2. Latitudes and Longitudes

1. Locating Places on the Earth

Geography fundamentally deals with the location and distribution of phenomena on the Earth. Because the Earth is a sphere without edges, locating places requires reference points or lines.

The method is similar to locating a point on a graph using X and Y coordinates. The X-coordinate represents the horizontal distance, and the Y-coordinate represents the vertical distance from a point of origin. On Earth, we use two primary reference lines: the Equator and the Prime Meridian. These are imaginary lines adopted internationally, and distances from them are measured in angles.

2. Latitudes (Parallels of Latitude)

Latitude is the angular distance of a place north or south of the Equator, measured from the center of the Earth.

• The Equator (0°): The starting point for measuring latitude. It is an imaginary circle midway between the North and South Poles, dividing the Earth into the Northern and Southern Hemispheres.
• Measurement: Latitudes are measured in degrees (°), minutes ('), and seconds ("). There are 90° of latitude north of the Equator (ending at the North Pole, 90°N) and 90° south (ending at the South Pole, 90°S).
• Parallels: Lines joining places with the same latitude form complete circles parallel to the Equator. These are called parallels of latitude. They gradually become shorter towards the poles, where they become just points.
• Distance: Since the distance from the Equator to either Pole is roughly 10,000 km, 1° of latitude is equivalent to approximately 111 km on the Earth's surface.

Important Parallels of Latitude

1. Equator (0°): Divides the Earth into two equal halves.
2. Tropic of Cancer (23½°N): The northernmost limit where the sun's rays can fall vertically (overhead) at noon, which happens on June 21st.
3. Tropic of Capricorn (23½°S): The southernmost limit where the sun's rays can fall vertically at noon, which happens on December 22nd.
4. Arctic Circle (66½°N): The latitude where there is 24 hours of daylight on June 21st. North of this line, summer days are longer than 24 hours.
5. Antarctic Circle (66½°S): The latitude where there is 24 hours of daylight on December 22nd. South of this line, summer days are longer than 24 hours.
6. North Pole (90°N) & South Pole (90°S): The extreme northern and southern points of the Earth's axis.

Uses of Latitudes

Latitudes serve two primary purposes:

• Finding Absolute Location: They define how far north or south a place is from the Equator.
• Measuring Distance: Knowing that 1° ≈ 111 km allows for quick distance calculations (e.g., a place at 30°N is roughly 30 × 111 = 3330 km from the Equator).

3. Longitudes (Meridians of Longitude)

Longitude is the angular distance of a place measured in degrees east or west of the Prime Meridian.

• To measure east-west distances, the Equator is divided into 360°. Lines drawn through these points from the North Pole to the South Pole (intersecting the Equator at right angles) are called Lines (or Meridians) of Longitude.
• All meridians are equal in length (half-circles).
• Prime Meridian (0°): By international agreement, the meridian passing through the old Royal Observatory at Greenwich (near London) is taken as 0° longitude.
• Measurement: Longitudes are numbered up to 180° East (Eastern Hemisphere) and 180° West (Western Hemisphere). The 180°E and 180°W lines are the same meridian.
• Distance between Meridians: Unlike latitudes, the distance between longitudes is not constant. It is maximum at the Equator (approx. 111 km) and decreases to zero at the Poles, where all meridians meet.

4. Longitude and Time

There is a direct relationship between longitude and time due to the Earth's rotation.

• The Earth completes one full rotation (360°) in 24 hours.
• Therefore, it rotates 15° in 1 hour (360 / 24).
• This means it takes 4 minutes for the Earth to rotate 1° (60 minutes / 15°).
• Since the Earth rotates from West to East, places to the East see the sun earlier and are ahead in time, while places to the West see the sun later and are behind in time.

Local Time vs. Standard Time

• Local Time: The time at a specific meridian based on the position of the sun. When the sun is at its highest point (zenith) in the sky over a meridian, it is 12:00 Noon local time for all places on that meridian. Local time changes by 4 minutes for every degree of longitude.
• Standard Time: If every town kept its own local time, it would create massive confusion for travel and communication. Therefore, a country usually selects a central meridian, and the local time of that meridian is adopted as the Standard Time for the entire country.
• Indian Standard Time (IST): India has selected the meridian of 82½°E (passing through Mirzapur, near Prayagraj) as its Standard Meridian.
  Calculation relative to GMT: 82.5° × 4 mins = 330 mins = 5 hours 30 minutes. Since India is East, IST is 5 hrs 30 mins ahead of Greenwich Mean Time.

Greenwich Mean Time (GMT)

The local time at the Prime Meridian (0° longitude) passing through Greenwich, England. It serves as the baseline for calculating standard times globally, ensuring international uniformity for travel and communication.

5. The International Date Line (IDL)

If you travel eastward from Greenwich, time is added. If you travel westward, time is subtracted. When you reach the 180° meridian, there is a total difference of 24 hours (12 hours gained moving East, 12 hours lost moving West).

• The International Date Line roughly follows the 180° meridian through the mid-Pacific Ocean.
• It is the line where the date changes by exactly one day.
• Crossing Westwards (e.g., from America to Asia): You lose a day (advance the calendar by one day, e.g., Sunday becomes Monday).
• Crossing Eastwards (e.g., from Asia to America): You gain a day (repeat the same date, e.g., Monday becomes Sunday).

6. Great and Small Circles

Great Circles

A Great Circle is any circle drawn on a globe that divides the Earth into two equal halves (hemispheres). Its center is the exact center of the Earth, and it represents the largest possible circle that can be drawn around the sphere.

• The Equator is the only line of latitude that is a Great Circle.
• ALL lines of longitude (meridians), when paired with their opposite meridian (e.g., 0° and 180°, or 90°E and 90°W), form Great Circles.

Small Circles

Any circle drawn on the globe that does not divide the Earth into two equal halves. All lines of latitude except the Equator are Small Circles.

Importance of Great Circles (Great Circle Routes)

The arc of a Great Circle represents the shortest distance between any two points on the spherical surface of the Earth.

• Navigators (ships) and aviators (aircraft) use Great Circle routes, especially over vast oceans like the Pacific and Atlantic, to save considerable fuel and time.
• Even though a straight line on a flat map (like following a line of latitude) might look shorter, the curved Great Circle route is actually shorter in reality due to the Earth's spherical shape.