Chapter 3 Latitude, Longitude And Time

The Earth is approximately a sphere, though not perfectly round due to bulging at the equator caused by its rotation. Its actual shape is best described as an oblate spheroid (or geoid). This curved shape makes it difficult to precisely locate features on its surface without a reference system.

To address this, a grid of imaginary lines is used on globes and maps to define locations. These lines are based on two natural reference points provided by Earth's rotation: the North Pole and the South Pole, which are the endpoints of the Earth's axis of rotation.

This geographical grid consists of two sets of perpendicular lines:

• Parallels of Latitudes: Horizontal lines running east-west.
• Meridians of Longitudes: Vertical lines running north-south.

These lines provide a systematic network called geographical coordinates, allowing us to pinpoint the position of any place on Earth and determine relative location, distance, and direction.

While an infinite number of these lines could theoretically be drawn, only a select number are typically shown on maps for clarity. Latitudes and longitudes are measured in degrees ($^\circ$), with each degree subdivided into 60 minutes ($'$) and each minute into 60 seconds ($''$).

Parallels Of Latitudes

The latitude of a place is its angular distance north or south of the Equator, measured from the Earth's center along the meridian passing through that place (Figure 3.1 illustrates parallels). Lines connecting all points on Earth with the same latitude are called parallels.

Diagram illustrating parallels of latitude as horizontal circles around the Earth, parallel to the Equator.

• The Equator is the fundamental parallel, with a value of $0^\circ$. It is the largest parallel and a 'great circle', dividing the Earth into the Northern and Southern Hemispheres.
• All other parallels are smaller circles that decrease in size towards the poles.
• Latitudes range from $0^\circ$ at the Equator to $90^\circ$ at the poles ($90^\circ$ N for the North Pole, $90^\circ$ S for the South Pole).
• If parallels are drawn at 1-degree intervals, there are 89 parallels in each hemisphere. Including the Equator ($0^\circ$), the total is 179 parallels.
• The letter N or S is added to the latitude value to indicate location in the Northern or Southern Hemisphere, respectively.

While the Earth is not a perfect sphere, the distance represented by one degree of latitude is nearly constant everywhere, approximately 111 km (ranging from 110.6 km at the equator to 111.7 km at the poles). Latitude can be determined by measuring the altitude of the Sun or the Pole Star.

Drawing parallels involves drawing lines parallel to the Equator at specific angular distances from the Earth's center (Figure 3.2 shows how to draw parallels). For example, to draw $20^\circ$S, you would measure an angle of $20^\circ$ downwards from the Equator line at the center of a circle representing a cross-section of the Earth and draw a line connecting the points where the angle's arms intersect the circle's circumference.

Diagram explaining the geometrical method for drawing parallels of latitude on a circular representation of Earth using angles from the center.

Meridians Of Longitude

Meridians of longitude are imaginary vertical lines that run north-south and connect the two poles (Figure 3.3 illustrates meridians). Unlike parallels of latitude (which are circles), meridians are semi-circles. All meridians are equal in length. If opposite meridians (e.g., $0^\circ$ and $180^\circ$) are taken together, they form a full circle, but they are treated as separate meridians.

Diagram illustrating meridians of longitude as semi-circular lines extending from the North Pole to the South Pole.

• Meridians intersect the Equator and all parallels of latitude at right angles ($90^\circ$).
• Unlike parallels, meridians are spaced farthest apart at the Equator and converge at the poles. The distance between two longitudes is approximately 111.3 km at the Equator and decreases to 0 km at the poles.
• By international agreement, the meridian passing through the Royal Observatory at Greenwich, near London, is designated as the Prime Meridian and assigned a value of $0^\circ$.
• Longitude values range from $0^\circ$ to $180^\circ$ eastward and $0^\circ$ to $180^\circ$ westward from the Prime Meridian. The area east of the Prime Meridian up to $180^\circ$ E is the Eastern Hemisphere; the area west of the Prime Meridian up to $180^\circ$ W is the Western Hemisphere.

Drawing The Meridians Of Longitude

Drawing meridians involves representing them as lines radiating from the North or South Pole (Figure 3.5 shows drawing from the North Pole perspective). Imagine looking down from directly above the North Pole:

Diagram showing how the 0 degree and 180 degree meridians meet at the North Pole when viewed from above.

Draw a circle representing the Equator, with the North Pole at its center. Draw a vertical line through the center; this represents the $0^\circ$ and $180^\circ$ meridians. To draw other meridians (e.g., $45^\circ$ E and $45^\circ$ W), place a protractor aligned with the $0^\circ/180^\circ$ line at the center. Measure $45^\circ$ angles on either side. Note that from the North Pole perspective, the directions are different; east is to your left, and west is to your right. The lines drawn at these angles from the center will represent the $45^\circ$ E and $45^\circ$ W meridians radiating from the North Pole towards the Equator.

Diagram explaining the geometrical method for drawing meridians of longitude on a circular representation of Earth viewed from the North Pole, using angles from the center.

Table 3.1 provides a comparison between the key characteristics of parallels of latitudes and meridians of longitudes.

Longitude And Time

The Earth's rotation on its axis from west to east causes the apparent movement of the Sun across the sky from east to west, resulting in day and night. The Earth completes one full rotation (360°) in approximately 24 hours. This means the Earth rotates $360^\circ / 24$ hours = $15^\circ$ of longitude per hour, or $1^\circ$ of longitude in $60 \text{ minutes} / 15 \text{ degrees} = 4$ minutes.

The local time of a place is determined by its longitude relative to the Sun's position. As Earth rotates, places to the east experience sunrise earlier than places to the west. For every degree of longitude eastward from a reference point, time increases by 4 minutes. For every degree of longitude westward, time decreases by 4 minutes.

This relationship is used to determine the local time of any place relative to the time at the Prime Meridian ($0^\circ$ Longitude), which is also known as Greenwich Mean Time (GMT) or Coordinated Universal Time (UTC).

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To maintain uniform time within a country, especially those with significant east-west extent, a Standard Meridian is chosen. This is usually a meridian passing near the center of the country, selected such that its longitude is a multiple of $7^\circ 30'$ (which corresponds to a time difference of 30 minutes, as $7.5^\circ \times 4 \text{ minutes} = 30$ minutes). The local time at the Standard Meridian is then adopted as the Standard Time for the entire country. This allows the difference between the country's standard time and GMT/UTC to be expressed in whole hours or half hours.

In India, the Standard Meridian is $82^\circ 30'$ East longitude, passing through Mirzapur. Indian Standard Time (IST) is calculated from the local time at this meridian. The time difference from GMT/UTC is $(82.5^\circ \times 4 \text{ minutes/degree}) / 60 \text{ minutes/hour} = 330 \text{ minutes} / 60 \text{ minutes/hour} = 5.5$ hours. Since India is east of Greenwich, IST is 5 hours and 30 minutes ahead of GMT/UTC.

Countries with very large east-west spans (like Russia, Canada, USA) may adopt multiple standard meridians, dividing the country into several time zones to better align local time with the Sun's position.

Figure 3.6 shows the major time zones of the world.

Map illustrating the division of the world into 24 major time zones, based on longitude.

International Date Line

The International Date Line (IDL) is an imaginary line that runs approximately along the $180^\circ$ meridian of longitude. It is the line where the calendar date changes as one travels across it. The IDL is necessary because the Earth rotates $360^\circ$ in 24 hours, meaning there's a full day's difference in time around the globe.

The time at the $180^\circ$ longitude is exactly 12 hours different from the time at the $0^\circ$ longitude (Prime Meridian). Time increases as you move east from the Prime Meridian and decreases as you move west. If you travel eastward from the Prime Meridian across all 180 degrees to reach the $180^\circ$ line, your local time will be 12 hours ahead of GMT. If you travel westward from the Prime Meridian across all 180 degrees to reach the $180^\circ$ line, your local time will be 12 hours behind GMT.

Consider someone traveling eastward across the IDL. They move into a time zone that is one full day earlier. So, if they cross the line on a Tuesday, they effectively go back in time and the day becomes Monday. Conversely, someone traveling westward across the IDL moves into a time zone that is one full day later. If they cross on a Monday, the day becomes Tuesday.

The IDL does not follow the $180^\circ$ meridian exactly; it zigzags in places to avoid passing through landmasses and to keep certain island groups or countries within the same time zone.

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