Below are the most basic and easily understandable concepts of longitude and latitude, as well as how to determine these coordinates on a map. Let’s take a look.
What is Longitude?
Prime Meridian, 180° Longitude (International Date Line) and the location of Greenwich Observatory.
Longitude is a half-circle on the surface of the Earth that connects the two poles, measuring approximately 20,000 km in length. It runs in a north-south direction and intersects the equator at a right angle. The plane of the 0° longitude (which passes through the Greenwich Observatory in London) and the 180° longitude divides the Earth into two hemispheres – the Eastern Hemisphere and the Western Hemisphere.
Longitudes connecting the magnetic poles are referred to as magnetic longitudes, while those connecting the geographic poles are known as geographic longitudes. The lines of longitude depicted on maps are called map longitudes.
This type of longitude is also known as geographic longitude to distinguish it from magnetic longitude, which is the intersection of the Earth’s surface and the plane that passes through the magnetic North and South poles.
What is Latitude?
On Earth or other spherical planets, latitude is an imaginary circle connecting all points that share the same latitude. On Earth, this circle runs from east to west. The position on a latitude line is determined by longitude. Each latitude line is always perpendicular to a longitude line at their intersection. Latitude lines closer to the poles have smaller diameters.
Five special latitude lines of Earth.
There are five special latitude lines on Earth. Four of these lines are defined based on the tilt of the Earth’s axis relative to its orbital plane around the Sun. The fifth line, the equator, lies between the two poles. They are:
- Arctic Circle (66° 33′ 38″ N)
- Tropic of Cancer (23° 26′ 22″ N)
- Equator (0° N)
- Tropic of Capricorn (23° 26′ 22″ S)
- Antarctic Circle (66° 33′ 38″ S)
The Tropic of Cancer and Tropic of Capricorn are the northern and southern boundaries of the area on Earth where the Sun can be seen directly overhead at least once during the year. The Arctic and Antarctic Circles mark the regions surrounding the poles where the Sun can be seen for at least one day during the summer solstice.
Latitude lines are often used as flight paths, but with the exception of the equator, they are not great circles and therefore do not represent the shortest distance between points. This contrasts with how they are depicted on some maps, where they appear as straight lines. Flights in the northern hemisphere between points of the same latitude will follow the shortest path, appearing as a curve that deviates northward on such maps.
Latitude lines on Earth are sometimes used as borders between countries and territories. Some notable latitude borders include:
- The border between Canada and the United States is almost entirely along the 49° N latitude, except for the section between Quebec and Vermont, which lies along the 45° N latitude.
- The 38° N latitude is used to separate North Korea and South Korea.
- The 17° N latitude is used to divide Vietnam as per the Geneva Accords.
- The 60° S latitude is used to delineate the border for Antarctica.
Currently, Earth has 181 latitude lines (including the equator as a special latitude).
How to Determine Longitude and Latitude on a Map
Open a map or rotate a globe, and you will notice there are lines running both horizontally and vertically in a systematic manner. Some lines are straight, while others are curved; these lines are referred to as longitude and latitude.
Their utility is significant; once we can establish the longitude and latitude lines, we can conveniently represent any location in the area. Especially when flying over vast oceans, deserts, or through dense forests, pinpointing the current position accurately relies on longitude and latitude lines.
So, how are longitude and latitude determined?
We know that the Earth rotates around its axis. The Earth’s axis is an imaginary line connecting the North and South Poles and passing through the center of the Earth. If we are at a location along this axis and create a plane perpendicular to it, similar to cutting a watermelon in half, each section represents the Northern and Southern Hemispheres. The intersection of this plane and the Earth’s surface is a large circle known as the largest latitude circle, which in geography is termed the equator. This allows us to orient towards the North and South Poles. On maps, many lines are drawn parallel to the equator, known as latitude lines. We consider the equator to be at 0° latitude, and each direction south and north corresponds to 90°. The equator moving south is referred to as southern latitude, while moving north is referred to as northern latitude. 90° northern latitude corresponds to the North Pole, and 90° southern latitude corresponds to the South Pole.
From the North Pole to the South Pole, many semicircles can be drawn, which represent longitudes. However, how are these longitudes divided? Initially, there was no consensus on this. Countries typically designated the longitude passing through their capital as 0°, using it as a starting point for calculating longitude. In 1884, countries attended the first International Longitude Conference held in Washington, which established the longitudinal line running through the observatory in Greenwich (Calini in the southeastern outskirts of London, England) as the common starting point for calculating longitude worldwide, also defining longitude 0°, or the Prime Meridian. From this line, equally dividing east and west gives 180°. Eastward is termed eastern longitude, while westward is termed western longitude. Thus, 180° eastern longitude and 180° western longitude are essentially on the same straight line, often referred to as the 180° longitude. On maps, this line is used to delineate the international date changes, fundamentally serving as the standard line.
For instance, if I tell you that the latitude of Beijing is 39°54′ N, and the longitude is 116°9′ E, you can immediately locate it on the map.
There is a method to roughly determine the longitude and latitude of an area. By observing the angle of the North Star above the horizon at night, you can approximate the latitude of that area. For example, the North Star in Beijing appears approximately 40° above the ground, indicating a latitude of about 40° N. The longitude can be calculated using the time difference between local time and world time. For instance, if Beijing is 7 hours and 46 minutes ahead of world time, we know that on Earth, a one-hour time difference corresponds to a 45° longitude difference. Thus, we can estimate Beijing’s longitude to be around 116.5° E.
Distinguishing Latitude and Longitude
Many people confuse the concepts of longitude and latitude, leading to difficulties in determining positions on a map. To differentiate them simply, you can rely on the following factors:
- There are 360 longitudes, while there are only 181 latitudes.
- Longitudes converge at the North and South Poles, while latitudes run parallel to each other and never intersect.
How to Determine Position Based on Latitude and Longitude
Determining a position based on longitude and latitude is quite simple. First, you need to identify where the North Pole, South Pole, and the equator are located in relation to the longitude and latitude.
Latitude lines run parallel to the equator; the equator is at 0° latitude, with each direction south and north being 90°. The equator extending south is termed southern latitude, while extending north is termed northern latitude.
The Prime Meridian is defined as the line running through the Greenwich Observatory in London, establishing it as longitude 0, also called the Prime Meridian. From this line, east and west are divided equally at 180°, with the east termed eastern longitude and the west termed western longitude. This line is also used to delineate the changes in international dates, serving as the standard line.
What are the Tropics?
The tropics refer to the two latitudes at +23°26’22” and -23°26’22” north and south of the equator. The Tropic of Cancer is also known as the Tropic of Cancer, while the Tropic of Capricorn refers to the Tropic of Capricorn.
On the Summer Solstice (June 21-22), the Sun is directly overhead at noon at the Tropic of Cancer, and on the Winter Solstice (December 21-22), the Sun is directly overhead at noon at the Tropic of Capricorn. Throughout the year, the Sun will be directly overhead twice within the area between the two tropics.
