1.5 - Geography Notes 07

Elaborate Notes

Comet

A comet is a small celestial body, often referred to as a “dirty snowball,” composed of a solid nucleus of rock, dust, and frozen gases such as water, carbon dioxide, ammonia, and methane. This model was first proposed by American astronomer Fred Whipple in 1950. Comets are relics from the outer regions of the solar nebula, remnants from the formation of the Solar System approximately 4.6 billion years ago.

• Origin and Orbits: Comets originate from two primary regions in the outer Solar System:

  • The Kuiper Belt: A disc-shaped region beyond the orbit of Neptune, extending from about 30 to 50 Astronomical Units (AU) from the Sun. It is home to short-period comets, which have orbital periods of less than 200 years. Halley’s Comet is a famous example, though its origin is now thought to be the Oort Cloud.
  • The Oort Cloud: A theoretical spherical cloud of icy bodies surrounding the Sun at a vast distance, perhaps up to 50,000 AU. It was hypothesised by Dutch astronomer Jan Oort in 1950. It is the source of long-period comets, whose orbits can take thousands or even millions of years to complete. Comet Hale-Bopp (1997) is a long-period comet from the Oort Cloud.
  • Comets travel in highly elliptical (elongated) orbits around the Sun, a fact established by Edmund Halley who, in 1705, used Newton’s laws of motion and gravity to correctly predict the return of the comet that now bears his name.

• Structure and Tail Formation: As a comet approaches the Sun, solar radiation causes its icy nucleus to heat up and sublimate (turn directly from solid to gas).

  • Nucleus: The solid, core part of the comet, typically a few kilometres in diameter.
  • Coma: The sublimated gas and dust form a vast, tenuous atmosphere around the nucleus, called the coma, which can extend for hundreds of thousands of kilometres.
  • Tail: The pressure from solar radiation and the solar wind (a stream of charged particles from the Sun) pushes the gas and dust of the coma away from the Sun, forming two distinct tails:
    • Ion Tail (Gas Tail): Composed of ionised gas, it is pushed directly away from the Sun by the solar wind. It often appears blue due to the emission from carbon monoxide ions.
    • Dust Tail: Composed of small solid particles, it is pushed by solar radiation pressure and tends to be broader and more curved. It reflects sunlight and appears whitish-yellow.
  • The tail is always pointed away from the Sun, regardless of the comet’s direction of travel. It is longest and brightest when the comet is at its perihelion (closest point to the Sun).

• Notable Examples:

  • Halley’s Comet: A short-period comet visible from Earth every 75–76 years. Its appearances have been recorded by astronomers since at least 240 BCE. Its last appearance was in 1986.
  • Comet Hale-Bopp: Discovered in 1995, it was one of the brightest comets of the 20th century, visible to the naked eye for a record 18 months in 1996-97.
  • Comet NEOWISE (C/2020 F3): A long-period comet that became spectacularly visible in July 2020, providing a significant astronomical event during that year.
  • Comet Leonard (C/2021 A1): Discovered in January 2021, it was the brightest comet of that year but disintegrated after passing its perihelion in early 2022.

Eclipse

An eclipse is an astronomical event that occurs when one celestial body is temporarily obscured by either passing into the shadow of another body or having another body pass between it and the viewer.

• Shadow Structure: The shadow cast by a celestial body has two distinct parts:

  • Umbra: The innermost and darkest part of the shadow, where the light source is completely blocked. An observer in the umbra experiences a total eclipse.
  • Penumbra: The outer region of the shadow where the light source is only partially obscured. An observer in the penumbra experiences a partial eclipse.

• Solar Eclipse: This occurs when the Moon passes between the Sun and Earth, and the Moon fully or partially blocks the Sun. This casts a shadow onto Earth. The alignment is Sun-Moon-Earth (SME). A solar eclipse can only take place during the New Moon phase. They do not occur every new moon because the Moon’s orbit is tilted at about 5 degrees to the plane of Earth’s orbit (the ecliptic).

  • Types of Solar Eclipse:
    • Total Solar Eclipse: The Sun is completely obscured by the Moon. This is only visible from the small area on Earth’s surface where the Moon’s umbra falls. During totality, the Sun’s faint outer atmosphere, the corona, and its middle layer, the chromosphere, become visible. A famous historical example is the eclipse of May 29, 1919, during which observations by Sir Arthur Eddington provided the first empirical evidence for Einstein’s theory of general relativity.
    • Partial Solar Eclipse: The Sun is only partially obscured by the Moon. This occurs when the observer is within the penumbra of the Moon’s shadow.
    • Annular Solar Eclipse: This occurs when the Moon is at or near its farthest point from Earth (its apogee). At this distance, the Moon’s apparent size is smaller than the Sun’s. Therefore, it cannot completely block the Sun’s disk, leaving a bright ring, or annulus, of the Sun’s photosphere visible. This is often referred to as the “ring of fire.”

• Lunar Eclipse: This occurs when the Moon passes directly behind Earth and into its shadow. This can occur only when the Sun, Earth, and Moon are exactly or very closely aligned, with Earth between the other two. The alignment is Sun-Earth-Moon (SEM). A lunar eclipse can only occur on the night of a Full Moon.

  • Types of Lunar Eclipse:
    • Total Lunar Eclipse: The entire Moon passes through Earth’s umbra. During this phase, the Moon does not become completely dark. Instead, it often takes on a reddish hue, known as a “Blood Moon.” This colour is caused by sunlight passing through Earth’s atmosphere, which filters out most of the blue light (a phenomenon known as Rayleigh scattering) and refracts the remaining reddish light onto the Moon’s surface.
    • Partial Lunar Eclipse: Only a part of the Moon passes through Earth’s umbra, while the rest remains in the penumbra.
    • Penumbral Lunar Eclipse: The Moon passes only through Earth’s penumbra. These eclipses are subtle events, causing only a slight dimming of the Moon’s surface, which can be difficult to observe with the naked eye.

• Various Types of Moon (Nomenclature):

  • Blood Moon: The term used to describe the reddish appearance of the Moon during a total lunar eclipse.
  • Supermoon: A colloquial term for a Full Moon that occurs when the Moon is at or near its closest approach to Earth (its perigee). It appears slightly larger and brighter than an average full moon.
  • Blue Moon: This term has two common definitions: 1) The second full moon that occurs within a single calendar month. 2) The traditional definition refers to the third of four full moons in a single astronomical season.

Difference Between Solar and Lunar Eclipses

Feature	Solar Eclipse	Lunar Eclipse
Alignment	Sun-Moon-Earth (SME)	Sun-Earth-Moon (SEM)
Moon Phase	New Moon	Full Moon
Time of Day	Daytime	Nighttime
Duration	Short (totality lasts a few minutes at most)	Long (totality can last over an hour)
Viewing Area	Visible from a very small, narrow path on Earth	Visible from the entire night side of Earth
Safety	Unsafe to view directly without specialised eye protection	Safe to view directly with the naked eye

Oceans

The global ocean is the interconnected body of saltwater that covers approximately 71% of Earth’s surface. Though it is one continuous body of water, for geographical and scientific purposes, it is divided into five principal oceans. This division is recognised by the International Hydrographic Organization (IHO).

• Pacific Ocean:

  • It is the largest and deepest ocean, covering about one-third of the planet’s surface. It is larger than all of Earth’s land area combined.
  • Its name was coined by the Portuguese explorer Ferdinand Magellan in 1520, who called it ‘Mar Pacífico’ (peaceful sea).
  • It contains the deepest known point on Earth, the Challenger Deep in the Mariana Trench, with a depth of approximately 10,984 meters. The trench’s depth was first measured by the HMS Challenger expedition (1872-1876).
  • It is connected to the Arctic Ocean by the Bering Strait, to the Atlantic by the Drake Passage (south of South America) and the Panama Canal, and to the Indian Ocean by the Strait of Malacca.

• Atlantic Ocean:

  • The second-largest ocean, it is characteristically “S”-shaped.
  • It is the youngest of the major oceans, having formed during the breakup of the supercontinent Pangaea.
  • It is dominated by the Mid-Atlantic Ridge, a massive submarine mountain range that runs down its centre, which is a site of active seafloor spreading. Iceland is a volcanic island that is a part of this ridge.

• Indian Ocean:

  • The third-largest ocean and the only one named after a country.
  • It is bounded by Asia to the north, Africa to the west, and Australia to the east. Its semi-enclosed nature in the north strongly influences regional climate systems, most notably the Indian Monsoon.
  • It has no direct surface connection to the Arctic Ocean.

• Southern Ocean:

  • Also known as the Antarctic Ocean, it is the ‘newest’ ocean, officially recognised by the IHO in 2000 and reaffirmed by National Geographic in 2021.
  • It comprises the waters south of 60 degrees South latitude that encircle Antarctica.
  • It is defined by the powerful Antarctic Circumpolar Current, which flows eastward and is the only current to circumnavigate the globe unimpeded, thermally isolating Antarctica.

• Arctic Ocean:

  • The smallest, shallowest, and coldest of the five oceans.
  • It is located in the Northern Hemisphere, mostly within the Arctic Circle.
  • Much of the ocean is covered by sea ice for a significant part of the year, though this is rapidly diminishing due to global warming, opening up potential new shipping routes like the Northwest Passage.

Continents

Continents are the world’s major continuous expanses of land. The arrangement and features of these landmasses are explained by the theory of plate tectonics, which evolved from Alfred Wegener’s theory of continental drift (proposed in 1912). In order of decreasing size, the seven continents are Asia, Africa, North America, South America, Antarctica, Europe, and Australia (often referred to as Oceania to include its surrounding islands).

• Asia:
  • The largest and most populous continent.
  • Its conventional boundary with Europe is not a distinct geological feature but a historical and cultural construct. It is generally demarcated by the Ural Mountains, the Ural River, the Caspian Sea, the Caucasus Mountains, and the Black Sea.
  • Asia is joined to Africa by the Isthmus of Suez, on which the Sinai Peninsula (part of Egypt) lies.
  • It contains the highest point on Earth, Mount Everest (8,848.86 meters), located in the Himalayas on the border of Nepal and China.
  • It also contains the lowest point on land, the shore of the Dead Sea (approximately -430.5 meters below sea level), located between Israel and Jordan.

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Prelims Pointers

• A comet’s nucleus is composed of frozen gases, rock, and dust.
• Comets originate from the Kuiper Belt (short-period comets) and the Oort Cloud (long-period comets).
• A comet’s tail always points away from the Sun due to solar wind and radiation pressure.
• Halley’s Comet has an orbital period of approximately 76 years.
• An eclipse is the obscuring of one celestial body by another.
• Umbra is the full, dark shadow causing a total eclipse.
• Penumbra is the partial shadow causing a partial eclipse.
• Solar Eclipse Alignment: Sun-Moon-Earth (SME).
• Solar eclipses occur during a New Moon.
• Annular Solar Eclipse (“ring of fire”) occurs when the Moon is at its apogee (farthest from Earth).
• The Sun’s corona is visible during a total solar eclipse.
• Lunar Eclipse Alignment: Sun-Earth-Moon (SEM).
• Lunar eclipses occur during a Full Moon.
• Blood Moon: A total lunar eclipse where the moon appears red due to Rayleigh scattering of light through Earth’s atmosphere.
• Supermoon: A full moon at or near its perigee (closest to Earth).
• Blue Moon: The second full moon in a calendar month.
• Water covers approximately 71% of the Earth’s surface.
• There are five oceans: Pacific, Atlantic, Indian, Southern, and Arctic.
• The Pacific Ocean is the largest and deepest.
• The deepest point on Earth is the Challenger Deep in the Mariana Trench.
• The Atlantic Ocean has an “S” shape and features the Mid-Atlantic Ridge.
• The Indian Ocean is the only ocean named after a country.
• The Southern Ocean is defined as the waters south of 60 degrees South latitude.
• The Arctic Ocean is the smallest and shallowest.
• Bering Strait: Connects the Arctic and Pacific Oceans.
• Drake Passage: Connects the Atlantic and Pacific Oceans.
• Strait of Malacca: Connects the Indian and Pacific Oceans.
• Continents in order of size: Asia > Africa > North America > South America > Antarctica > Europe > Australia/Oceania.
• Ural Mountains separate Europe from Asia.
• Sinai Peninsula connects Asia and Africa.
• Highest point on land: Mount Everest.
• Lowest point on land: Shore of the Dead Sea.

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Mains Insights

Comets and Eclipses: Scientific and Cultural Significance (GS-I, GS-III)

• Scientific Advancement:
  • Comets: They are pristine samples from the early Solar System. Studying their composition (e.g., through missions like ESA’s Rosetta to Comet 67P/Churyumov–Gerasimenko) provides invaluable data on the conditions under which planets formed. They support the panspermia hypothesis, suggesting they may have delivered water and organic compounds essential for life to early Earth.
  • Eclipses: Historically, eclipses have been crucial scientific tools. The 1919 solar eclipse validated Einstein’s theory of general relativity by showing the bending of starlight by the Sun’s gravity. The element Helium was first discovered by observing the Sun’s chromosphere during an 1868 eclipse. They remain important for studying the Sun’s corona.
• Cultural and Historical Impact:
  • Eclipses and comets were often interpreted as omens or divine messages in ancient civilizations, influencing historical events and cultural beliefs. Their predictability, established by astronomers like Halley, marked a significant shift from superstition to scientific reasoning, embodying the spirit of the Enlightenment.
• Planetary Defense:
  • Comets and asteroids are classified as Near-Earth Objects (NEOs). The impact of Comet Shoemaker-Levy 9 on Jupiter in 1994 served as a stark reminder of the potential threat. This has spurred international efforts in planetary defense, including tracking NEOs and developing deflection technologies (e.g., NASA’s DART mission).

Geopolitics and Environmental Concerns of Oceans (GS-I, GS-II, GS-III)

• Strategic Importance:
  • Oceans are the highways of global trade. Over 80% of world trade by volume is carried by sea. Control over strategic Sea Lanes of Communication (SLOCs) and choke points like the Strait of Malacca, Strait of Hormuz, and Suez Canal is of immense geopolitical importance, leading to naval competition and strategic alliances (e.g., the Quad in the Indo-Pacific).
  • The UN Convention on the Law of the Sea (UNCLOS) provides the legal framework for marine activities, but disputes over Exclusive Economic Zones (EEZs) and territorial waters persist, as seen in the South China Sea.
• Climate Change Nexus:
  • Arctic Ocean: The melting of Arctic sea ice due to global warming is a double-edged sword. It presents economic opportunities (new shipping routes, access to fossil fuels and minerals) but also poses grave environmental risks, threatens indigenous livelihoods, and creates a new arena for geopolitical competition among Arctic nations.
  • Southern Ocean: The Antarctic Circumpolar Current is a critical driver of global ocean circulation (thermohaline circulation). Changes in this system due to warming waters could have far-reaching impacts on global climate patterns.
  • Ocean as a Carbon Sink: Oceans absorb about a quarter of anthropogenic CO2 emissions, which mitigates global warming but leads to ocean acidification, threatening marine ecosystems, particularly coral reefs and shellfish.

Continents: Geographical Realities and Human Constructs (GS-I)

• Physical vs. Cultural Boundaries:
  • The division of the great landmass of Eurasia into Europe and Asia is a prime example of a boundary based more on history, culture, and politics than on clear geographical separation. The Ural and Caucasus mountains are conventional markers, not impassable barriers. This illustrates how human geography often overlays and reinterprets physical geography.
• Plate Tectonics and Hazard Management:
  • The theory of plate tectonics explains not only the shape and location of continents but also the distribution of major physical features like mountain ranges (e.g., the Himalayas formed by the collision of the Indian and Eurasian plates) and the location of seismic and volcanic activity (e.g., the Pacific “Ring of Fire”).
  • This understanding is fundamental for disaster management in GS-III, as it allows for the mapping of high-risk zones for earthquakes, tsunamis, and volcanic eruptions, informing building codes, early warning systems, and emergency preparedness.