📑 Topics on this page (Jump to Topic):
📌 1. Earth: Basic Facts
| Fact | Value |
|---|---|
| Shape | Oblate spheroid / Geoid — flattened at the poles |
| Equatorial diameter | ≈ 12,756 km |
| Polar diameter | ≈ 12,714 km |
| Equatorial circumference | ≈ 40,075 km |
| Mean distance from Sun | ≈ 149.6 million km (1 AU) |
| Axial tilt | 23.5° from the perpendicular (66.5° with orbital plane) |
| Water / Land | ≈ 71% / 29% |
📌 2. Rotation & Revolution
- Rotation: west to east, ≈ 23 h 56 min 4 s (sidereal day) — causes day and night. That is why the Sun appears to rise in the east.
- Revolution: ≈ 365 days 6 hours — with the tilted axis it causes seasons. The extra quarter day gives a leap year every 4 years (29 Feb).
- Perihelion ≈ 3 January (closest to the Sun); Aphelion ≈ 4 July (farthest).
| Date | Event | Sun overhead at |
|---|---|---|
| 21 June | Summer solstice — longest day in N. Hemisphere | Tropic of Cancer (23.5° N) |
| 22 December | Winter solstice — shortest day in N. Hemisphere | Tropic of Capricorn (23.5° S) |
| 21 March / 23 September | Equinox — equal day and night | Equator |
📌 3. Latitudes & Longitudes
- Latitude: angular distance north/south of the Equator (0°); parallels are parallel circles; 181 in total. The Equator is the largest circle.
- Key parallels: Tropic of Cancer 23.5° N, Tropic of Capricorn 23.5° S, Arctic Circle 66.5° N, Antarctic Circle 66.5° S.
- Longitude: east/west of the Prime Meridian (0°, Greenwich, London); 360 in total; all meet at the poles. 1° = 4 minutes, 15° = 1 hour.
- Moving east, time is ahead (add); moving west, behind (subtract).
- International Date Line ≈ 180° (Pacific Ocean), zig-zag to avoid land; crossing it changes the date by one day.
- Indian Standard Time (IST) = 82.5° E (passes near Mirzapur) → GMT + 5:30.
💡 Memory Tip: Time sums: longitude difference × 4 minutes. E.g., 75° difference = 300 min = 5 hours.
📌 4. Interior of the Earth
| Layer | Depth / feature | Main elements |
|---|---|---|
| Crust | Continental ≈ 30–70 km, oceanic ≈ 5–10 km; thinnest layer | SIAL (silica + aluminium) / SIMA below |
| Mantle | Down to ≈ 2,900 km; ≈ 84% of volume; upper asthenosphere is the source of magma | SIMA (silica + magnesium) |
| Outer core | 2,900–5,150 km; liquid — causes Earth's magnetism | NIFE (nickel + iron) |
| Inner core | 5,150–6,371 km; solid (extreme pressure) | NIFE |
- Discontinuities: Moho (crust-mantle), Gutenberg (mantle-core), Lehmann (outer-inner core).
- Most abundant in crust: oxygen > silicon > aluminium > iron. In the whole Earth: iron.
- Seismic waves: P-waves (primary, fastest, through solids, liquids, gases), S-waves (secondary, only solids — do not pass the outer core → proof it is liquid), L/surface waves (most destructive). S-wave shadow zone beyond 105°.
⚠️ Exam Traps:
- The thinnest layer is the crust; the largest by volume is the mantle.
- At perihelion (January) Earth is closest to the Sun, yet it is winter in the north — seasons come from the tilt, not distance.
- S-waves cannot travel through liquids; P-waves travel through all media.
📌 5. Practice MCQs
📝 Practice MCQs (prepared by SF Prep — not PYQs)
Q1. In which direction does Earth rotate on its axis? Practice
Correct answer: (B)
Because Earth spins west to east, the Sun appears to rise in the east.
Because Earth spins west to east, the Sun appears to rise in the east.
Q2. What is the main cause of the change of seasons? Practice
Correct answer: (C)
Revolution with a 23.5° tilted axis changes the angle of sunlight and day length.
Revolution with a 23.5° tilted axis changes the angle of sunlight and day length.
Q3. When is the longest day in the Northern Hemisphere? Practice
Correct answer: (D)
On 21 June (summer solstice) the Sun is overhead at the Tropic of Cancer.
On 21 June (summer solstice) the Sun is overhead at the Tropic of Cancer.
Q4. On an equinox the Sun is overhead at — Practice
Correct answer: (A)
On 21 March and 23 September the Sun is overhead at the Equator and day equals night.
On 21 March and 23 September the Sun is overhead at the Equator and day equals night.
Q5. When is Earth closest to the Sun (perihelion)? Practice
Correct answer: (B)
Perihelion is about 3 January and aphelion about 4 July.
Perihelion is about 3 January and aphelion about 4 July.
Q6. A difference of 1° longitude equals a time difference of — Practice
Correct answer: (C)
360° = 24 h = 1440 min, so 1° = 4 min.
360° = 24 h = 1440 min, so 1° = 4 min.
Q7. Indian Standard Time is based on which longitude? Practice
Correct answer: (D)
Based on 82.5° E (near Mirzapur), IST = GMT + 5:30.
Based on 82.5° E (near Mirzapur), IST = GMT + 5:30.
Q8. The International Date Line roughly follows which longitude? Practice
Correct answer: (A)
The Date Line runs along ≈ 180° in the Pacific Ocean.
The Date Line runs along ≈ 180° in the Pacific Ocean.
Q9. The Prime Meridian (0° longitude) passes through — Practice
Correct answer: (B)
0° longitude passes through the Royal Observatory, Greenwich (adopted 1884).
0° longitude passes through the Royal Observatory, Greenwich (adopted 1884).
Q10. Which is the thinnest layer of the Earth? Practice
Correct answer: (C)
The crust is only 5–10 km thick under oceans.
The crust is only 5–10 km thick under oceans.
Q11. Earth's core is called "NIFE" because it is mainly made of — Practice
Correct answer: (D)
Ni (nickel) + Fe (iron) = NIFE.
Ni (nickel) + Fe (iron) = NIFE.
Q12. Which element is most abundant in Earth's crust? Practice
Correct answer: (A)
Oxygen ≈ 46% of the crust, then silicon ≈ 28%.
Oxygen ≈ 46% of the crust, then silicon ≈ 28%.
Q13. The boundary between the crust and the mantle is called — Practice
Correct answer: (B)
The Mohorovičić (Moho) discontinuity separates crust from mantle.
The Mohorovičić (Moho) discontinuity separates crust from mantle.
Q14. Which seismic waves cannot pass through liquids? Practice
Correct answer: (C)
S-waves are transverse and travel only through solids; their absence in the outer core proves it is liquid.
S-waves are transverse and travel only through solids; their absence in the outer core proves it is liquid.
Q15. The main cause of Earth's magnetic field is believed to be — Practice
Correct answer: (D)
Motion of liquid iron-nickel creates the "geodynamo".
Motion of liquid iron-nickel creates the "geodynamo".
Q16. If it is 12 noon at Greenwich, what is the time at 90° E? Practice
Correct answer: (A)
90 × 4 = 360 min = 6 h; east is ahead → 12 + 6 = 6 PM.
90 × 4 = 360 min = 6 h; east is ahead → 12 + 6 = 6 PM.
Q17. The Arctic Circle lies at which latitude? Practice
Correct answer: (B)
Arctic Circle 66.5° N, Antarctic Circle 66.5° S.
Arctic Circle 66.5° N, Antarctic Circle 66.5° S.
Q18. Earth's true shape is best described as — Practice
Correct answer: (C)
Earth is slightly flattened at the poles and bulges at the Equator — a geoid.
Earth is slightly flattened at the poles and bulges at the Equator — a geoid.
Q19. The partly molten upper mantle that is the source of volcanic magma is the — Practice
Correct answer: (D)
The asthenosphere lies ≈ 100–400 km deep; plates float on it.
The asthenosphere lies ≈ 100–400 km deep; plates float on it.
Q20. One rotation of Earth (sidereal day) takes about — Practice
Correct answer: (A)
One rotation relative to stars ≈ 23 h 56 min 4 s; the solar day is 24 h.
One rotation relative to stars ≈ 23 h 56 min 4 s; the solar day is 24 h.