Chapter 3 Latitude, Longitude And Time

The Earth, being roughly a sphere (more accurately, an **oblate spheroid**, bulging slightly at the equator due to rotation), presents challenges in precisely locating features on its surface. Unlike a flat plane, there's no obvious starting point or reference from which to measure positions. To overcome this, a system of imaginary lines forming a grid is used on globes and maps to provide a reference system for locating any point on Earth.

The Earth's rotation on its axis provides two natural fixed points: the **North Pole** and the **South Pole**. These poles serve as the basis for constructing the geographical grid, a network of intersecting lines that allows for the systematic location of places.

The geographical grid consists of two sets of imaginary lines:

• **Horizontal lines** running in an east-west direction, called **parallels of latitude**.
• **Vertical lines** running in a north-south direction, joining the poles, called **meridians of longitude**.

The line drawn exactly midway between the North and South Poles is the **Equator**. It is the largest possible circle that can be drawn on the Earth's surface and divides the Earth into two equal halves: the Northern Hemisphere and the Southern Hemisphere. The Equator is considered a **great circle**. All other parallels of latitude are smaller circles, becoming progressively smaller towards the poles, and divide the Earth into two unequal halves (except the Equator itself).

Meridians of longitude are semi-circles that extend from the North Pole to the South Pole. They are farthest apart at the Equator and converge to a single point at each pole. Unlike parallels of latitude, all meridians of longitude are equal in length.

These parallels of latitude and meridians of longitude are together known as **geographical coordinates**. They form a systematic grid that allows for the precise representation of any location on the Earth's surface. Using these coordinates, we can easily determine the location, distance from other points, and direction of various features.

While conceptually, an infinite number of parallels and meridians could be drawn, only a selected number are typically shown on maps to maintain clarity.

Latitudes and longitudes are measured in **degrees ($^\circ$)** because they represent angular distances from specific reference lines. Each degree is further subdivided into 60 **minutes ($'$)**, and each minute into 60 **seconds ($''$)**.

Glossary Terms:

Parallels Of Latitudes

The **latitude** of a place is its angular distance north or south of the Equator. It is measured as the angle from the Earth's center to the point on the surface, along the meridian passing through that place. Lines connecting all places with the same latitude value are called **parallels of latitude**. These lines are parallel to the Equator and to each other.

The Equator has a latitude of **0°**. The North Pole is at **90°N** latitude, and the South Pole is at **90°S** latitude (Figure 3.1). If parallels are drawn at one-degree intervals, there would be 89 parallels in the Northern Hemisphere and 89 in the Southern Hemisphere. Including the Equator (0°), the total number of parallels typically considered is 179 (89 N + 89 S + 1 Equator). The value of a latitude is always accompanied by 'N' or 'S' to indicate whether the place is north or south of the Equator.

If the Earth were a perfect sphere, the distance between two consecutive degrees of latitude would be constant at approximately 111 km everywhere. This distance is also close to the length of a degree of longitude at the Equator. However, due to Earth's oblate shape, the length of a degree of latitude varies slightly, being about 110.6 km at the Equator and increasing to about 111.7 km at the poles. The latitude of a place can be determined by observing the altitude of the sun at noon or the altitude of the Pole Star (in the Northern Hemisphere).

Drawing The Parallels Of Latitudes

To visualize how parallels of latitude are drawn on a globe or map (representing a cross-section of the Earth):

1. Draw a circle to represent a cross-section of the Earth.
2. Draw a horizontal line through the center, dividing the circle into two halves. This represents the Equator (0° latitude). The center of the circle represents the center of the Earth.
3. To draw a parallel of latitude, say $20^\circ$ South: Place a protractor with its baseline coinciding with the Equator line and its center at the Earth's center. Measure an angle of $20^\circ$ downwards from the Equator line (into the lower half of the circle).
4. Draw lines from the Earth's center at $20^\circ$ angle to the Equator on both the left and right sides (east and west). The points where these lines intersect the circle's circumference represent points on the $20^\circ$S parallel.
5. Draw a line connecting these two points. This line will be parallel to the Equator and represents the $20^\circ$ South parallel of latitude. Figure 3.2 illustrates this method for drawing a parallel in the Southern Hemisphere. Repeat the process measuring angles upwards from the Equator for Northern Hemisphere latitudes.

Meridians Of Longitude

Unlike the parallels of latitude, which are complete circles (except for the poles), the **meridians of longitude** are semi-circles that extend from the North Pole to the South Pole. All meridians converge at the poles and are equal in length. If two opposite meridians (e.g., 0° and 180°) are taken together, they form a complete circle, but they are treated as two distinct meridians for numbering purposes.

Meridians of longitude intersect the Equator and all parallels of latitude at right angles (90°).

To establish a reference point for numbering longitudes, the meridian passing through the **Greenwich observatory** near London, UK, was internationally agreed upon as the **Prime Meridian**. It is assigned the value of **0° longitude**. Longitude values are measured in degrees eastward or westward from the Prime Meridian, ranging from 0° to 180° in each direction. The area east of the Prime Meridian is the Eastern Hemisphere, and the area west is the Western Hemisphere (Figure 3.3). The 180° East meridian and the 180° West meridian are the same line, approximately coinciding with the International Date Line.

Drawing The Meridians Of Longitude

To visualize drawing meridians of longitude on a globe or map (representing a view from a pole):

1. Draw a circle. Let the center of this circle represent one of the poles (e.g., the North Pole). The circumference of the circle represents the Equator.
2. Draw a vertical line passing through the center of the circle (the North Pole). This line represents both the 0° (Prime) Meridian and the 180° Meridian, which meet at the pole (Figure 3.4).

When looking at a map, East is typically to your right and West to your left. However, when drawing meridians from a polar perspective, imagine standing at the North Pole. East would then be to your left and West to your right relative to the 0° meridian. (Figure 3.5 illustrates this view and drawing).

3. To draw a meridian, say $45^\circ$ East: Place a protractor along the vertical 0°/180° line with its center at the North Pole. Measure an angle of $45^\circ$ to the left (which is East from the North Pole perspective). Draw a line from the North Pole along this angle to the Equator. This is the $45^\circ$ East meridian.
4. To draw a meridian, say $45^\circ$ West: From the same 0°/180° line, measure an angle of $45^\circ$ to the right (West from the North Pole perspective). Draw a line from the North Pole along this angle to the Equator. This is the $45^\circ$ West meridian.

Comparison between Parallels of Latitudes and Meridians of Longitudes (Table 3.1):

Longitude And Time

The Earth's rotation from **west to east** on its axis is responsible for the apparent movement of the sun across the sky, causing sunrise in the east and sunset in the west. The Earth completes one full rotation ($360^\circ$ of longitude) in approximately 24 hours.

This means that the Earth rotates $360^\circ / 24 \text{ hours} = 15^\circ$ of longitude per hour. Equivalently, it takes $24 \text{ hours} / 360^\circ = 1/15 \text{ hours}$ or $(1/15) \times 60 = 4 \text{ minutes}$ for the Earth to rotate through 1 degree of longitude.

So, the time difference for every 1 degree of longitude is **4 minutes**.

Since the Earth rotates from west to east, places located to the east experience sunrise and noon earlier than places to the west. Therefore, local time increases as we move eastward and decreases as we move westward from a reference meridian (like the Prime Meridian).

The relationship between longitude and time is used to determine the **local time** of any place on Earth based on its longitudinal position relative to a known reference time (e.g., the time at the Prime Meridian or Greenwich Mean Time - GMT).

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Answer:

To avoid different local times within a single country (due to its longitudinal extent), most countries adopt a **Standard Time**. The Standard Time is the local time of a designated **Standard Meridian** within the country's territory. This meridian is usually chosen so that its longitude is a multiple of $7^\circ30'$ (since $15^\circ/2 = 7^\circ30'$), ensuring that the time difference from GMT is in whole or half hours.

India's Standard Time (IST) is based on the meridian of $82^\circ30'$ East longitude, which passes near Mirzapur. The time difference from GMT is calculated as $(82^\circ30' \times 4 \text{ minutes/degree}) / 60 \text{ minutes/hour}$. $82^\circ30'$ East = $82.5^\circ$ East Time difference = $82.5 \times 4 = 330$ minutes. 330 minutes = $330 / 60 = 5.5$ hours. So, IST is **5 hours and 30 minutes ahead of GMT (+5:30 GMT)**.

Countries with a very large east-west extent (like Russia, Canada, the USA) may have multiple standard meridians, resulting in multiple **time zones** to better reflect the local solar time across their vast territories. The world is divided into 24 major time zones, each roughly spanning $15^\circ$ of longitude, corresponding to a one-hour time difference. Figure 3.6 shows these major time zones.

International Date Line

The **International Date Line (IDL)** is an imaginary line on the Earth's surface that roughly follows the $180^\circ$ meridian of longitude. It serves as the boundary between one calendar day and the next. Crossing the IDL changes the date.

The time at the $180^\circ$ longitude is exactly 12 hours different from the time at the 0° longitude (Prime Meridian). Specifically, the time at $180^\circ$ East is 12 hours ahead of GMT (+12:00 GMT), and the time at $180^\circ$ West is 12 hours behind GMT (-12:00 GMT). This creates a 24-hour difference in time across the $180^\circ$ line.

If a person travels eastward across the International Date Line, they gain a day. For example, crossing the IDL eastward on a Tuesday means they would consider it Monday on the other side of the line, effectively repeating a day. Conversely, if a person travels westward across the International Date Line, they lose a day. Crossing the IDL westward on a Tuesday means they would consider it Wednesday on the other side, skipping a day. The IDL is not a perfectly straight line; it zigzags slightly in places to avoid cutting through island groups or landmasses and causing countries to have two different calendar dates.

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