Locating places on the earth

Overview & Fundamental Concept

Pointwise AISSEE Class 9 Social Science guide for Locating places on the earth (Topic 5). Explains parallels of latitude, Earth heat zones, Prime Meridian, time conversion mathematics (1° = 4 min), solved models, and 10 practice drills.

• The Globe: A true miniature spherical model of the Earth. The imaginary line around which the Earth rotates from west to east is its Axis, passing through the North Pole and South Pole.
• Parallels of Latitude:
  • All imaginary parallel circles running from the Equator to the poles.
  • The Equator (0°): Divides the Earth into the Northern Hemisphere and Southern Hemisphere.
  • Latitudes are measured in degrees. The distance from the Equator to either of the poles is one-fourth of a circle around the Earth: $\frac{1}{4} \times 360^circ = 90^circ$. Hence, $90^circ\text{ N}$ is the North Pole and $90^circ\text{ S}$ is the South Pole.
• 4 Important Parallels of Latitude:
  • 1. Tropic of Cancer: 23½° N (23°30' N) in the Northern Hemisphere.
  • 2. Tropic of Capricorn: 23½° S (23°30' S) in the Southern Hemisphere.
  • 3. Arctic Circle: 66½° N (66°30' N) north of the Equator.
  • 4. Antarctic Circle: 66½° S (66°30' S) south of the Equator.
• The Three Major Heat Zones of the Earth:
  • 1. Torrid Zone (Between 23½° N and 23½° S): The mid-day sun is exactly overhead at least once a year on all latitudes in this belt. Receives the maximum heat.
  • 2. Temperate Zones (23½° to 66½° in both hemispheres): The mid-day sun never shines directly overhead beyond the tropics. The angle of the sun's rays slants, resulting in moderate temperatures.
  • 3. Frigid Zones (66½° to 90° Poles): Extremely cold. The sun does not rise much above the horizon; rays are always severely slanting. Land is perennially snow-covered.

• Meridians of Longitude: Imaginary semi-circles running from the North Pole to the South Pole. Unlike latitudes, all meridians of longitude are of equal length. The distance between longitudes is maximum at the Equator (~111 km) and decreases progressively poleward until it becomes zero at the poles.
• The Prime Meridian (0° Longitude):
  • Passes through the Royal Astronomical Observatory at Greenwich, near London.
  • Divides the Earth into the Eastern Hemisphere and Western Hemisphere.
  • $180^circ\text{ East}$ and $180^circ\text{ West}$ meridians meet at the same line directly opposite 0°.
• The Mathematical Law of Time Calculation:
  • The Earth completes one full rotation of $360^circ$ in 24 hours (1,440 minutes).
  • Degrees rotated per hour: $360^circ / 24\text{ hours} = 15^circ\text{ per hour}$.
  • Time required to rotate through $1^circ$ of longitude: $60\text{ minutes} / 15^circ = \mathbf{4\text{ minutes}}$.
• East-Gain / West-Lose Rule (E.G.A. / W.L.S.):
  • Since the Earth rotates from West to East, places located East of Greenwich see the sun earlier and are ahead of Greenwich time (+4 min per degree).
  • Places located West of Greenwich see the sun later and are behind Greenwich time (-4 min per degree).
• Indian Standard Time (IST):
  • Standard Meridian of India = $82^circ 30'\text{ E} = 82.5^circ\text{ E}$.
  • Calculation of Time: $82.5 \times 4\text{ minutes} = 330\text{ minutes}$.
  • Converting to hours: $330 / 60 = \mathbf{5\text{ hours } 30\text{ minutes}}$.
  • Therefore, IST is exactly 5:30 hours ahead of GMT (+5:30).

• The Geographic Grid: The criss-cross network formed by the intersection of parallels of latitude and meridians of longitude on the globe.
• How to Pinpoint Exact Position: If you know the latitude and longitude of any place, you can identify its exact global position at the point where those two lines intersect.
• NCERT Benchmark Example:
  • Dhubri in Assam is situated at $26^circ\text{ N}$ latitude and $90^circ\text{ E}$ longitude.
  • Finding the intersection point of the $26^circ\text{ N}$ parallel and $90^circ\text{ E}$ meridian pinpoints the exact location of Dhubri on Earth.