among the coldest planets, with cloud-top temperatures around −224°C.

Natural Satellites

• Uranus has 28 known moons (current count), including:
  • Titania
  • Oberon
  • Umbriel
  • Ariel
  • Miranda

Rotation

• Uranus rotates from east to west (retrograde rotation).
• Its axis is tilted by about 98°, causing it to appear as a Planet Lying on Its Side.

Neptune

Position

• Neptune is the eighth and farthest recognised planet from the Sun.

Temperature

• Neptune has an extremely cold atmosphere, with temperatures close to −214°C.

Similarity

• Because of its comparable size, composition, and appearance, Neptune is often regarded as the twin of Uranus.

Rings

• Neptune possesses faint ring systems composed mainly of ice particles and dust.

Additional Fact

• Neptune experiences the fastest planetary winds in the Solar System, reaching speeds of over 2,000 km/h.

Pluto

Reclassification of Pluto

Decision of the IAU

• In August 2006, the International Astronomical Union (IAU) introduced a new definition of a planet during its General Assembly held in Prague, Czech Republic.
• Based on these revised criteria, Pluto was reclassified from a planet to a Dwarf Planet.

Why was Pluto Demoted?

According to the IAU, a celestial body must satisfy three conditions to be recognised as a planet:

• It must revolve around the Sun.
• It must possess sufficient mass to attain a nearly spherical shape.
• It must have cleared the neighbourhood around its orbit by becoming the dominant gravitational body.

Pluto fulfils only the first two conditions, but it shares its orbital region with numerous Kuiper Belt objects. Hence, it does not qualify as a planet.

Impact of the Decision

• The number of recognised planets in the Solar System was reduced from nine to eight.
• The eight classical planets are:
  • Mercury
  • Venus
  • Earth
  • Mars
  • Jupiter
  • Saturn
  • Uranus
  • Neptune

Discovery

• Pluto was discovered in 1930 by Clyde Tombaugh.

Pluto’s New Status

Dwarf Planet

• Following its reclassification, Pluto was officially recognised as a Dwarf Planet.
• It is one of the major members of the Kuiper Belt, a region beyond Neptune containing numerous icy bodies.

Minor Planet Number

• Pluto has been assigned the Minor Planet Number 134340 by the Minor Planet Center (MPC).

Natural Satellites

Pluto has five known moons:

• Charon (Largest)
• Nix
• Hydra
• Kerberos
• Styx

Charon is exceptionally large relative to Pluto, and together they are often considered a binary dwarf planet system.

Facts and Figures About the Planets

Note: The number of satellites is based on the latest confirmed discoveries and may change as new moons are identified.

Asteroids (Planetoids)

• Asteroids, also called Minor Planets or Planetoids, are small rocky celestial bodies that revolve around the Sun.

Location

• Most asteroids are concentrated in the Asteroid Belt, situated between the orbits of Mars and Jupiter.

Origin

• They are believed to be leftover remnants from the formation of the Solar System that never combined to form a planet because of Jupiter’s strong gravitational influence.

Characteristics

• More than 1.3 million asteroids have been identified.
• They may be spherical, elongated, or irregular in shape.
• Most asteroids complete one rotation in a few hours to a day.
• Some asteroids possess natural satellites.

Trojan Asteroids

• Trojan Asteroids occupy stable gravitational regions (Lagrange Points L₄ and L₅) along Jupiter’s orbit, moving ahead of and behind the planet.

Meteors and Meteorites

Meteoroids

• Meteoroids are small fragments of rock or metal travelling through space, usually originating from asteroids or comets.

Meteors

• When a meteoroid enters the Earth’s atmosphere, friction with atmospheric gases causes it to heat up and glow, producing a streak of light known as a Meteor or Shooting Star.

Meteorites

• If part of the meteoroid survives atmospheric entry and reaches the Earth’s surface, it is called a Meteorite.

Origin

• Most meteoroids originate from:
  • Asteroid collisions
  • Comet debris
  • Fragments left during Solar System formation

Comets

• Comets are icy celestial bodies that revolve around the Sun in highly elliptical orbits.
• They are often called the “Dirty Snowballs” of the Solar System because they consist of ice, frozen gases, dust, and rocky material.

Orbit

• Many comets require decades, centuries, or even thousands of years to complete one revolution around the Sun.

Composition

Comets mainly contain:

• • Water Ice
  • Carbon Dioxide
  • Methane
  • Ammonia
  • Rocky particles
  • Cosmic dust

Visibility

• A comet becomes clearly visible only when it approaches the Sun, where increasing temperatures cause its frozen material to sublimate.

Structure of a Comet

Nucleus

• The solid central core composed of ice, dust, and rock.

Coma

• A cloud of gas and dust surrounding the nucleus, formed due to solar heating.

Tails

Comets generally develop two tails:

    Ion Tail

• • Made of ionised gases.
  • Always points away from the Sun due to the Solar Wind.

    Dust Tail

• • Composed of fine dust particles pushed away by solar radiation pressure.
  • Usually appears curved.

Important: A comet’s tail always points away from the Sun, irrespective of the direction in which the comet is travelling.

Stars

• Stars are self-luminous celestial bodies composed mainly of hot gases, primarily Hydrogen and Helium.
• They produce their own light and heat through the process of Nuclear Fusion occurring in their cores.

Apparent Motion

• Although stars appear fixed relative to one another in the night sky, they are actually moving continuously through space.
• Their enormous distance from the Earth makes these movements almost imperceptible over short periods.

Nearest Star

To the Earth

• • The Sun is the nearest star to the Earth.

Beyond the Solar System

• • Proxima Centauri is the nearest known star after the Sun, situated approximately 4.24 light-years away.

Examples of Important Stars

Some prominent stars include:

• Polaris (Pole Star)
• Sirius
• Vega
• Capella
• Alpha Centauri
• Beta Centauri
• Proxima Centauri
• Spica
• Regulus
• Pleiades
• Aldebaran
• Arcturus
• Betelgeuse
• The Sun

Facts About Stars

Number of Stars

• The observable universe contains an estimated 10²² to 10²⁴ stars.
• Only about 2,000–3,000 stars are visible to the unaided human eye under ideal atmospheric conditions.

Naked-Eye Visibility

• Approximately 6,000 stars can be seen from the Earth’s surface under perfect conditions.
• At any one location and at one time, only about 3,000 stars are visible because the Earth blocks the remaining half of the celestial sphere.

Why Stars are Invisible During the Day

• During daytime, the Sun’s intense brightness scatters light in the atmosphere, making distant stars invisible to the naked eye.

Constellations

• A Constellation is a recognised group of stars that appears to form a particular pattern or figure in the night sky.
• Constellations help astronomers identify the position of celestial objects.

Recognition

• The International Astronomical Union (IAU) officially recognises 88 constellations.

Importance

Constellations are used for:

• Astronomical mapping
• Celestial navigation
• Locating stars and planets
• Studying seasonal changes in the sky

Some Well-Known Constellations

Note: The 12 Zodiac Constellations lie along the Ecliptic, the apparent path of the Sun across the sky.

Galaxy

• A Galaxy is a vast gravitationally bound system consisting of billions of stars, gas, dust, dark matter, and various celestial bodies.

Number of Galaxies

• The observable universe is estimated to contain nearly 100 billion to 2 trillion galaxies, according to recent astronomical studies.

Our Galaxy

• The Milky Way Galaxy is the home galaxy of the Solar System.
• It is classified as a Barred Spiral Galaxy.

Earth’s Galaxy – The Milky Way

General Features

• The Milky Way has a barred spiral structure with several spiral arms.

Size

• Diameter: Approximately 100,000–120,000 light-years
• Thickness: Around 1,000 light-years in the stellar disk (much larger including the halo).

Why it is Called the Milky Way

• It appears as a faint milky band of light stretching across the night sky because of the combined light of billions of distant stars.

Rotation

• The Milky Way completes one rotation around its centre in approximately 225–250 million years, known as a Galactic Year (Cosmic Year).

Andromeda – Earth’s Closest Major Galactic Neighbour

• The Andromeda Galaxy (M31) is the nearest major spiral galaxy to the Milky Way.

Characteristics

• It contains nearly one trillion stars, making it larger than the Milky Way.
• It can be observed with the naked eye under exceptionally dark skies.

Future Event

• Astronomers predict that the Milky Way and Andromeda will merge approximately 4–5 billion years from now.

Local Group and Galaxy Clusters

Local Group

• The Milky Way, Andromeda, and more than 100 smaller galaxies together form the Local Group.

Virgo Cluster

• The Local Group is a part of the much larger Virgo Cluster.

Superclusters

• Galaxy clusters combine to form enormous Superclusters, which are among the largest known structures in the Universe.

Nebulae

• A Nebula is a vast interstellar cloud of gas and dust, often serving as the birthplace of stars or the remnants of dying stars.

Appearance

Nebulae may appear as:

• • Bright luminous clouds
  • Dark dust clouds
  • Colourful glowing regions

Types of Nebulae

   Emission Nebula

• • Glows because nearby hot stars ionise Hydrogen gas, causing it to emit light.

   Reflection Nebula

• • Does not emit its own light.
  • Appears bright because it reflects the light from nearby stars.

   Dark Nebula

• • Dense clouds of dust that block the light from background stars, appearing as dark patches.

   Planetary Nebula

• • Formed when a Sun-like star sheds its outer layers during the final stage of its life.

   Supernova Remnant

• • Created after the explosion of a massive star, leaving behind expanding clouds of gas and dust.

Importance of Nebulae

• They are the stellar nurseries where new stars and planetary systems originate.
• They also play a crucial role in the chemical enrichment of the Universe by distributing heavy elements produced inside stars.

The Earth: Shape and Size

Shape of the Earth

Pythagoras’ View

• Pythagoras (c. 570–495 BCE) was among the earliest Greek philosophers to propose that the Earth is spherical rather than flat.
• This idea was later supported by observations made by Aristotle and scientifically confirmed by later astronomers.

The Earth is Not Flat

Several natural observations prove that the Earth cannot be a flat surface.

Different Time of Sunrise and Sunset

• If the Earth were flat, the Sun would rise and set simultaneously at all places.
• In reality, places located in the east experience sunrise earlier than places situated in the west due to the Earth’s rotation.

Appearance of Ships

• When a ship approaches the coast, the mast or funnel becomes visible first, followed gradually by the rest of the ship.
• If the Earth were flat, the entire ship would appear at the same time.

The Earth is a Sphere

Circular Shadow During Eclipse

• During a Lunar Eclipse, the Earth always casts a circular shadow on the Moon.
• A consistently circular shadow is possible only if the Earth is nearly spherical.

Pole Star Observation

• The Pole Star (Polaris) appears almost 90° above the horizon at the North Pole.
• As one travels towards the Equator, its apparent altitude gradually decreases, becoming nearly 0° at the Equator.
• This variation confirms the Earth’s curved surface.

Horizon Observation

• Travellers observe that new landforms and stars gradually appear or disappear over the horizon while moving.
• Such observations are possible only on a curved Earth.

Celestial Bodies

• The Sun, Moon, and most other planets appear spherical because gravity pulls matter equally towards the centre.
• The Earth also possesses the same characteristic.

Evidence from Space

• Satellite images and photographs taken from space missions conclusively demonstrate that the Earth is nearly spherical.

The Earth as an Oblate Spheroid

Actual Shape

• The Earth is not a perfect sphere.
• It is slightly flattened at the poles and bulges at the Equator because of its rotation.

Scientific Term

• This shape is known as an Oblate Spheroid or Oblate Ellipsoid.

Cause of the Equatorial Bulge

• The Earth’s rotation generates centrifugal force, which is greatest at the Equator and causes a slight outward bulge.

Factors Making the Earth Suitable for Life

Suitable Atmosphere

• The Earth possesses an atmosphere rich in Nitrogen, Oxygen, Carbon Dioxide, and other gases essential for sustaining life.

Moderate Temperature

• The Earth’s distance from the Sun ensures a moderate climate, allowing liquid water to exist.

Abundant Water

• Nearly 71% of the Earth’s surface is covered by water, making it unique among the known planets.

Oxygen-rich Environment

• The atmosphere contains about 21% Oxygen, which supports respiration and the survival of living organisms.

Protective Ozone Layer

• The Ozone Layer absorbs harmful Ultraviolet (UV) radiation, protecting life from excessive solar radiation.

Biodiversity

• Earth supports an extraordinary range of plants, animals, and microorganisms.
• Tropical regions possess the highest biodiversity because of favourable climatic conditions.

Composition of the Earth’s Crust

Note: These are the modern accepted values for the composition of the Earth’s crust, replacing the outdated figures in the original notes.

Statistical Data of the Earth

Physical Dimensions of the Earth

Important Facts About the Earth

Distribution of Land and Water

Land

• Approximately 29% of the Earth’s total surface consists of continents and islands.

Water

• About 71% of the Earth’s surface is covered by oceans, seas, lakes, rivers, glaciers, and groundwater.

Hydrosphere

• The collective water present in oceans, rivers, lakes, ice sheets, groundwater, and atmospheric moisture is known as the Hydrosphere.
• The Hydrosphere occupies nearly 71% of the Earth’s surface and plays a vital role in regulating the global climate and sustaining life on Earth.