Chapter 2 Globe : Latitudes And Longitudes

As discussed in the previous chapter, our Earth is not a perfect sphere. It is slightly flattened at the North and South Poles and bulges in the middle. A globe is a miniature, true model of the Earth. Globes come in various sizes and types and can be rotated on an axis.

Locating a specific point on a spherical body like the Earth is challenging. To pinpoint locations, we need specific points of reference and lines.

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Axis and Poles

A needle is typically fixed through the globe in a tilted manner. This needle represents the Earth's axis. The two points on the globe where the needle passes through are the North Pole and the South Pole.

The globe can be rotated around this axis from west to east, simulating the Earth's rotation. However, it's important to remember that the Earth's axis is an imaginary line; the real Earth does not have a physical needle.

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Equator and Hemispheres

Another imaginary line that runs on the globe and divides it into two equal halves is called the equator. The half of the Earth north of the equator is known as the Northern Hemisphere, and the half south of the equator is called the Southern Hemisphere. Both hemispheres are equal in size.

The equator is a crucial reference point for locating places on Earth. It is an imaginary circular line representing zero degrees (0°) latitude.

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Parallels of Latitudes

All imaginary circular lines running parallel to the equator, extending from the equator up to the poles, are called parallels of latitudes (Figure 2.2). Latitudes are measured in degrees (°).

The distance from the equator to either the North or South Pole is one-fourth of the Earth's circumference (a circle). Therefore, the angle from the equator to the poles measures ¼th of 360°, which is 90°. The North Pole is at 90° North latitude (90° N), and the South Pole is at 90° South latitude (90° S).

Parallels located north of the equator are called 'north latitudes', and parallels south of the equator are called 'south latitudes'. To specify a location's latitude, its degree value is followed by 'N' for North or 'S' for South (e.g., 20° N or 20° S).

As you move away from the equator towards the poles, the size of the parallels of latitude decreases.

Do you know? You can determine the latitude of your location by measuring the angle of the Pole Star from your position in the Northern Hemisphere.

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Important Parallels Of Latitudes

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Besides the equator (0°), the North Pole (90° N), and the South Pole (90° S), there are four other important parallels of latitude (Figure 2.3):

1. Tropic of Cancer: Located at 23½° North (23.5° N) in the Northern Hemisphere.
2. Tropic of Capricorn: Located at 23½° South (23.5° S) in the Southern Hemisphere.
3. Arctic Circle: Located at 66½° North (66.5° N) of the equator.
4. Antarctic Circle: Located at 66½° South (66.5° S) of the equator.

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Heat Zones Of The Earth

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Based on the amount of heat received from the sun, the Earth is divided into three heat zones (Figure 2.3):

1. Torrid Zone: This zone lies between the Tropic of Cancer (23½° N) and the Tropic of Capricorn (23½° S). The mid-day sun is directly overhead at least once a year on all latitudes within this zone. As a result, this area receives the maximum amount of heat.
2. Temperate Zones: There are two Temperate Zones. The North Temperate Zone is located between the Tropic of Cancer (23½° N) and the Arctic Circle (66½° N) in the Northern Hemisphere. The South Temperate Zone is located between the Tropic of Capricorn (23½° S) and the Antarctic Circle (66½° S) in the Southern Hemisphere. In these zones, the mid-day sun never shines directly overhead. The angle of the sun's rays is moderate. Therefore, these zones experience moderate temperatures. (Figure 2.4 illustrates how the angle of light affects brightness and heat).
3. Frigid Zones: There are two Frigid Zones, known for being very cold. The North Frigid Zone lies between the Arctic Circle (66½° N) and the North Pole (90° N) in the Northern Hemisphere. The South Frigid Zone lies between the Antarctic Circle (66½° S) and the South Pole (90° S) in the Southern Hemisphere. In these regions, the sun does not rise much above the horizon. Consequently, the sun's rays are always very slanting, providing very little heat.

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What Are Longitudes?

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Knowing only the latitude of a place is not enough to determine its exact location. For precise positioning, we also need to know its position east or west of a specific reference line running from the North Pole to the South Pole.

These reference lines are called meridians of longitude (Figure 2.5). Distances between meridians are measured in 'degrees of longitude'. Each degree is further divided into minutes, and minutes into seconds.

Meridians of longitude are semi-circles. The distance between them is greatest at the equator and decreases steadily towards the poles, becoming zero at the poles where all meridians meet.

Unlike parallels of latitude, all meridians of longitude are of equal length. This made it difficult to establish a numbering system. Therefore, by international agreement, it was decided to begin the numbering from the meridian passing through Greenwich, near London, where the British Royal Observatory is located. This meridian is called the Prime Meridian.

The value of the Prime Meridian is 0° longitude. From the Prime Meridian, longitudes are counted 180° eastward and 180° westward. The Prime Meridian (0°) and the 180° meridian (which is on the opposite side of the Earth) divide the Earth into two equal halves: the Eastern Hemisphere and the Western Hemisphere.

The longitude of a place is indicated by its degree value followed by the letter 'E' for East or 'W' for West. The 180° East and 180° West meridians are, in fact, the same line on the opposite side of the globe from the Prime Meridian.

Together, the network of parallels of latitude and meridians of longitude form a grid on the globe (Figure 2.6). This grid allows us to locate any point on the Earth's surface if we know its latitude and longitude. For example, if Dhubri in Assam is at 26° N latitude and 90° E longitude, finding the intersection of these two lines on the grid gives its location.

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Longitude And Time

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The rotation of the Earth is the most reliable basis for measuring time. The regular rising and setting of the sun provides a natural time-keeper globally. Local time at any place can be determined by the position of the sun in the sky and the length of the shadow it casts; the shadow is shortest at noon (when the sun is highest) and longest at sunrise and sunset.

When the sun is at its highest point in the sky along the Prime Meridian (Greenwich, 0° longitude), it is **mid-day or noon** at all places located on that meridian.

Since the Earth rotates from west to east, places located to the east of Greenwich will experience sunrise earlier and thus be ahead of Greenwich Mean Time (GMT), while places to the west will be behind GMT (Figure 2.8).

The rate of time difference can be calculated based on the Earth's rotation:

• The Earth rotates 360° in approximately 24 hours.
• This means it rotates 15° every hour (360° / 24 hours = 15°/hour).
• Equivalently, it rotates 1° every 4 minutes (60 minutes / 15° = 4 minutes/°).

Therefore, for every 15° of longitude eastward, time is one hour ahead of Greenwich. For every 15° of longitude westward, time is one hour behind Greenwich.

For example, when it is 12 noon at Greenwich (0°), the time at 15° East longitude will be 1 hour ahead (1 p.m.), and the time at 15° West longitude will be 1 hour behind (11 a.m.). At 180° longitude (both East and West), it will be midnight when it is 12 noon at Greenwich.

A watch at any specific location can be set to 12 o'clock when the sun is highest in the sky, giving the local time for that place. All places situated on the same meridian of longitude will have the same local time.

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Why Do We Have Standard Time?

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Because local time varies with longitude, places on different meridians have different local times. This creates practical difficulties, especially for coordinating activities across a large country with significant longitudinal extent, like preparing train schedules that cross multiple longitudes.

To avoid this confusion, countries adopt a standard time based on the local time of a central meridian. This chosen meridian's local time is used as the official time for the entire country or a specific region within it.

In India, the longitude of 82½° East (82° 30' E) is designated as the Standard Meridian of India (Figure 2.9). The local time at this meridian is taken as the **Indian Standard Time (IST)** for the entire country. This means that regardless of their local solar time, all places in India use the time of the 82°30' E meridian as their official time.

The difference between IST and GMT can be calculated. India's Standard Meridian is 82.5° East of Greenwich. Since the Earth rotates 1° in 4 minutes, the time difference is 82.5 × 4 minutes = 330 minutes. 330 minutes is equal to 5 hours and 30 minutes (330 / 60 = 5.5). Since India is east of Greenwich, its time is 5 hours and 30 minutes ahead of GMT.

Answer:

Some large countries with significant longitudinal extent have multiple standard times to reduce the difference between local solar time and standard time within their borders. Russia, for instance, has eleven standard times. The Earth is generally divided into 24 time zones, each covering approximately 15° of longitude, corresponding to a one-hour time difference.