If our planet is “cut” through the axis of rotation and perpendicular to it by many planes, then vertical and horizontal circles - meridians and parallels - will appear on the surface.
The meridians will converge at two points - at the North and South Poles. Parallels, as the name suggests, are parallel to each other. Meridians serve to measure longitude, parallels - latitude.
An action so simple at a superficial glance - “ruling out” the Earth - became the greatest discovery in the study of the planet. It made it possible to use coordinates and accurately describe the location of any object. Without parallels and meridians it is impossible to imagine a single map or a single globe. And they were invented... in the 3rd century BC by the Alexandrian scientist Eratosthenes.
Reference. Eratosthenes had encyclopedic knowledge in all areas at that time. He was in charge of the legendary Library of Alexandria, wrote the work “Geography” and became the founder of geography as a science, compiled the first map of the world and covered it with a degree grid of verticals and horizontals - he invented a coordinate system. He also introduced names for lines - parallel and meridian.
Meridian
In geography, a meridian is half a sectional line of the earth's surface drawn through any point on the surface. All imaginary meridians, of which there can be an infinite number, connect at the poles - North and South. The length of each of them is 20,004,276 meters.
Although you can mentally draw as many meridians as you like, for ease of movement and mapping, their number and location have been regulated by international treaties. In 1884, at the International Meridian Conference in Washington, it was decided that the prime meridian (zero) would be the one that passes through Greenwich, a county in southeast London.
However, not everyone immediately agreed with this decision. For example, in Russia, even after 1884 until the beginning of the twentieth century, the zero meridian was considered to be its own - Pulkovsky: it “passes” through the Round Hall of the Pulkovo Observatory.
Prime Meridian
The prime meridian is the starting point of geographic longitude. He himself, accordingly, has zero longitude. This was the case before the creation of the world's first satellite navigation system, Transit. 
With its appearance, the prime meridian had to be shifted slightly - 5.3" relative to Greenwich. This is how the International Reference Meridian appeared, which is used as a reference point for longitude by the International Earth Rotation Service.
Parallel
In geography, parallels are lines of an imaginary section of the surface of the planet by planes that are parallel to the equatorial plane. The parallels depicted on the globe are circles parallel to the equator. They are used to measure geographic latitude.
By analogy with the Greenwich prime meridian, there is also a zero parallel - this is the equator, one of the 5 main parallels, which divides the Earth into hemispheres - southern and northern. Other main parallels are the tropics North and South, the polar circles - North and South.
Equator
The longest parallel is the equator - 40,075,696 m. The rotation speed of our planet at the equator is 465 m/s - this is much greater than the speed of sound in air - 331 m/s.
Southern and Northern tropics
The Tropic of the South, also called the Tropic of Capricorn, is located south of the equator and is the latitude above which the noon sun is at its zenith on the day winter solstice.
The Northern Tropic, also known as the Tropic of Cancer, is located north of the equator and, similar to the southern Tropic, represents the latitude above which the midday sun is at its zenith on the day of the summer solstice.
Arctic Circle and Antarctic Circle
The Arctic Circle is the boundary of the polar day region. To the north of it, in any place at least once a year the sun is visible above the horizon 24 hours a day or not visible for the same amount of time.
The Southern Arctic Circle is similar to the Northern Circle in every way, only it is located in the southern hemisphere.
Degree grid
The intersections of meridians and parallels form a degree grid. Meridians and parallels are spaced at intervals of 10° - 20°; smaller divisions, as in angles, are called minutes and seconds. 
Using a degree grid, we determine the exact location of geographic objects - their geographic coordinates, calculating longitude using meridians, and latitude using parallels.
In the 4th century. BC e. greatest thinker In ancient times, Aristotle proved that our planet has a shape very close to the shape of a sphere.
Around the same time, observing the visible movement of the stars and the Sun while traveling in various places, ancient scientists established certain conventional lines for orientation on the earth's surface.
Let's go on a mental journey across the surface of the Earth. The position above the horizon of the imaginary axis of the world, around which the daily rotation of the heavenly vault occurs, will change for us all the time. In accordance with this, the pattern of movement of the starry sky will change.
Traveling north, we will see that the stars in the southern part of the sky rise to a lower altitude every night. And the stars in the northern part - at the lower culmination - have a greater height. If we move long enough, we will get to the North Pole. Here, not a single star rises or falls at all. It will seem to us that the whole sky is slowly spinning parallel to the horizon.
Ancient travelers did not know that the apparent movement of stars was a reflection of the rotation of the Earth. And they have not been to the Pole. But they needed to have a landmark on the earth's surface. And for this purpose they chose the north-south line, easily determined by the stars. This line is called the meridian.
Meridians can be drawn through any points on the surface of the Earth. Many meridians form a system of imaginary lines connecting the North and South Poles of the Earth, which are convenient to use for determining location.
Let's take one of the meridians as the initial one. The position of any other meridian in this case will be known if the reference direction is indicated and the dihedral angle between the desired meridian and the initial one is specified.
Currently, by international agreement, it has been agreed to consider the initial meridian to be the one that passes through one of the oldest astronomical observatories in the world - the Greenwich Observatory, located on the outskirts of London. The angle formed by any meridian with the initial one is called longitude. The longitude, for example, of the Moscow meridian is 37° east of Greenwich.
To distinguish points lying on the same meridian from each other, we had to enter a second geographic coordinate - latitude. Latitude is the angle that a plumb line drawn at a given location on the Earth’s surface makes with the plane of the equator.
The terms longitude and latitude came to us from ancient sailors who described the length and width of the Mediterranean Sea. The coordinate that corresponded to the measurements of the length of the Mediterranean Sea became longitude, and the one that corresponded to the width became modern latitude.
Finding latitude, like determining the direction of the meridian, is closely related to the movement of the stars. Already ancient astronomers proved that the height of the celestial pole above the horizon is exactly equal to the latitude of the place.
Let's assume that the Earth has the shape of a regular ball, and let's dissect it along one of the meridians, as in the figure. Let the person depicted in the picture as a light figure stand at the North Pole. For him, the upward direction, i.e. the direction of the plumb line, coincides with the axis of the world. The celestial pole is directly above his head. The height of the celestial pole here is 90.
Since the apparent rotation of stars around the axis of the world is a reflection of the real rotation of the Earth, then at any point on the Earth, as we already know, the direction of the axis of the world remains parallel to the direction of the axis of rotation of the Earth. The direction of the plumb line changes when moving from point to point.
Let's take, for example, another person (a dark figure in the picture). The direction of the world axis remained the same as the first one. And the direction of the plumb line has changed. Therefore, the height of the celestial pole above the horizon here is not 90°, but much less.
From simple geometric considerations it is clear that the height of the celestial pole above the horizon (angle ft in the figure) is indeed equal to latitude (angle φ).
The line connecting points with the same latitudes is called a parallel.
Meridians and parallels form the so-called system geographical coordinates. Each point on the earth's surface has a well-defined longitude and latitude. And vice versa, if latitude and longitude are known, then one parallel and one meridian can be constructed, at the intersection of which one will get one single point.
Understanding the features of the daily movement of stars and the introduction of a system of geographical coordinates made it possible to carry out the first determination of the radius of the Earth. It was made in the second half of the 3rd century. BC e. famous mathematician and geographer Eratosthenes.
The principle of this definition is as follows. Suppose we were able to measure the difference in latitude of two points lying on the same meridian (see figure). Thus, we became aware of the angle Df with the vertex in the center of the Earth, which corresponds to the arc of the meridian L on the surface of the Earth. If we can now also measure the arc L, then we will obtain a sector with a known length of the arc and the corresponding central angle. This sector is shown separately in the figure. By simple calculations, you can obtain the radius of this sector, which is the radius of the Earth.
Eratosthenes, a Greek by nationality, lived in the rich Egyptian city of Alexandria. South of Alexandria there was another city - Siena, which today is called Aswan and where, as is known, with the help Soviet Union The famous high dam was built. Eratosthenes knew that Syene had interesting feature. At noon on one June day, the Sun over Siena is so high that its reflection is visible at the bottom of even very deep wells. From this Eratosthenes concluded that the altitude of the Sun in Syene on that day was exactly 90°. In addition, since Siena lies strictly south of Alexandria, then they are on the same meridian.
For an unusual measurement, Eratosthenes decided to use a scaphis - a cup-shaped sundial with a pin and divisions inside it. Installed vertically, these sundial using the shadow from the pin makes it possible to measure the height of the Sun above the horizon. And at noon of that very day when the Sun rose so high above Siena that all objects stopped casting shadows. Eratosthenes measured its height in the city square of Alexandria. The altitude of the Sun in Alexandria, according to the measurements of Eratosthenes, turned out to be 82° 48". Therefore, the difference in latitude between Alexandria and Syene is 90° 00" - 82° 48" = 7° 12".
All that remained was to measure the distance between them. But how to do that? How to measure a distance on the Earth's surface that is approximately 800 km in modern units?
The difficulties of such an undertaking were then literally innumerable.
Indeed, how to make such a giant ruler with which one could make measurements? How can we ensure that for 800 km this ruler is laid strictly along the meridian, without any distortions?
The necessary data about the distance between the cities had to be taken from the stories of merchants who led trade caravans from Alexandria to Siena. The merchants said that the distance between them was approximately 5,000 Greek stadia. Eratosthenes accepted this value as true and, using it, calculated the radius of the Earth.
If we compare the value obtained by Eratosthenes with modern data, it turns out that he was mistaken relatively little - only by 100 km.
So, from the 3rd century. BC BC, from the time of Eratosthenes, the paths of astronomy and geodesy intertwined - another ancient science that studies the shape and size of both the entire Earth as a whole and its individual parts.
Methods for astronomical determinations of latitudes developed and improved. This was especially important, in particular, precisely in connection with the need to more carefully determine the size of the Earth. For, starting with the same Eratosthenes, it was clear that the task of determining the size of the Earth falls into two parts: astronomical, i.e., determining the difference in latitude, and geodetic, i.e., determining the length of the meridian arc. Eratosthenes managed to solve the astronomical part of the problem, and many of his followers followed essentially the same path.
We will still have occasion to talk about more accurate measurements of the size of the Earth, but for now, having become accustomed to determining latitudes, we will tackle a much more complex matter - determining geographic longitudes.
Globe and geographic Maps“entangled” in a kind of grid consisting of intersecting lines. These lines did not appear on maps immediately, since in ancient times maps resembled simple plans.
The globe and its section planes
The Earth is a ball slightly flattened at the poles. The ball can be cut by planes in different directions. It can be cut, firstly, in the same way as an orange is divided into slices, and, secondly, in the same way as an orange is cut across the slices with a knife. With any method of dissecting a ball with planes, circles are obtained, the boundaries of which are circles. The diameter of the circles is greatest if the section planes pass through the center of the ball. The diameters of such circles are equal to the diameter of the ball.
Let's turn to and mentally dissect Earth planes perpendicular to the Earth's axis of rotation. Circles parallel to each other appear on the surface of the globe. These circles are called parallels (from the Greek word parallclos - walking next to each other). The longest and main parallel is the equator, its length is 40,076 kilometers.
The equator is located at equal distances from the planet's poles and divides the Earth into the Northern and Southern Hemispheres. The length of other parallels decreases towards the south and north from the equator. All points lying on the same parallel are equally distant from the equator. The parallel lines show the west-east direction.
If you cut the globe with planes that pass through the axis of rotation of the Earth, then meridians will appear on the surface of the globe - semicircles connecting the North and South poles of the Earth. They are perpendicular to the parallels and show the north-south direction. The word “meridian” itself means “midday” (from the Latin word meridianus), since the direction of all meridians coincides with the direction of the shadow of objects at noon.
All meridians have the same length - 20,005 kilometers. By agreement between the countries, the main, prime meridian is considered to be the meridian passing through the Greenwich Observatory in the suburbs of London. Therefore, this meridian is also called Greenwich. Greenwich meridian and its continuation on the opposite side
The globe divides the Earth into the Western and Eastern Hemispheres.
Parallels and meridians on maps
The parallels on the globe are circles, and the meridians are semicircles. But due to distortions when transferring the convex surface of the Earth onto a plane, the image of these lines looks different. Whatever the appearance of the parallels and meridians, on any map the directions to the east and west are determined only by the direction of the parallels, and to the north and south - only by the direction of the meridians. Thus, parallels and meridians allow one to orient themselves, that is, determine directions to the sides of the horizon.
You can draw as many lines of parallels and meridians on the globe and maps as you like. But only one meridian and one parallel pass through one point on the surface. The position of any point on a flat sheet can be characterized by two coordinate numbers that show the position of this point relative to the edges of the sheet.
On a spherical surface, the coordinates of points are determined with respect to the equator and the prime meridian. For this, a system of parallels and meridians is used.
The magnetic meridian is a curved line in the form of an arc, vertically enveloping the geomagnetic sphere of the Earth, located along the lines of force of the earth's geomagnetic field. All magnetic meridians converge at a single point of the north and south magnetic poles, which do not coincide with the geographical poles.
Basic Concepts
At any point on the planet, the compass needle will be located precisely along the magnetic meridian, and not the geographical one, if there are no interference or anomalies nearby. The angle that makes up the difference in direction between these meridians is called magnetic declination. It will be different for each location.
The difference between the magnetic meridian and the geographical one
The compass needle in working condition does not indicate the exact north direction, but only an approximate one - this is sometimes very important to take into account. Since the force poles are deviated from the geographic ones, the meridian indicated by the compass was defined as magnetic, and the geographic one as true, so as not to confuse these values.
The azimuth found using a compass will similarly differ in degrees from the true azimuth, since the calculation takes into account the angle between the position of the arrow and the direction to the object. And true azimuth is the angle between the geographic meridian and the existing direction to the same object. 
When making navigational calculations for military or sailors, this difference is significant, but also in travel conditions it will not be superfluous to know how to correct the degrees of angle for the true azimuth, that is, by the value of the geomagnetic declination. This is necessary when the route azimuth was initially calculated from the map and turned out to be true, but on the way, when orienting to an object using a compass needle, it is the geomagnetic one that will be calculated.
Many maps indicate the geomagnetic declination value for areas outside the boundaries, but sometimes they do not print it. Then you need to find out it in advance before the trip, having studied the data in the directory, and then put it on a map of the area through which the path will lie. These data are constantly coming from geomagnetic observatories that regularly monitor changes in the Earth’s geomagnetic field, as well as displacements of power peaks.
- Declension can be Western or Eastern. With an eastern declination, the needle will deviate east of the true meridian, with a western declination, it will deviate to the west.
- When making an adjustment for the eastern declination, the degrees of deviation are subtracted from the azimuth calculated on the ground using a compass in order to reach the value of the true azimuth calculated at the beginning of the journey with the map. Usually the eastern declination is marked with a “-”.
- When making an adjustment for the western declination, the degrees of deviation are added, and it is marked as such - with a “+” sign.
- Thus, the route will pass strictly according to the azimuth calculated using the map.
Advice! If for the desired area the declination value does not exceed 10º, and the route is not very long, it is not necessary to make an orientation correction.
North and south magnetic poles
Our Earth has two geographic poles, indicating both ends of the planet's rotation axis - north and south. It is to them that the geographic meridians converge at one point, drawing arcs on the globe or straight lines on an enlarged map of the area.
However, our planet is shrouded in a geomagnetic field, the lines of force of which similarly have poles - north and south. Their points are constantly shifting relative to geographic coordinates; moreover, during the day they describe a kind of oval.
- IN this moment coordinates geographical location of the north magnetic pole are located in the Canadian Arctic under a layer of ice, at a northern latitude of 86º and a western longitude of 147º. Since 2001, the shift of the geomagnetic northern peak in latitude has been 5º, in longitude - 37º.
- The geographic coordinates of the south magnetic pole are located near the very edge of Antarctica, at latitude 64º south and longitude 137º east. Since 1998, the southern peak has shifted in latitude by fractions of a degree - only 11 minutes, and in longitude - by only 1º.
By the way! From the point of view of magnetic physics, the north pole of force is the south pole, since the south tail of the magnetized compass needle is repelled from it. Accordingly, the south power pole, according to the laws of physics, is the north one, since, on the contrary, it attracts the south tail of the magnetized compass needle.
Today there is not a single area left on Earth that has not been studied by man or at least visited! How more information appeared about the surface of the planet, the more pressing the question arose of determining the location of this or that object. Meridians and parallels, which are elements of the degree grid, help to find the geographic address of the desired point and facilitate the process of orienting on the map.
History of cartography
Humanity did not immediately come to this simple way determining the coordinates of an object, such as calculating its longitude and latitude. Familiar to all of us from school, the main lines gradually appeared in the sources of cartographic knowledge. Below is information about several key stages in the history of the formation of such sciences as geography and astronomy, which led civilization to the creation of a modern map with a convenient degree grid.
- One of the “founders” of natural sciences is Aristotle, who was the first to prove that our planet has a spherical shape.
- The ancient travelers of the Earth were very observant, and they noticed that in the sky (according to the stars), the direction N (north) - S (south) can be easily traced. This line became the first “meridian”, an analogue of which today can be found on the simplest map.
- Eratosthenes, who is better known as the “father of the science of geography,” made many small and large discoveries that influenced the development of geodesy. He was the first to use a skafis (ancient sundial) to calculate the height of the sun over an area different cities and noticed a significant difference in my measurements, which depended on the time of day and season. Eratosthenes identified the connection between sciences such as geodesy and astronomy, thereby making it possible to carry out many studies and measurements of terrestrial territories using celestial bodies.
Degree grid
Numerous meridians and parallels, intersecting on a map or globe, are connected into a geographical grid consisting of “squares”. Each of its cells is limited by lines that have their own degree. Thus, using this grid you can quickly find the desired object. The structure of many atlases is designed in such a way that different squares are considered on separate pages, which allows you to systematically study any territory. With the development of geographical knowledge, the globe also improved. Meridians and parallels are available on the very first models, which, although they did not contain all the reliable information about the objects of the Earth, already gave an idea of the approximate location of the desired points. Modern maps have mandatory elements that make up the degree grid. Using it, coordinates are determined.
Elements of the degree grid
- The poles - North (above) and South (below) - are the points at which the meridians converge. They are the exit points of a virtual line called an axis.
- Polar circles. The boundaries of the polar regions begin with them. The Arctic Circles (Southern and Northern) are located beyond the 23rd parallel towards the poles.
- It divides the surface of the Earth into Eastern and and has two more names: Greenwich and Primary. All meridians have the same length and connect the poles on the surface of a globe or map.
- Equator. It is oriented from W (west) to E (east), which divides the planet into the Southern and Northern hemispheres. All other lines parallel to the equator have different sizes- their length decreases towards the poles.
- Tropics. There are also two of them - Capricorn (Southern) and Cancer are located on the 66th parallel south and north of the equator.
How to determine the meridians and parallels of the desired point?
Any object on our planet has its own latitude and longitude! Even if it is very, very small or, conversely, quite large! Determining the meridians and parallels of an object and finding the coordinates of a point is the same action, since it is the degree of the main lines that determines the geographic address of the desired territory. Below is a plan of action that can be used when calculating coordinates.
Algorithm for the address of an object on the map
- Check the correctness geographical name object. Annoying mistakes happen due to simple inattention, for example: a student made a mistake in the name of the desired point and determined the wrong coordinates.
- Prepare an atlas, a sharp pencil or pointer and a magnifying glass. These tools will help you more accurately determine the address of the desired object.
- Select the largest-scale map from the atlas on which the desired geographical point. The smaller the map scale, the more errors appear in the calculations.
- Determine the relationship of the object to the main mesh elements. The algorithm for this procedure is presented after the point: “Calculating the size of the territory.”
- If the desired point is not located directly on the line marked on the map, then find the nearest ones, which have a digital designation. The degree of lines is usually indicated along the perimeter of the map, less often - on the equator line.
- When determining coordinates, it is important to find out how many degrees the parallels and meridians are located on the map and correctly calculate the required ones. It must be remembered that the elements of the degree grid, except for the main lines, can be drawn through any point on the Earth’s surface.
Calculating the size of the territory
- If you need to calculate the size of an object in kilometers, then you need to remember that the length of one degree of grid lines is 111 km.
- To determine the extent of an object from W to E (if it is completely located in one of the hemispheres: Eastern or Western), it is enough to use greater value latitude of one of extreme points, subtract the smaller one and multiply the resulting number by 111 km.
- If you need to calculate the length of a territory from N to S (only if it is all located in one of the hemispheres: Southern or Northern), then you need to subtract the smaller one from the larger degree of longitude of one of the extreme points, then multiply the resulting amount by 111 km .
- If the Greenwich meridian passes through the territory of an object, then to calculate its length from W to E, the degrees of latitude of the extreme points of a given direction are added, then their sum is multiplied by 111 km.
- If the equator is located on the territory of the designated object, then to determine its extent from N to S it is necessary to add the degrees of longitude of the extreme points of this direction, and multiply the resulting sum by 111 km.
How to determine the relationship of an object to the main elements of the degree grid?
- If the object is located below the equator, then its latitude will be only southern, if above - northern.
- If the desired point is located to the right of the prime meridian, then its longitude will be eastern, if to the left - western.
- If an object is located above the 66th degree north or south parallel, then it enters the corresponding polar region.
Determining the coordinates of mountains
Since many mountain systems have a large extent of different directions, and the meridians and parallels intersecting such objects have different degree values, then the process of determining their geographical address is accompanied by many questions. Below are options for calculating the coordinates of the high territories of Eurasia.
Caucasus
The most picturesque mountains are located between two water areas of the mainland: from the Black Sea to the Caspian Sea. Meridians and parallels have different degrees, so which ones should be considered determining for the address of a given system? In this case, we focus on the highest point. That is, the coordinates of the Caucasus mountain system are the geographical address of Elbrus peak, which is equal to 42 degrees 30 minutes north latitude and 45 degrees east longitude.
Himalayas
The most high system mountains on our continent - the Himalayas. Meridians and parallels, having different degrees, intersect this object as often as the above-mentioned one. How to correctly determine the coordinates of this system? We do the same as in the case of the Ural Mountains, we focus on the highest point of the system. Thus, the coordinates of the Himalayas coincide with the address of the Qomolungma peak, and it is 29 degrees 49 minutes north latitude and 83 degrees 23 minutes and 31 seconds east longitude.
Ural Mountains
The longest on our continent are the Ural Mountains. Meridians and parallels, having different degrees, intersect a given object in various directions. To determine coordinates Ural mountains you need to find their center on the map. This point will be the geographic address of this object - 60 degrees north latitude and the same eastern longitude. This method of determining the coordinates of mountains is acceptable for systems that have a large extent in one of the directions or in both.
