Understand Earth’s shape, size, and geoid properties.
Learn Earth’s composition, surface area, and density.
Recognize different methods to measure Earth’s age.
Identify Earth’s motions and their impact on seasons.
Understand latitude, longitude, and time distribution.
Shape and Size of the Earth
The Earth is an oblate spheroid, slightly bulging at the equator and flattened at the poles, which gives it the name geoid. The study of Earth’s size and shape is known as geodesy.
Key Features:
Equatorial Bulge: Due to Earth’s rotation, a centrifugal force is generated, causing a bulge at the equator. The equatorial circumference is 83 miles larger than the polar circumference.
Gravitational Pull: Strongest at the poles and weakest at the equator, resulting in weight variation – highest at the poles, lowest at the equator.
Size Comparison: The Earth is the third closest planet to the Sun and the fifth largest in our solar system. It is the largest among the inner planets.
Surface Area: Approximate 510 million km²: 70.8% water (361 million km²) and 29.2% land (149 million km²). Water concentration is greater in the Southern Hemisphere, while landmass is more concentrated in the Northern Hemisphere.
Density: Earth’s density is about 5.52 grams/cm³, the greatest in the solar system.
Antipodal Arrangement: Land and water are arranged in such a way that they are directly opposite each other on the globe (antipodal points).
Note: Antipodal points on Earth are calculated by converting North latitude to South latitude and subtracting the longitude from 180°.
Erosion: Measures rock erosion rates. Estimation is around 1 foot every 5,000 years, providing age estimation for geological formations.
Sedimentation: Tracks the sediment deposit rate, typically between 500 and 10,000 years for one foot of sediment.
Salinity of the Oceans: Calculates age by determining the salt content in oceans and its yearly accumulation from rivers.
Radioactive Method: Uses uranium’s slow breakdown into lead. Half of uranium takes about 5 billion years to convert to lead. Provides an estimated Earth age of 4.5 billion years.
Carbon-14 Dating: Used to date organic remains up to 15,000 years old. Measures the breakdown of carbon-14 in fossils.
Important Concept: Carbon-14 dating is used for relatively recent fossils, while the radioactive method is employed for older geological formations.
Motions of the Earth
Earth exhibits various spatial motions, influencing phenomena like day, night, and seasons.
Rotation
Definition: Earth’s spinning around its axis from West to East, causing the Sun, Moon, and stars to rise in the East and set in the West.
Speed: Varies from 1,670 km/h at the equator to 1,120 km/h at 45° latitude.
Day Types:
Solar Day: 24 hours, the interval between two successive midnights.
Sidereal Day: 23 hours, 56 minutes, the interval between the passage of a star over a meridian twice.
Effects: Creates day-night cycles, deflects ocean currents and winds (Coriolis force), and causes alternate ocean wave rise and fall.
Revolution
Definition: Earth’s elliptical orbit around the Sun at about 29.6 km/sec.
Seasons: Earth’s tilt and revolution result in seasonal changes.
Distance Variation: The closest point to the Sun (perihelion) is around 147 million km, occurring around January 3rd. The farthest point (aphelion) is around 152 million km, occurring around July 4th.
Axial Precession
Definition: Earth’s rotational axis exhibits a slow, conical motion around the orbital axis due to gravitational forces.
Implications: Influences the Earth’s climate over long periods.
Note: Earth’s tilt changes periodically from 24.5° to 21.5° over approximately 40,000 years, affecting climate.
Latitude and Longitude
Earth’s coordinate system for location and time calculation.
Latitudes
Definition: Angular distance of a place north or south of the equator, measured in degrees.
Important Lines:
Equator (0°): Divides Earth into Northern and Southern hemispheres.
Tropic of Cancer (23½° N), Tropic of Capricorn (23½° S): Define the tropics.
Arctic Circle (66½° N), Antarctic Circle (66½° S): Define polar regions.
Total Latitudes: 181, drawn parallel to the equator.
Distance: Each degree of latitude is approximately 111 km.
Longitudes
Definition: Semi-circular lines joining the poles and measured angular distance east or west of the Prime Meridian (0°).
Prime Meridian: Passes through Greenwich, London, chosen as the starting point for measuring longitude.
Total Longitudes: 360, extending east and west from the Prime Meridian.
International Date Line: Located at 180° longitude; crossing changes the date by one day.
Important Note: Earth’s rotation implies that moving 15° eastward advances local time by 1 hour, while moving 15° westward retards it by 1 hour.
Great and Small Circles
Great Circles: Divide the Earth into two equal halves (e.g., equator).
Small Circles: Do not pass through Earth’s center, dividing it into unequal parts.
Latitude and Longitude Facts Table:
Line
Countries/Regions Passed Through
Equator
Ecuador, Brazil, Congo, Indonesia, Kenya
Tropic of Cancer
Mexico, India, Egypt, China
Tropic of Capricorn
Australia, Brazil, South Africa
Prime Meridian
UK, Spain, Algeria, Ghana
Distribution of Date and Time
To standardize global time:
Time Zones: Earth is divided into 24 time zones, each differing by 15° in longitude or 1 hour.
Greenwich Mean Time (GMT): Reference point for international time reckoning.
Indian Standard Time (IST): Based on 82½° E longitude, 5 hours 30 minutes ahead of GMT.
Seasons
Earth’s tilt and revolution around the Sun result in distinct seasonal changes.
Summer Solstice (21st June): Sun shines directly on the Tropic of Cancer, marking summer in the Northern Hemisphere.
Winter Solstice (22nd December): Sun shines directly on the Tropic of Capricorn, indicating winter in the Northern Hemisphere.
Spring Equinox (21st March): Sun crosses the equator, leading to equal day and night durations.
Autumn Equinox (23rd September): Similar equatorial alignment, resulting in balanced day and night.
Concept: If Earth had no axial tilt, every region would experience uniform day and night throughout the year.
Earth’s Motions and Their Effects
Temperature Variations: Sun oscillates within the tropics, leading to concentrated sunlight and higher temperatures in those regions.
Heat Gain and Loss: Tropical regions are net gainers of heat, while areas beyond these latitudes lose heat.
Permanent Darkness and Light: Areas within the Arctic and Antarctic Circles experience 24-hour darkness or daylight depending on the solstices.
The Earth and the Moon
Comparison:
Size: Moon’s diameter is about 27% of Earth’s, surface area about 7.4%, and mass about 1.2%.
Orbit: The Moon revolves around Earth in an **elliptical path
**.
Phases: Changing angles between Earth, Sun, and Moon result in phases from New Moon to Full Moon.
Eclipse:
Solar Eclipse: Moon obstructs the Sun.
Lunar Eclipse: Earth obstructs sunlight reaching the Moon.
Earth’s Magnetic Field
Earth behaves like a bar magnet with a magnetic field axis tilted relative to its rotational axis. The magnetic field, also known as the magnetosphere, shields Earth from solar wind.
Note: Magnetic poles do not align precisely with geographic poles.
Magnetic Field Table:
Property
Description
Axis Tilt
About 11.5° inclination
Magnetic Poles
Near geographical poles, spread over an area
Magnetic Field Strength
Strongest near poles, weakest near equator
Van Allen Belts
High concentration of charged particles, affected by solar wind
Important Concept: The dynamo effect in Earth’s core generates the magnetic field, which reverses every few million years.
Multiple Choice Question: Which of the following motions of the Earth primarily contributes to the change of seasons?
Rotation
Revolution
Axial Precession
Changes in Earth’s orbit Correct Answer: 2. Revolution