The times when the Earth was considered flat and located on the backs of elephants are long gone. Many scientists and astronomers, even in ancient times, argued that the Earth has the shape of a ball and rotates around its axis.
And even today this fact is known to everyone almost from childhood. And if you answer the question why our Earth is round, it will be necessary to consider several significant facts and factors that influence the shape of the planet.
The influence of the composition of planet Earth on its shape
The Earth has the shape of a ball, like all other cosmic bodies that have a large mass. And this phenomenon is directly related to the force of gravity, which governs the movement of almost all space objects. In this case, the greater mass of the cosmic body corresponds to a greater force of attraction.
All large planets of near-Earth space (Moon, Sun, etc.) have enormous mass, which also implies an increased gravitational force. The surface of our planet is exposed to gravitational force, due to which the Earth acquires the spherical shape we observe. Moreover, the same gravitational force ensures that each point on the Earth’s surface is equally distant from its center.
No less important is the presence of one of the components that make up planet Earth, namely, hot magma located under the crust and periodically appearing on the surface of the Earth in the form. Without this, the force of gravity would not have such an impact on creating the shape of our planet - for this, the cosmic body must be optimally plastic, for example, gaseous or liquid.
But here you can make a small amendment, clarifying that in fact calling the Earth round would also not be entirely correct. And there is some significant evidence for this.
Justification why the Earth is round
The Earth's polar radius is 6357 kilometers, its equatorial radius is 6378 kilometers, which is a difference of as much as 19 kilometers. Therefore, calling the planet an absolute sphere would be a little incorrect, since it rather has the shape of a sphere, slightly flattened at the poles and stretched along the Equator line. The movement of the Earth around its axis and the resulting presence of centrifugal force play a role here.
The increase in centrifugal force, which confidently resists the force of the earth's extension, depends on the distance of specific points from the poles. And thanks to the considerable speed of the planet’s natural rotation around its axis, the speed of any point on the earth’s equator can be compared to the speed of a supersonic aircraft.
Also, the Earth cannot be perfectly round due to the fact that hot magma, as a type of liquid, is present only under the crust of the earth’s surface, and the crust itself is a solid substance. If the Earth's surface were entirely composed of liquid, it might well have the exact shape of a sphere.
But it is worth noting that the liquid located on the surface of the Earth is also affected by certain phenomena - more precisely, the gravitational force of other celestial objects. For example, the force of gravity, which can create ebbs and flows, slightly bending the shape of the liquid shell of the globe.
The Sun, stars, Earth, Moon, all the planets and their large satellites are “round” (spherical) because they have a very large mass. Their own force of gravity (gravity) tends to give them the shape of a ball.
If some force gives the Earth the shape of a suitcase, then at the end of its action the gravitational force will again begin to collect it into a ball, “pulling in” the protruding parts until its entire surface is established (i.e., stabilized) at an equal distance from the center.
Why doesn't the suitcase take the shape of a ball?
In order for a body to become spherical under the influence of its own gravitational force, this force must be sufficiently large, and the body must be sufficiently plastic. Preferably liquid or gaseous, since gases and liquids most easily take the shape of a ball when they accumulate a large mass and, as a result, gravity. Planets, by the way, are liquid inside: under a thin layer of solid crust they have liquid magma, which even sometimes pours out onto their surface - during volcanic eruptions.
All stars and planets have a spherical shape from birth (formation) and throughout their existence - they are quite massive and plastic. For smaller bodies - for example, asteroids - this is not the case. Firstly, their mass is much less. Secondly, they are entirely solid. If, for example, the asteroid Eros had the mass of the Earth, it would also be round.
The earth is not quite a ball
Firstly, the Earth rotates around its axis, and at a fairly high speed. Any point on the earth's equator moves at the speed of a supersonic plane (see the answer to the question “Is it possible to overtake the sun?”). The further from the poles, the greater the centrifugal force opposing the force of gravity. Therefore, the Earth is flattened at the poles (or, if you prefer, stretched out at the equator). It is flattened, however, quite a bit, by about one three-hundredth: the equatorial radius of the Earth is 6378 km, and the polar radius is 6357 km, only 19 kilometers less.
Secondly, the surface of the earth is uneven, there are mountains and depressions on it. Still, the earth's crust is solid and retains its shape (or rather, changes it very slowly). True, the height of even the highest mountains (8-9 km) is small compared to the radius of the Earth - a little more than one thousandth.
For more information about the shape and size of the Earth, see (you will find out what geoid, ellipsoid of revolution And Krasovsky's ellipsoid).
Thirdly, the earth is subject to gravitational forces from other celestial bodies - for example, the Sun and the Moon. True, their influence is very small. And yet, the gravitational force of the Moon is capable of slightly (several meters) bending the shape of the liquid shell of the Earth - the World Ocean - creating ebbs and flows.
Have you ever thought why is the earth round? Why is the Earth not flat, as previously thought, or, say, not square...? Why a ball? And finally, what gave our planet its spherical shape?
We need to start with the fact that a ball is not a rare shape at all; on the contrary, a ball is practically the most common shape of objects in Universe. All stars, planets, planetary satellites, large asteroids are round, or rather spherical. This is due to one of the fundamental forces operating in the Universe - gravity.
The force of gravity.
Gravity is a very interesting force. It dominates in the macrocosm, controlling the movements of planets, stars and even entire galaxies, but is almost completely absent in the microcosm, and does not have any effect on microobjects, for example atoms. This is explained by the fact that the force of attraction (gravity) directly depends on the mass of the object; the greater the mass, the greater the force, and vice versa.
It is thanks to the force of gravity that all large objects in the Universe have ball shape, since their attractive force is so great that it seems to pull in and/or push out individual parts of the body until the entire surface is established at the same distance from the center. Moreover, this force is constant and acts throughout the entire existence of the object, in other words, if for some incredible reason the Earth acquires any other shape other than a ball, for example a cube, the force of gravity will eventually give the Earth a spherical shape again.
Why aren't all objects round?
If you carefully read the two previous paragraphs, you should understand that only those objects that have a very large mass and, accordingly, gravitational force become round (spherical). But there is one more nuance here. Astronomers know a large number of huge asteroids and dwarf planets that have sufficient mass, but for some reason have a spherical shape. This can be explained quite simply, asteroids, unlike stars and planets, consist entirely of stone and/or metal (stars and planets almost entirely consist of liquid matter: molten metals, gases..., and only in rare cases are planets covered with a thin solid substance ). This makes it much more difficult for gravity to change the shape of a solid object, but even then, gravity will tend to make the body round, but it will take much longer.
The earth is not completely round.
Well, this is no longer a secret: the Earth is not a perfect sphere! The shape of the Earth is more like an ellipse slightly flattened at the poles; in the scientific world this “figure” is called geocide. In addition, individual parts of the Earth's surface are elevated or depressed against the background of the general level. The reason for this is also gravity, but not of the Earth, but of its nearest neighbor - Moon. The moon constantly rotates around our planet and also constantly attracts the earth's surface to itself, causing ebbs and flows in the sea, and uneven terrain on land.
We live in amazing times. Most of the celestial bodies of the Solar System have been explored by NASA probes, GPS satellites are circling above the Earth, ISS crews are steadily flying into orbit, and returning rockets are landing on barges in the Atlantic Ocean.
Nevertheless, there is still a whole community of people who are sure that the Earth is flat. Reading their statements and comments, you sincerely hope that they are all just trolls.
Here are some simple proofs that our planet is round.
Ships and horizon
If you visit any port, look at the horizon and watch the ships. As the ship moves away, it doesn't just get smaller and smaller. It gradually disappears over the horizon: first the hull disappears, then the mast. Conversely, approaching ships do not appear on the horizon (as they would if the world were flat), but rather emerge from the sea.
But the ships do not emerge from the waves (with the exception of the “Flying Dutchman” from “”). The reason approaching ships look like they are slowly rising over the horizon is because the Earth is not flat, but round.
Varying Constellations
Paranal Observatory in ChileDifferent constellations are visible from different latitudes. This was noticed by the Greek philosopher Aristotle back in 350 BC. e. Returning from a trip to Egypt, Aristotle wrote that “in Egypt and<…>there are stars in Cyprus that are not visible in the northern regions.”
The most striking examples are the constellations Ursa Major and the Southern Cross. Ursa Major, a bucket-shaped constellation of seven stars, is always visible at latitudes above 41° north latitude. Below 25° south latitude you will not see it.
Meanwhile, you will discover the Southern Cross, a small constellation of five stars, only when you reach 20° north latitude. And the further south you move, the higher above the horizon the Southern Cross will be.
If the world were flat, we could see the same constellations from anywhere on the planet. But that's not true.
You can repeat Aristotle's experiment when you go on a trip. These for Android and iOS will help you discover constellations in the sky.
Lunar eclipses
Stages of a lunar eclipse / wikimedia.org Another proof of the sphericity of the Earth, found by Aristotle, is the shape of the Earth's shadow on the Moon during an eclipse. During an eclipse, the Earth comes between the Moon and the Sun, blocking the Moon from sunlight.
The shape of the Earth's shadow that falls on the Moon during eclipses is completely round. This is why the Moon becomes a crescent.
Shadow length
The first person to calculate the circumference of the earth was a Greek mathematician named Eratosthenes, who was born in 276 BC. e. He compared the length of the shadows on the day of the summer solstice in Siena (this Egyptian city is today called Aswan) and Alexandria located to the north.
At noon, when the sun was directly above Siena, there were no shadows. In Alexandria, a stick placed on the ground cast a shadow. Eratosthenes realized that if he knew the angle of the shadow and the distance between cities, he could calculate the circumference of the globe.
On a flat Earth there would be no difference between the lengths of shadows. The position of the Sun would be the same everywhere. Only the sphericity of the planet explains why the position of the Sun is different in two cities at a distance of several hundred kilometers from each other.
Observations from above
Another obvious proof of the spherical shape of the Earth: the higher you go, the further you can see. If the Earth were flat, you would have the same view no matter your elevation. The curvature of the Earth limits our viewing range to about five kilometers.
Traveling around the world
View from the Concorde cockpit / manchestereveningnews.co.uk The first trip around the world was made by the Spaniard Ferdinand Magellan. The voyage lasted three years, from 1519 to 1522. To circumnavigate the globe, Magellan needed five ships (of which two returned) and 260 crew (of which 18 returned). Fortunately, nowadays, in order to make sure that the Earth is round, you just need to buy a plane ticket.
If you've ever traveled by plane, you may have noticed the curvature of the Earth's horizon. It is best seen when flying over the oceans.
According to the article Visually discerning the curvature of the Earth, published in the journal Applied Optics, the curve of the Earth becomes visible at an altitude of about 10 kilometers, provided that the observer has a field of view of at least 60°. From the window of a passenger airliner the view is still less.
The curvature of the horizon is more clearly visible if you fly above 15 kilometers. It is best seen in photographs from Concorde, but, unfortunately, this supersonic aircraft has not made flights for a long time. However, high-altitude aviation is being revived in the passenger rocket plane from Virgin Galactic - Space Ship Two. So in the near future we will see new photographs of the Earth taken in suborbital flight.
An airplane can easily fly around the globe without stopping. Around the world trips by plane have been carried out more than once. At the same time, the planes did not detect any “edges” of the Earth.
Weather balloon observations
Image from weather balloon / le.ac.uk Regular passenger airliners do not fly that high: at an altitude of 8–10 kilometers. Weather balloons rise much higher.
In January 2017, students at the University of Leicester strapped several cameras to a hot air balloon and launched it into the sky. It rose to an altitude of 23.6 kilometers above the surface, significantly higher than passenger airliners fly. In the photographs taken by the cameras, the curve of the horizon is clearly visible.
Shape of other planets
Photo of Mars / nasa.gov Our planet is pretty ordinary. Of course, there is life on it, but otherwise it is no different from many other planets.
All our observations show that the planets are spherical. Since we have no compelling reason to believe otherwise, our planet is also spherical.
A flat planet (ours or any other) would be an incredible discovery that would contradict everything we know about planet formation and orbital mechanics.
Time Zones
When it is seven in the evening in Moscow, it is noon in New York, and midnight in Beijing. In Australia at the same time it is 1:30 am. You can tell what time it is anywhere in the world, and make sure that the time of day is different everywhere.
There is only one explanation for this: the Earth is round and rotates around its axis. On the side of the planet where the Sun shines, it is currently day. The opposite side of the Earth is dark and night there. This forces us to use time zones.
Even if we imagine that the Sun is a directional searchlight that runs over a flat Earth, we would not have a clear day and night. We would still observe the Sun, even if we were in the shadow, just as we can see spotlights shining on the stage in a theater, while being in a dark hall. The only explanation for the change in time of day is the sphericity of the Earth.
Center of gravity
It is known that gravity always pulls everything towards the center of mass.
Our Earth is spherical in shape. The center of mass of the sphere is, logically, at its center. Gravity pulls all objects on the surface towards the Earth's core (that is, straight down) regardless of their location, which is what we always see.
If we imagine that the Earth is flat, then gravity should attract everything on the surface to the center of the plane. That is, if you find yourself at the edge of a flat Earth, gravity will not pull you down, but towards the center of the disk. It is hardly possible to find a place on the planet where things fall not down, but sideways.
Images from space
Photo from ISS / nasa.gov The first photograph of the Earth from space was taken in 1946. Since then, we have launched many satellites, probes and astronauts (or astronauts, or taikonauts - depending on the country) there. Some satellites and probes have returned, some remain in Earth's orbit or fly through the solar system. And in all photographs and videos transmitted by spacecraft, the Earth is round.
The curvature of the Earth is clearly visible in photographs from the ISS. In addition, you can see photographs of the Earth taken every 10 minutes by the Japan Meteorological Agency's Himawari-8 satellite. It is constantly in geostationary orbit. Or here are real-time photos from the DSCOVR satellite, NASA.
Now, if you suddenly find yourself in the company of flat earthers, you will have several arguments to argue with them.
If Gagarin is not an authority for your child, and all the pictures from the ISS, in his opinion, are fakes, you will have to be patient and prove the sphericity of the Earth, using a minimum of technical means - just like the ancient Greeks did. This process will be long, but extremely instructive.
1. We prove that the Earth is a disk or a ball
Let's start by deciding on the outlines of our home planet. Is it shaped like a suitcase or is there a turtle and elephants down there? There is a very simple way to understand that the Earth is a disk or a sphere. To do this, just wait for a total lunar eclipse (in Europe, the closest one can be observed on July 27, 2018; they occur every year. Go with your child to where the sky will definitely be clear on that day, and watch how the round shadow of the Earth slowly covers the Moon. Before that demonstrate how the shape of a shadow depends on the shadow of an object - show a wolf or an elk with the shadows of hands on the wall. If the shadow is round, then the body that casts it is round.
After this, all that remains is to understand whether the earth has the shape of a disk or the shape of a ball.
2. Choose between a disk and a sphere
To answer the question of whether the Earth is flat or spherical, we will need: to get out of the city, a ball and an ant (beetle, ladybug or cockroach - your choice).
First, we need to find a tall, free-standing structure on flat terrain (for example, a power line pylon) and go from there. Just like a ship at sea, the support will not disappear from sight immediately, but gradually - first the “legs”, then the middle part and, finally, the top with the wires.
Now let's interpret the observation results. If we were dealing with a tall tower on a plane, then, moving away, it would become smaller and smaller, but, even remaining barely noticeable, it would be completely visible. On the surface of the sphere, objects gradually disappear from view.
We take a ball and put an insect on it. We bring the ball very, very close to the eyes so that the insect is half behind the “horizon” - the far visible edge of the ball. Only part of the animal’s body will be visible, just as only part of the tower is visible from afar. Now we can confidently conclude that we live on the surface of the earth (jokes aside).
3. Once again about the ball
Another great way to make sure the earth is round is to go out into the field at dawn. Take your watch with you and face the brightest edge of the sky. As soon as the edge of the Sun (or the Moon - it doesn’t matter) appears below the horizon, lie down on the Earth and note the time. Look in the same direction. For a few seconds the star will disappear behind the horizon again. Why? Because you changed your viewing angle, and for a short time the Sun (or Moon) was hidden from you by the convex surface of the Earth.
The same can be done at sunset or watching the moon set, but only in the reverse order: first watch while lying down, and then while standing.
4. Determine the size of the ball
For the first time, the circumference of the equator was calculated by the librarian of the Library of Alexandria, Eratosthenes of Cyrene. The ancient sage compared the deviation of the Sun from the zenith on the same day of the year in two cities located at a distance of 800 kilometers from each other - Alexandria and Siena.
It is easy to catch the sun at its zenith: at this moment its rays fall even on the bottom of deep pits (Eratosthenes was guided by wells), and objects do not cast shadows. On the same day, the Sun cast sheer rays on Alexandria, but not on Sienna. It deviated from the zenith by 7.2°. Seven degrees from 360 is two percent. We multiply 800 by 50 and get 40 thousand (kilometers): this is the length of the Equator, this is confirmed by modern high-precision measurements.
Repeating Eratosthenes' experiment is quite simple, but you will have to enlist the help of friends in another city. Wait for the moment when the Sun is at its zenith (you can slack off and look on the Internet, you can navigate by a sundial - a stick stuck into the Earth. When the shadow is the shortest, then the Sun is closest to the zenith). Above the middle zone, the Sun is never at its zenith, but this does not matter. It is important at the moment when the shadow from your stick reaches its minimum, call your friends in a city located quite far from you - from Moscow, for example, to St. Petersburg, and ask them to measure the length of their shadow (and the height of the stick). Calculate the value of the acute angle between the stick and an imaginary straight line from the end of the stick to the end of the shadow in your place and in a distant city. Next - pure arithmetic: it should be about 40 thousand kilometers.
5. Once again measure the size of the ball
Let's return to experiments with clocks and sunrises (sunsets). We measured time for a reason: knowing it and your own height, you can solve the problem about the radius of the globe.
First, let's find the angle by which the Earth turned in the interval between the time you saw the edge of the rising Sun or Moon at dawn while standing and lying down. To do this, solve a simple proportion. If the Earth rotates 360° in 24 hours, what angle did it rotate during the time you recorded? Calculate and call it angle α.
Imagine that it was not you who fell and got up. Instead, the sunrise was observed by two people: Ivan 1 and Ivan 2, at such a distance from each other that the first saw the Sun later than the other by exactly the same time T. Two radii R to Ivan 1 and Ivan 2 form an isosceles triangle with angle α.
Complete the radius to Ivan 2 with a segment equal to your height h, and connect its end to the point where Ivan 1 stands. We get a right triangle with a hypotenuse R+h and a known acute angle. A little trigonometry and we calculate the radius of the Earth.