The question of what the exact circumference of our planet is has worried humanity since the ancient Greeks first suggested that the Earth was spherical. The answer to it is not an unambiguous constant that can simply be memorized, since our planet is not an ideal geometric ball, but a complex body of irregular shape. Equator length and the distance across the poles differ due to the oblateness at the poles caused by the Earth's rotation on its axis.
Modern science uses the term geoid to describe the shape of the Earth, which implies deviations from the ideal ellipsoid of rotation. This is why the numbers we see in geography textbooks are averages or calculated for a specific model adopted as a standard (for example, WGS-84). Understanding these nuances is critical for navigation, cartography and satellite communications systems.
In this article, we will take a closer look at where the numbers of 40,075 kilometers come from, why the meridian is shorter than the equator, and how ancient scientists managed to calculate these values with amazing accuracy without the use of space technology.
Historical methods of measurement: from Eratosthenes to the present day
The first scientific measurement of the Earth's circumference known to science was carried out by Eratosthenes of Cyrene back in the 3rd century BC. He noticed that on the day of the summer solstice in Siena (now Aswan) the Sun at noon was exactly at its zenith and illuminated the bottom of deep wells without casting a shadow. At the same time, in Alexandria, located to the north, the sun cast a shadow, the angle of deviation of which was 1/50 of a circle.
Knowing the distance between the cities, which at that time was estimated at 5,000 stadia, Eratosthenes multiplied this value by 50 and received 250,000 stadia. By converting ancient measures of length into modern ones, we get a value surprisingly close to real ones 40,000 kilometers. This achievement was made possible thanks to simple but ingenious geometric logic.
⚠️ Attention: Eratosthenes' method was based on the assumption that the Earth is a perfect sphere and Siena and Alexandria are located on the same meridian. In reality, these cities are shifted relative to each other, which introduced an error into the calculations, but the final result turned out to be amazingly accurate.
In later eras, until the advent of satellites, surveyors used the triangulation method. They created chains of triangles on the ground, measuring the bases with high accuracy and calculating the angles between the vertices. This method made it possible to clarify the parameters Krasovsky ellipsoid, which for a long time served as the basis for cartography in the USSR and Russia.
Why the Earth is not a perfect sphere: the equatorial bulge
The main reason why one cannot give one single figure for the “circumference of the Earth” lies in the physics of rotation. Our planet rotates around its axis, and the centrifugal force acting on matter is maximum at the equator and zero at the poles. This causes the planet to “flatten” at the poles and swell in the equatorial region.
The difference between the equatorial and polar radii is about 21 kilometers. It would seem that this is not much on a planetary scale, but for accurate calculations of the circumference this is a colossal value. Equatorial radius is approximately 6378 kilometers, while the polar one is about 6357 kilometers.
If the Earth were liquid and spinning faster, this bulge would be even more noticeable. The shape of the planet is in a state of hydrostatic equilibrium, where gravity tends to give the body a spherical shape, and centrifugal force tends to stretch it. The balance of these forces determines the current parameters geoid.
What would happen if the Earth rotated faster?
If the Earth's rotation rate were to increase, the centrifugal force would increase, causing the poles to flatten even more and the equator to increase in length. In the extreme, the planet could break apart or turn into a ring.">
Exact data: length of the equator and meridian in kilometers
For practical tasks, such as routing aircraft or ships, standardized models are used. The most common is the WGS-84 system used in GPS navigation. According to these data, the circumference at the equator is 40,075.017 km.
However, if we measure the distance around the Earth through the poles (meridional circle), the figure will be smaller. It is approximately 40,008 kilometers. This difference of 67 kilometers may seem insignificant as a percentage (less than 0.2%), but in absolute terms this distance is greater than the distance from Moscow to Tula.
Below is a table comparing the main parameters of the Earth's ellipsoid, so that you can clearly assess the difference between the “wide” and “narrow” circumferences of our planet:
| Parameter | Value (km) | Description |
|---|---|---|
| Equatorial circle | 40 075 | Line length of the widest part of the planet |
| Meridional circle | 40 008 | Length across poles |
| Average circumference | 40 041 | Average value for a sphere |
| Radius difference | 21.3 | Flattening at the poles |
That is why, when they say “how many kilometers around the globe,” most often they mean the equatorial value, since it is the maximum and most often used in reference books.
Relief influence and gravity anomalies
If you go down from the level of idealized mathematical models to the real surface, the picture becomes even more complex. Relief of the Earth - mountains, ocean basins, ice sheets - makes its own adjustments. However, compared to the overall radius of the planet, even Everest (8.8 km) is only a microscopic roughness that does not significantly affect the overall circumference.
A more important factor is the distribution of masses within the planet. The Earth's gravitational field is not uniform: in some regions the gravity is higher, in others it is lower. This phenomenon, known as gravity anomalies, affects ocean levels, which in turn are the main reference for measuring height and shape. geoid.
⚠️ Attention: The sea level is not perfectly smooth. Due to gravitational anomalies, the ocean surface has “humps” and “hollows” up to 100 meters high relative to the ellipsoid, which must be taken into account in high-precision measurements.
Modern satellites such as GOCE and GRACE, created detailed maps of the gravitational field, allowing us to refine the shape of the Earth down to centimeters. This data is used not only for geography, but also for studying climate and ocean currents.
☑️ Factors influencing the shape of the Earth
Practical application: navigation and aviation
Knowing the exact circumference of the Earth is critical for aviation. Pilots and controllers use the concept of "great circle" - the shortest distance between two points on the surface of a sphere. Airplane routes often pass through the poles or high latitudes, where map distortion is greatest, and calculating straight-line distances on a flat map will produce a huge error.
The unit of measurement used in maritime affairs is the nautical mile, which was historically defined as the length of one minute of the meridian arc. Since the Earth is not a perfect sphere, the length of a minute of arc varies with latitude, so the standard international nautical mile was adopted to be exactly 1852 meters.
Global positioning systems (GPS, GLONASS) operate on the basis of complex mathematical models of an ellipsoid. If a navigator considered the Earth to be a perfect sphere, the error in determining the coordinates would accumulate with each kilometer of travel, making navigation in the open ocean or desert impossible.
Comparison with other celestial bodies of the Solar System
To better understand the scale of our planet, it is useful to compare its size with other objects. Earth is the largest terrestrial planet, but pales in comparison to the gas giants. For example, Jupiter's equator circumference is about 449,000 kilometers, which is more than 11 times larger than Earth's.
The Moon, our natural satellite, has an equator length of only about 10,921 kilometers. This means that the Earth could be “wound” around the Moon almost four times. Mars, often called the Earth's twin, is almost two times smaller in circumference than our planet (about 21,344 km).
Venus is almost identical in size to Earth; it is often called the “sister” of our planet. The difference in equator length between Earth and Venus is less than 2%, making them twins on the scale of the Solar System, despite radical differences in atmosphere and surface temperature.
Earth is the densest and largest of the terrestrial planets, but its shape is far from a perfect sphere, which distinguishes it from small bodies like asteroids, which can have irregular shapes.
Why is the equatorial bulge not felt by us?
A person does not feel the convexity of the Earth or the rotation of the planet due to the enormous scale. At a distance of several kilometers, the surface appears completely flat. In addition, we rotate with the planet at a constant speed, so we do not feel inertia, just as a passenger in a uniformly flying airplane does not feel a speed of 900 km/h without turbulence.
Does the length of the equator change over time?
Yes, but extremely slowly. Tectonic processes, melting glaciers (post-glacial isostasy) and even large earthquakes can slightly change the distribution of masses and, therefore, the shape of the planet. For example, the 2011 earthquake in Japan shifted the Earth's rotation axis and slightly changed the length of the day, which indirectly indicates a change in the geometry of the planet.
What value for the length of the Earth is used in schools?
School textbooks most often give a rounded value of 40,000 kilometers. This number is convenient for remembering and making approximate calculations. It was actually laid down in the definition of the meter during the French Revolution, when the meter was defined as one forty millionth of the length of the meridian.