The Earth, with its geographic and climatic complexity, has long required precise systems to locate any point on its surface, understand climates, and organize social and economic life. Meridians, along with parallels, form the basis of the geographic coordinate system that makes it possible and are essential in determining time, navigation, and climate zoning.
Today, understanding the role of meridians goes far beyond memorizing imaginary lines on the globe: it helps us understand the weather, orient ourselves on a map, or understand why time changes from one country to another. In the following lines, we will explore in depth the origin, function, and significance of meridians in geography and climate, developing all the aspects that make them a fundamental component for global functioning.
What are meridians and why do they exist?
Meridians are imaginary lines that run across the planet from the North Pole to the South Pole, dividing it into longitudinal segments. Unlike parallels, which are horizontal and circle the Earth from east to west, meridians extend vertically and all have the same length because they meet at both poles, forming perfect semicircles.
The original purpose of the meridians was to serve as a reference for measuring geographic longitude.This measurement was essential for navigation, the creation of accurate maps, and the organization of territories and trade routes. But its importance expanded greatly over time, as it also made it possible to define time zones and coordinate daily life and the economy on a global scale.
The origin and historical evolution of the meridians
The concept of meridian comes from the Latin "meridies", which means noon, since the sun reaches its highest point on a given meridian at solar noon. Since ancient civilizations, such as the Greeks, attempts have been made to establish reference lines to understand the position of places relative to the movement of the sun. Eratosthenes, for example, proposed a prime meridian as early as 200 BC.
Before the 19th century, there was no universal agreement on what the zero or reference meridian would be. Each country could choose its own meridian for drawing maps and defining longitudes, which caused confusion, especially as international trade and navigation intensified. For example, France used the Paris meridian, while other countries used different references.
The International Meridian Conference of 1884, held in Washington DC, was a turning point: Twenty-five countries agreed to adopt as their prime meridian the one that passes through Greenwich, a London suburb where the Royal Astronomical Observatory was located. This agreement meant that the Greenwich meridian would become the world reference, with longitude 25°, and all other meridians would be measured in degrees east or west up to 0°. This decision was practical and responded to the reality of the time: most sea routes and time systems already used Greenwich as their starting point.
The structure of the geographic network: Meridians and parallels
The geographic coordinate system is based on the combination of meridians (longitude) and parallels (latitude). The parallels, which cross the Earth from east to west, determine the distance from the equator (latitude), while the meridians determine the distance from the Greenwich meridian (longitude).
- Meridians: There are 360, 180 eastward, and 180 westward of the Greenwich Meridian. All the meridians meet at the poles and divide the Earth into the eastern and western hemispheres.
- Parallels: They are horizontal circles that decrease in size as they approach the poles. The equator is the principal parallel (latitude 0º).
The intersection between a meridian and a parallel defines the exact position of a point on Earth, which can be expressed by its coordinates (latitude, longitude).
The Greenwich Meridian and World Time
The Greenwich meridian (0º longitude) is the reference for establishing time zones on a planetary level. It determines time differences in 15-degree increments, equivalent to one hour, given that the Earth rotates 360 degrees in 24 hours. Thus, each time zone covers approximately 15 degrees of longitude, although in practice, time zone boundaries are adapted for political and geographical reasons to avoid fragmenting countries or regions.
The adoption of a standard meridian allowed for international time synchronization, which was fundamental to the Industrial Revolution, the expansion of railway systems, and the improvement of communications. Previously, each city adjusted its clock based on local solar noon, causing significant discrepancies. The creation of universal time (Greenwich Mean Time or GMT) and the subsequent incorporation of Coordinated Universal Time (UTC) have made international flights, scientific coordination, and global trade possible today.
The International Date Line and the Antimeridian
On the opposite side of the Greenwich Meridian, at 180º longitude, is the antimeridian, also known as the International Date Line. This line runs across the Pacific Ocean and is the official point of the day: crossing it from west to east subtracts one day, and crossing it from east to west adds one day. This maintains consistency in the global calendar.
In some regions, such as Samoa and Tokelau, deliberate adjustments have been made to the location of this line to accommodate economic interests and better synchronize with their major trading partners. In other cases, the route also deviates to respect national borders and avoid absurd situations such as the same country having two different days within its territory.
Meridians, parallels and climatic zones
The parallels, together with the meridians, divide the Earth into different climatic zones that determine the meteorological characteristics of each region. Latitude, that is, the distance from the equator, directly influences the climate, the length of the seasons, and the incidence of sunlight.
- Intertropical zones: Between the equator and the Tropics of Cancer and Capricorn (23,5° north and south, respectively). These are warm regions with little annual temperature variation and frequent rainfall.
- Temperate zones: Between the tropics and the polar circles (66,5° north and south). They have more distinct seasons and variable climates.
- Polar zones: Beyond the Arctic and Antarctic Circles, phenomena such as the polar night and the midnight sun occur here, with wide variations in brightness and extremely low temperatures.
Parallels mark the boundaries of these climates, but meridians are also important in determining how solar hours and local time vary across the planet's surface.
Natural phenomena linked to parallels and meridians
Precise knowledge of parallels and meridians explains the appearance of unique phenomena such as the midnight sun and the polar night, which occur near the polar circles. These events occur when the tilt of the Earth's axis prevents the sun from setting or rising during certain periods, generating entire days of light or darkness in the northern and southern extremes of the planet.
Also, the Tropics of Cancer and Capricorn are the only places where the sun reaches its zenith (vertical) right on the summer or winter solstice, an essential phenomenon for the study of the seasons and the agricultural calendar.
Geographic coordinates: precise location of any point
Thanks to the network of meridians and parallels, any place in the world can be accurately located using the geographic coordinate system. Each coordinate consists of two values:
- Latitude: measures the distance from the equator, from 0° to 90° north or south.
- Length: It measures the distance from the Greenwich meridian, from 0° to 180° east or west.
The combination of both values allows us to identify the unique position of cities, geographical features or remote points on the planet.
Modern applications: GPS, navigation and digital mapping
Current navigation systems, both the GPS systems we use in mobile phones and cars and those used in aviation and maritime navigation, are based on the coordinate system defined by meridians and parallels. Thanks to this global network, satellites can determine a receiver's position with a margin of error of just a few meters.
Geographic Information Systems (GIS) and digital maps, which are key tools in territorial management, urban planning, and environmental analysis, also use geographic coordinates to represent spatial data and phenomena.
The relationship between meridians, parallels and social and economic organization
The unification of time systems through meridians was key to the development of modern societies. From synchronizing rail transport to coordinating international markets and sporting events, standard time is made possible by the global division into time zones based on meridians.
Furthermore, the use of meridians and parallels in cartography has allowed for planning urban expansion, delimiting borders, and optimizing the logistics of global trade. Current conventions, such as the choice of the Greenwich Meridian or the adaptation of time zones to political factors, have facilitated daily life and international collaboration.
Diversity of meridians: local meridian and magnetic meridian
In addition to the Greenwich meridian, there is the concept of the "local meridian," which is the meridian that passes through any point where an observer is located. This concept is useful for determining solar noon in each location and adjusting clocks traditionally.
On the other hand, the "magnetic meridian" is used in compass navigation, taking magnetic North as a reference. The angular difference between the geographic and magnetic meridian is called magnetic declination and is crucial for navigators, as it affects heading and orientation on the ground.
Practical examples of the use of meridians and parallels
To better understand the relevance of these concepts, it is enough to look at their practical application:
- In aviation and maritime navigation, pilots and sailors calculate routes based on latitudes and longitudes, optimizing journeys and avoiding risks.
- Many emergency and rescue systems use geographic coordinates to accurately locate isolated people or locations.
- In the educational field, school maps and globes incorporate the network of meridians and parallels to teach students about the structure of the planet.
- In climatology, division by parallels helps define agricultural zones adapted to different thermal and sunlight conditions.
The study and application of meridians is not merely an academic matter; it is an inseparable part of daily life, from the functioning of the internet and satellites to weather forecasting and time management in every corner of the planet. As technology advances and interdependence between regions increases, the role of meridians, along with parallels, is becoming more established as one of the great human inventions for understanding and organizing the world we inhabit.
