Latitude, Longitude And Time (Mapping Aspects)

Parallels Of Latitudes

Parallels of Latitude are imaginary horizontal lines that circle the Earth parallel to the Equator. They measure distance north or south of the Equator.

Characteristics:

• Circumferences: They are full circles, decreasing in circumference from the Equator towards the poles.
• Equator: The largest parallel of latitude is the Equator (0° latitude), which divides the Earth into the Northern Hemisphere and the Southern Hemisphere.
• Measurement: Measured in degrees (°), minutes ('), and seconds ('').
• Direction: Indicate distance north or south of the Equator.
• Parallel: All parallels of latitude are parallel to each other and to the Equator.
• Distance: The distance between any two parallels of latitude is roughly constant (about 111 km per degree).
• Key Parallels:
• Tropic of Cancer: 23.5° N
• Tropic of Capricorn: 23.5° S
• Arctic Circle: 66.5° N
• Antarctic Circle: 66.5° S
• North Pole: 90° N
• South Pole: 90° S

Purpose:

• Location: Used to determine the position of a place north or south of the Equator.
• Climate Zones: Help define climate zones (e.g., tropical, temperate, polar zones).
• Mapping: Form horizontal grid lines on maps and globes.

Mapping Aspect: On maps, parallels of latitude are typically drawn as horizontal lines, which are parallel to each other and perpendicular to the meridians.

Meridians Of Longitude

Meridians of Longitude are imaginary vertical lines that run from the North Pole to the South Pole, converging at the poles. They measure distance east or west of the Prime Meridian.

Characteristics:

• Semi-circles: They are semi-circles, not full circles.
• Convergence: All meridians converge at the North and South Poles.
• Measurement: Measured in degrees (°), minutes ('), and seconds ('') east or west of the Prime Meridian.
• Prime Meridian: The Prime Meridian (0° longitude) passes through Greenwich, London, UK. It divides the Earth into the Eastern Hemisphere and the Western Hemisphere.
• Great Circles: Meridians are part of great circles, which are the largest circles that can be drawn on the Earth's surface.
• Distance: The distance between meridians decreases towards the poles. At the Equator, the distance between two meridians 1° apart is about 111.3 km, but at the poles, this distance becomes zero.

Purpose:

• Location: Used to determine the position of a place east or west of the Prime Meridian.
• Time Calculation: Crucial for calculating time zones and local time around the world.
• Mapping: Form vertical grid lines on maps and globes.

Drawing The Meridians Of Longitude

On a Globe: Meridians are drawn as semi-circles connecting the North Pole and the South Pole. They are spaced equally apart at the Equator and converge at the poles.

On a Map:

• Projection Dependent: The way meridians are drawn on a map depends on the map projection used.
• Cylindrical Projection (e.g., Mercator): Meridians are drawn as parallel vertical lines, equally spaced across the map. This preserves direction but exaggerates areas at higher latitudes.
• Conical Projection: Meridians are drawn as straight lines converging towards the map's apex (representing a pole).
• Azimuthal Projection: Meridians are drawn as straight lines radiating outwards from a central point (representing a pole).
• Common Representation: On most standard maps, meridians are shown as vertical lines, spaced according to the map's longitude coverage and projection.

Longitude And Time

The rotation of the Earth on its axis is the basis for our measurement of time. The Earth rotates from west to east, causing the sun to appear to rise in the east and set in the west. The system of time zones is based on the Earth's longitude and rotation.

Earth's Rotation and Time:

• Rotation Period: The Earth completes one full rotation (360°) in approximately 24 hours.
• Speed of Rotation: This means the Earth rotates at a speed of 360° / 24 hours = 15° per hour.
• Time Difference per Degree: For every 15° of longitude, there is a time difference of 1 hour. For every 1° of longitude, there is a time difference of 4 minutes (60 minutes / 15° = 4 minutes per degree).

Local Time:

• Based on the Sun: Local time at any point on Earth is determined by the position of the Sun in the sky. High noon is when the Sun is at its highest point (zenith).
• Longitude and Local Time: As you move east from a reference point, the time becomes earlier. As you move west, the time becomes later.

Standard Time:

• Problem of Local Time: If every place used its own local time, it would be incredibly confusing for travel and communication.
• Solution: Countries establish a standard time, usually based on the local time of a central meridian within the country.
• Time Zones: The world is divided into 24 standard time zones, each theoretically spanning 15° of longitude.

Indian Standard Time (IST):

• Meridian: Based on the longitude 82°30' E, which passes through Allahabad (Prayagraj) and Mirzapur in India.
• Calculation: Since 82.5° E is east of the Prime Meridian (0°), India's time is ahead of Greenwich Mean Time (GMT).
• Time Difference from GMT: 82.5° * 4 minutes/degree = 330 minutes. 330 minutes / 60 minutes/hour = 5.5 hours.
• IST = GMT + 5:30 hours.

Mapping Aspect: Maps showing longitude often include grid lines of meridians. Understanding the longitude of a place is key to determining its local time and its time zone relative to GMT or IST.

International Date Line

The International Date Line (IDL) is an imaginary line that roughly follows the 180° meridian of longitude. It is where the calendar date changes. Crossing the IDL has a direct impact on the date.

Concept:

• Westward Movement: When you travel westward across the IDL, you gain a day (e.g., if it's Monday on one side, it's Tuesday on the other).
• Eastward Movement: When you travel eastward across the IDL, you lose a day (e.g., if it's Tuesday on one side, it's Monday on the other).

Location and Deviations:

• General Position: It generally follows the 180° meridian.
• Deviations: The IDL deviates significantly from the 180° meridian in several places to avoid dividing landmasses into different dates. For example:
• It zigzags around Kiribati and Samoa to keep them in the same date.
• It also deviates to accommodate Russia and Alaska.

Purpose:

• Calendar Continuity: It ensures that the calendar date is consistent within any given territory.
• Global Timekeeping: It serves as the reference line for coordinating dates across the globe.

Relationship with Time Zones:

• The IDL separates two consecutive calendar days.
• If you travel west from the Americas towards Asia, you cross the IDL and advance the date.
• If you travel east from Asia towards the Americas, you cross the IDL and set the date back.

Mapping Aspect: The IDL is shown on world maps as a distinct line, often depicted as a dashed or thick blue line, showing its irregular path across the Pacific Ocean.