These Earth notes for OPSC OAS cover the origin of the planet, its shape and size, rotation and revolution, the Coriolis effect, seasons and equinoxes, latitude, longitude and time, and the interior structure with seismic discontinuities. Each section is written in the fact pattern in which OPSC and UPSC actually ask the topic, and the page closes with previous year practice questions.
Origin of the Earth and Early Theories
The Earth accreted about 4.54 billion years ago from the solar nebula, a rotating cloud of gas and dust. The older hypotheses are no longer accepted as complete explanations, but OPSC and UPSC ask them as matching and assertion questions, so their propounders must be memorised.
| Theory | Propounder | Core idea |
|---|---|---|
| Nebular Hypothesis | Immanuel Kant (1755), revised by Laplace (1796) and Roche (mid-19th century) | Solar System formed from a rotating cloud of gas and dust |
| Planetesimal Hypothesis | T.C. Chamberlin | Small solid bodies called planetesimals accreted to form planets |
| Tidal Hypothesis | James Jeans (1919), with Jeffreys | A passing star raised a tidal filament of matter from the Sun |
| Gas–Dust Accretion | Otto Schmidt (1940s) | Earth formed from a cloud of gas and dust particles |
IAS (Pre) 1999 asked who suggested the Earth's origin from gases and dust particles — Otto Schmidt.
About 4.5 billion years ago Earth's interior crossed the melting point of iron (1,538°C). Molten iron and nickel sank to the centre while lighter silicates rose, a process called planetary differentiation that produced the core, mantle and crust.
Shape and Size of the Earth
The Earth is an oblate spheroid, flattened at the poles and bulging at the equator because of rotation; more precisely it is a geoid, the equipotential surface of mean sea level. The equatorial diameter is about 12,756 km and the polar diameter about 12,714 km, a difference of roughly 43 km, giving an equatorial bulge of about 21 km. The equatorial circumference is about 40,075 km, and the mean density of the Earth is about 5.5 g/cm³.
- Weight increases from the equator to the poles, because the poles are closer to the Earth's centre and centrifugal force is zero there. The weight-decreases-with-latitude assertion is false.
- Earth's mean diameter of about 12,742 km is often confused with the equatorial circumference of about 40,000 km.
- Earth ranks fifth in size among the planets, after Jupiter, Saturn, Uranus and Neptune.
Rotation, Revolution and the Coriolis Effect
| Motion | Period | Direction | Main effects |
|---|---|---|---|
| Rotation | 23 h 56 min 4.9 s (sidereal day); 24 h solar day | West to east | Day and night, 15° of rotation per hour, Coriolis deflection, tidal rhythm |
| Revolution | 365.25 days | West to east | Year, seasons with axial tilt, changing day length |
Earth's orbital speed averages about 29.8 km/s, rounded to 30 km/s. The orbit is elliptical: Earth is nearest the Sun (perihelion) around 3 January and farthest (aphelion) around 4 July. Johannes Kepler established that planetary orbits are ellipses, not circles.
Coriolis effect for OPSC OAS
The Coriolis force arises from Earth's rotation. It is zero at the Equator and maximum at the poles, and it deflects moving objects to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. It shapes trade winds, the monsoon circulation, cyclones and ocean gyres.
NDA 2017 asked what the Coriolis effect results from — Earth's rotation.
Rotation gives day and night, the 15° per hour time rule and the Coriolis effect. Revolution together with the 23.5° axial tilt gives seasons. Do not swap these causes.
IMAGE (Diagram): Earth's orbit around the Sun at four positions, with the axis tilt drawn at each.
Labels: 21 March, 21 June, 23 September, 22 December; axis tilted 23.5°; perihelion 3 January; aphelion 4 July; Sun at one focus.
Caption: Revolution on a tilted axis produces solstices, equinoxes and seasons.
Find it: NCERT Class 6 Geography chapter on Motions of the Earth, or Wikimedia Commons ‘Earth seasons diagram’.
Seasons, Solstices and Equinoxes
Seasons are caused by the revolution of the Earth on an inclined axis (23.5° to the perpendicular of the orbital plane, or 66.5° to the plane itself), not by changing distance from the Sun. The Northern Hemisphere has summer when Earth is near aphelion, which proves the point.
| Date | Event | Sun overhead at | Northern Hemisphere |
|---|---|---|---|
| 21 March | Vernal equinox | Equator | Spring; day and night equal |
| 21 June | Summer solstice | Tropic of Cancer (23.5°N) | Longest day; summer |
| 23 September | Autumnal equinox | Equator | Autumn; day and night equal |
| 22 December | Winter solstice | Tropic of Capricorn (23.5°S) | Shortest day; winter |
The difference between the longest and shortest day is least at the Equator and increases towards the poles, where it reaches six months of continuous day or night. At the December solstice the North Pole is in darkness for 24 hours while the South Pole has continuous daylight.
On 21 June the noon Sun is overhead at the Tropic of Cancer, so noon shadows in Delhi are shortest. On 22 December the overhead Sun is at the Tropic of Capricorn, so the Northern Hemisphere has its shortest day.
Latitude, Longitude and Time Calculation
Latitude is the angular distance of a point north or south of the equator, measured from the Earth's centre. One degree of latitude is about 111 km everywhere. The Equator (0°) is the longest parallel and a great circle. The Tropics lie at 23.5°N and 23.5°S and the polar circles at 66.5°N and 66.5°S.
Longitude is the angular distance east or west of the Prime Meridian (0°), which passes through Greenwich. Meridians converge at the poles and are farthest apart at the Equator; they are not parallel to one another. The International Date Line broadly follows 180° longitude.
Earth rotates 360° in 24 hours, so it turns 15° per hour and 1° in 4 minutes. Moving east, time is ahead; moving west, time is behind. India's standard meridian is 82°30′E, passing near Mirzapur in Uttar Pradesh, giving IST = GMT + 5:30. India's longitudinal extent from 68°7′E to 97°25′E produces a difference of about two hours between its eastern and western tips.
Indira Point, India's southernmost point, lies at about 6°45′N. The intersection of 0° latitude and 0° longitude lies in the Atlantic Ocean, not at Greenwich, which is at 51.5°N. The Equator passes through Colombia, Brazil, Gabon, Kenya and Indonesia, but not through Mexico, Somalia or Rwanda.
- Places on the same meridian share the same local time even if they are hundreds of kilometres apart along it.
- The Equator, Tropic of Cancer and Tropic of Capricorn all cross Africa only.
- All three lines pass through Africa, but Asia is crossed only by the Tropic of Cancer.
Interior of the Earth Notes for OPSC OAS: Layers and Discontinuities
Sources of information about the interior
Direct sources include deep mines such as the South African gold mines 3–4 km deep, deep-sea drilling projects, and magma thrown out by volcanoes. Indirect sources include seismic waves, gravity anomalies, magnetic surveys, temperature, pressure and density estimates, and meteorites, whose composition resembles the Earth's interior.
Seismic waves and the shadow zone
P-waves are longitudinal, fastest, and travel through solids, liquids and gases. S-waves are transverse, slower, and travel only through solids. The disappearance of S-waves beyond about 103° from the epicentre, and the P-wave shadow zone between about 103° and 142°, proved that the outer core is liquid. The velocity of seismic waves also changes sharply at the boundaries between layers, which are called discontinuities.
Crust, mantle and core
| Layer | Depth / extent | Composition | Key point |
|---|---|---|---|
| Crust | 5–70 km | Oceanic: basaltic (Sima), about 2.9 g/cm³; continental: granitic (Sial), about 2.7 g/cm³ | Thinnest layer; about 1% of Earth's mass |
| Mantle | Up to about 2,900 km | Silicates rich in olivine, pyroxene and garnet; about 3.9 g/cm³ | 84% of Earth's volume and 67% of its mass |
| Outer core | 2,900–5,150 km | Liquid iron and nickel | Geodynamo generates the magnetic field; S-waves cannot pass |
| Inner core | 5,150–6,371 km | Solid iron and nickel | Solid despite extreme heat because of pressure; densest layer |
Crustal abundance by weight runs oxygen (46.6%) > silicon (27.7%) > aluminium (8.1%) > iron (5.0%). Aluminium is the most abundant metal, and iron is the second. The core is called Nife because it is nickel plus iron; the core contains about 90% of Earth's sulphur.
Discontinuities inside the Earth
| Discontinuity | Boundary |
|---|---|
| Conrad | Upper and lower crust, that is oceanic and continental crust |
| Mohorovicic (Moho) | Crust and mantle |
| Repetti | Upper and lower mantle |
| Gutenberg | Mantle and outer core |
| Lehmann (Bullen) | Outer core and inner core |
Lithosphere and asthenosphere
The lithosphere is the rigid outer shell of crust plus uppermost solid mantle, about 100 km thick and up to 300 km under mountain belts. Below it lies the asthenosphere, a weak, semi-molten, ductile zone between roughly 100 km and 350–650 km depth, composed of peridotite. It is called the low-velocity zone because seismic waves slow down in it, and it allows the lithospheric plates to move.
NDA 2019 asked what the lithosphere consists of — crust and the uppermost solid mantle.
IMAGE (Diagram): Cut-away cross-section of the Earth from surface to centre, with layers and discontinuities labelled.
Labels: crust, Moho, upper mantle, Repetti discontinuity, lower mantle, Gutenberg discontinuity, outer core, Lehmann discontinuity, inner core; depths 5–70 km, 2,900 km, 5,150 km, 6,371 km.
Caption: Chemical layers and seismic discontinuities of the Earth's interior.
Find it: NCERT Class 11 Fundamentals of Physical Geography Fig. 3.3, or Wikimedia Commons ‘Earth cutaway’.
IMAGE (Map): World outline map marking the Equator, Tropics, polar circles, Prime Meridian and International Date Line.
Labels: 0° Equator, 23.5°N Tropic of Cancer, 23.5°S Tropic of Capricorn, 66.5°N and 66.5°S circles, 0° Prime Meridian, 180° International Date Line, 82°30′E standard meridian of India.
Caption: The grid of parallels and meridians used for location and time.
Find it: NCERT Class 6 Geography globe map, or Wikimedia Commons ‘World map with tropics’.
Odisha Angle: Pathani Samanta and Śatānanda
Odisha has a continuous astronomical tradition. Ācārya Śatānanda composed the Bhāsvatī in 1099 CE, introducing centesimal (base-100) calculations that simplified astronomical computation. Pathani Samanta (Samanta Chandra Sekhar, 1835–1904), born at Khandapada, was the last great naked-eye astronomer of the classical Indian tradition. He completed the Siddhanta Darpana in 1869 and designed instruments such as the Svayambhu-yantra and Chapa-yantra, correcting the lunar calendar through sustained observation. The Pathani Samanta Planetarium in Bhubaneswar is named after him, and his birth anniversary is observed on 13 December.
Odisha-specific questions usually test the association: Pathani Samanta with Siddhanta Darpana (1869) and naked-eye astronomy, Śatānanda with Bhāsvatī (1099 CE) and centesimal calculation. Neither is linked to the telescope or to the Big Bang.
Practice Questions
Assertion (A): Moving from one place on Earth to another place of higher latitude, the weight of an object decreases.
Reason (R): Earth is not a perfect sphere. Which one of the following is correct?
- Both A and R are true and R is the correct explanation of A
- Both A and R are true, but R is not the correct explanation of A
- A is true, but R is false
- A is false, but R is true
Show answer
(d) – Weight actually increases towards the poles because the poles are closer to the centre and the centrifugal effect is zero there; the reason is true but the assertion is false. IAS (Pre) 2003
The Coriolis effect is the result of:
- Pressure gradient
- Earth's axis of inclination
- Earth's rotation
- Earth's revolution
Show answer
(c) – The Coriolis force arises from Earth's rotation; it is zero at the Equator and maximum at the poles. NDA 2017
Which of the following statements regarding the duration of day and night is correct?
- Difference is least near the Equator and progressively increases away from it
- Difference is maximum at the Equator and progressively decreases away from it
- Difference is least at the Tropics and progressively increases towards the Equator and Poles
- Difference is maximum at the Tropics and progressively decreases towards the Equator and Poles
Show answer
(a) – At the Equator day and night are nearly equal all year; the gap grows towards the poles. CDS 2014
Identify the correct pairs for the Northern Hemisphere: I. Winter Solstice — 22 December; II. Autumn Equinox — 23 September; III. Summer Solstice — 21 June; IV. Spring Equinox — 21 March.
- I and II
- II and III
- III and IV
- All are correctly matched
Show answer
(d) – All four dates are correct for the Northern Hemisphere. WBCS 2020
In the internal structure of planet Earth, below the mantle, the core is mainly made up of which one of the following?
- Aluminium
- Chromium
- Iron
- Silicon
Show answer
(c) – The core is predominantly iron with some nickel (Nife); aluminium and silicon are crustal elements. IAS (Pre) 2009
The waves that help scientists understand the internal structure of the Earth are:
- Primary waves
- Secondary waves
- Surface waves
- Longitudinal waves
Show answer
(b) – S-waves cannot pass through liquids, and their shadow zone revealed the liquid outer core. CDS 2012
Among the following, the most abundant metal in the Earth's crust is:
- Calcium
- Iron
- Magnesium
- Aluminium
Show answer
(d) – Aluminium is the most abundant metal at about 8.1%, followed by iron at about 5%. SSC 2017; JPSC 2011
The Equator does not pass through which of the following countries? I. Gabon; II. Somalia; III. Equatorial Guinea; IV. Rwanda.
- I and IV
- II and IV
- II and III
- III and IV
Show answer
(b) – Gabon and Equatorial Guinea lie on the Equator; Somalia and Rwanda do not. UPPSC 2020
Assertion (A): The Equator is a great circle.
Reason (R): The Equator encircles the whole Earth and its plane divides the Earth into two equal halves. Which one of the following is correct?
- Both A and R are true and R is the correct explanation of A
- Both A and R are true, but R is not the correct explanation of A
- A is true, but R is false
- A is false, but R is true
Show answer
(a) – A great circle passes through the Earth's centre and divides it equally; the reason correctly explains the assertion. NDA 2009
Pathani Samanta, the 19th-century Odia astronomer, is best known for:
- Discovering the moons of Jupiter
- Writing the astronomical treatise Siddhanta Darpana
- Proposing the Big Bang theory
- Discovering the rings of Saturn
Show answer
(b) – Samanta Chandra Sekhar (1835–1904) completed the Siddhanta Darpana in 1869 and designed instruments such as the Svayambhu-yantra and Chapa-yantra.