Meridian

We explain what a meridian is and what the Greenwich meridian is. In addition, we tell you how they relate to time zones and parallels.

What is a meridian?

a meridian It is one of the imaginary lines into which the globe is divided and that connects the north pole with the south pole. The meridians are semicircle-shaped and divide the planet into longitudinal segments of 15° sexagesimal each (like the segments of a tangerine), which allows determining the longitude of any point on the globe and also establishing the time zones of each region.

The word “meridian” comes from Latin meridianuswhich means “related to noon” (meridies), since all people located along the imaginary line of the same meridian, regardless of the hemisphere in which they are located, will experience solar noon at the same time (although not with the same intensity). This is due to the rotation of the Earth.

Together with the parallels, the meridians allow the planet's surface to be divided into quadrants and thus establish a geographic coordinate system, useful for locating any point on the planetary surface. For this, however, it was necessary to choose a main meridian or “zero” from which to start counting (which is the Greenwich meridian), and a main parallel or “zero” (which is the equator) that divides the Earth in two hemispheres.

These determinations were carried out in a conventional manner at the end of the 19th century, due to the need to organize time zones in an increasingly internationalized economy.

See also: Cardinal points

Greenwich Meridian

The Greenwich meridian, also called the zero meridian or origin meridian, It is the main or base, from which the counting of the other meridians of the planet begins. That is, it is the starting point of 0° sexagesimals, so that the meridians that follow it in an easterly direction accumulate degrees in a positive direction (15°, 30° and so on) and those that follow in a westerly direction accumulate degrees in a negative sense (-15°, -30° and so on).

The Greenwich Meridian It was chosen to be the zero meridian at the International Meridian Conference held in 1884 in Washington, United States, since it was necessary to standardize time zones and choose a single main meridian, so that a “universal day” existed and the times of different places could be determined.

The choice of the meridian that passes through Greenwich, a district of London where the Royal Greenwich Observatory was located, was due to the fact that It was already used by most sailors and it suited the powerful nations of the time. France and some other nations disagreed, and continued to use the meridian passing through the Paris Observatory on their maps for some time.

The extension of the Greenwich meridian to the other side of the planet, that is, the continuation of this line to create a circumference, determines in the Pacific Ocean region the international date line, also known as the “antimeridian”. This means that at that point on the planet each new day of the calendar begins.

Standard meridian and time zones

Meridians divide the Earth into 24 identical regions, each of which is considered a time zone that is, a region that shares a time slot of Coordinated Universal Time (UTC) or world standard time. Between two meridians there is a 15° separation on the circumference of the planet.

This planetary division allows the times of each region to be comparable and convertible with each other and in this way have a way to calculate the time in any region of the planet. This calculation is especially useful for international trade and for countries with large territories, whose populations have time differences.

So, Each time zone corresponds to a specific time determined by your standard meridian and calculated from the zero meridian (the Greenwich meridian), adding or subtracting hours depending on how close or far away you are geographically. Thus, the time zone increases in an easterly direction (+1, +2, +3 hours and so on) and decreases in a westerly direction (-1, -2, -3 hours and so on) starting from the zero meridian.

This is why international times are expressed with the abbreviation GMT (Greenwich Meridian Time“Greenwich Mean Time). For example:

• Argentina: GMT -3.
• Bolivia: GMT -4.
• Colombia: GMT -5.
• Mexico: GMT -6.
• Hawaii: GMT -10.
• Norway: GMT +1.
• Madagascar: GMT +3.
• Pakistan: GMT +5.
• China: GMT +8.
• Japan: GMT +7.
• Marshall Islands: GMT +12.

These determinations, however, more than a strictly geographical reason, often respond to political conventions. The territory of some large nations, such as the United States, Russia, or Canada, often spans two or more different time zones.

Meridians and parallels

If the meridians are the imaginary lines that we draw from one pole to the other (that is, in a longitudinal or vertical sense), Parallels are similar imaginary lines but drawn in a latitudinal direction (transverse or horizontal).

That is, the parallels are semicircles that divide the globe in two, and that are separated from each other by sexagesimal degrees: 15°, 30°, 45°, 60° and 75°, both in a positive sense (towards the north pole) and in a negative sense (towards the south pole), using the zero parallel as a starting point, that is, the earth's equator.

The result of this segmentation of the globe are the climatic zones of the planet, that is, the latitudinal regions that present similar climatic features, and that are delimited by the main parallels: the equator, the tropic of Cancer, the tropic of Capricorn, the arctic circle and the antarctic circle. These climatic zones are:

• The intertropical zones located between the equator and the tropics of Cancer and Capricorn.
• The temperate zones located between the Tropics of Cancer and Capricorn, and the Arctic and Antarctic Circles respectively.
• The glacial zones located above the respective polar lines of the Arctic and Antarctic.

Latitude and longitude

From the division of the terrestrial globe by meridians and parallels, it is possible to determine the location of any point through two specific geographical data: longitude (determined by meridians) and latitude (determined by parallels).

• Length. It is the distance (expressed in an arc or an angle) between the selected point and the Greenwich meridian, determined by the meridian that passes through said point. Points located to the east of the zero meridian will have positive longitude (or east, E) and those to the west will have negative longitude (or west, W), on a scale from 0° to 180°.
• Latitude. It is the distance (expressed in an arc or an angle) between the selected point and the line drawn by the equator, determined by the parallel that passes through said point. Points located north of the equator will have North latitude (N) and those located south of the equator will have South latitude (S), on a scale from 0° to 90°.

These two data are combined to form the coordinate system, written in parentheses (latitude, longitude). To refine the location, units smaller than the sexagesimal degree are used, such as minutes (') and seconds (''). For example, the coordinates of some of the main capitals of Europe are:

• London, England: latitude of 51° 30' 30” north and longitude of 0° 7' 32” west (51° 30' 30” N; 0° 7' 32” W).
• Berlin, Germany: latitude of 52° 31′ 28» north and longitude of 13° 24′ 38» east (52° 31′ 28» N; E).
• Paris, France: latitude of 48° 51' 12″ north and longitude of 2° 20' 56″ east (48° 51' 12″ N; 2° 20' 56″ E).
• Madrid, Spain: latitude of 40° 24′ 59» north and longitude of 3° 42′ 09» west (40° 24′ 59» N; 03° 42′ 09» W).
• Rome, Italy: latitude of 41° 53′ 30» north and longitude of 12° 29′ 39» east (41° 53′ 30» N; 12° 29′ 39» E).

References

• “Meridian” on Wikipedia.
• “Parallel” in Wikipedia.
• “Etymology of Meridian” in the Online Spanish Etymological Dictionary.
• “Meridians and parallels” in Rebumbios of the Government of the Canary Islands (Spain).
• “Meridian (geography)” in The Encyclopaedia Britannica.