The question is how many kilometers is the globe along the equatorseems simple only at first glance. The school curriculum often gives a rounded value of 40,000 kilometers, but for navigation, cartography and geodesy such accuracy is categorically unacceptable. Our planet is not an ideal geometric object, but a complex body whose shape has changed over billions of years.
Modern science operates with the most accurate figures obtained through satellite measurements and gravimetric data. Equatorial circle is a key parameter for creating global positioning systems, without which it is impossible to imagine the operation of GPS navigators in your car or smartphone. Understanding the real length of this line helps to understand the scale of our planet.
In this article we will look at why the numbers in different sources may differ, and what value is the reference. You will learn the difference between an ideal sphere and a real one geoid, as well as how the rotation of a planet affects its size. This knowledge is necessary not only for geographers, but also for everyone who is interested in the structure of the world.
Why is the Earth not a perfect sphere?
The ancient Greeks assumed the Earth was spherical, but considered it an ideal sphere. However, Isaac Newton theoretically substantiated that a rotating body cannot maintain the shape of an ideal ball. The centrifugal force arising during rotation “flattens” the planet at the poles and “inflates” it in the equatorial region. This phenomenon is called equatorial swelling.
As a result, the Earth has the shape ellipsoid of revolution. The difference between the equatorial and polar radii is approximately 21 kilometers. This means that if you follow the equator, your path will be longer than if you try to go around the planet through the poles. That is why the question about the circumference requires clarification: which line is the measurement along?
⚠️ Attention: Using the value of 40,000 km for precise engineering calculations or navigation may lead to the accumulation of critical errors over large distances. For professional tasks, use the parameters of reference ellipsoids.
The compression of the planet at the poles directly affects the gravitational field and even the passage of time, albeit on a microscopic scale. For an ordinary person these differences are invisible, but for satellite communications they are fundamental. Satellites hanging above the equator move taking into account precisely this increased circumference.
Exact length of the equator in kilometers
So what is the final figure? When it comes to the length of the equator, the International Bureau of Weights and Measures and geodetic organizations use several standards. The most common value used in the system WGS-84 (World Geodetic System), is the length 40,075.017 km. This figure should be considered the most relevant for modern calculations.
However, in Soviet and Russian geodesy, Krasovsky’s ellipsoid was used for a long time. According to its parameters, the equatorial radius is slightly different, giving a circumference of about 40,075.7 km. The difference seems minimal, but on a planetary scale it is significant. Equatorial radius in the WGS-84 system it is taken to be 6378.137 km.
For comparison, we can cite the length of the meridian (the circle through the poles), which is only about 40,008 km. Thus, the “girth” of the Earth at the waist (equator) is greater than the “girth” across the head (the poles) by more than 60 kilometers. This confirms the theory that the planet is oblate.
When planning a trip around the world on a yacht or plane, always keep in mind that the actual route along the equator will be longer than the 40 thousand kilometers indicated in old textbooks.
Differences in coordinate systems and ellipsoids
Why are the numbers different? The whole point is what mathematical model of the Earth we take as a basis. A geoid is a figure whose surface is everywhere perpendicular to the direction of gravity. It is difficult to calculate. Therefore, scientists have created simplified models - reference ellipsoids.
Different countries and different eras used different ellipsoids, which led to inconsistencies in maps. For example, the American system WGS-84 and Russian PZ-90 have slight differences in parameters. This affects the coordinates of points: the same point on the ground may have different coordinates in different systems.
- 🌍 WGS-84 - used in GPS navigation around the world, standard for Google Maps.
- 🇷🇺 Krasovsky's ellipsoid - used in the USSR and is used in some old CIS maps.
- 🛰️ PZ-90 - a modern Russian system used in GLONASS.
- 📏 1975 International Ellipsoid — used for international settlements before the introduction of WGS-84.
When working with navigation equipment, it is important that the map coordinate system coincides with the receiver coordinate system. Otherwise, the positioning error can amount to tens of meters, which is critical when approaching a port or in a narrow fairway.
Comparative table of Earth parameters
To structure information about the size of our planet, it is convenient to use comparative data. Below is a table showing the differences between equatorial and polar parameters, as well as the values for different models.
| Parameter | Equatorial radius (km) | Polar radius (km) | Circumference (km) |
|---|---|---|---|
| Sphere (simplified) | 6371,0 | 6371,0 | 40 030 |
| WGS-84 (GPS) | 6378,137 | 6356,752 | 40 075,017 |
| Krasovsky's ellipsoid | 6378,245 | 6356,863 | 40 075,7 |
| Average value | 6378,1 | 6356,8 | 40 075,5 |
The table shows that the difference in radii is more than 21 km. This is a significant indicator that confirms that the Earth is far from an ideal sphere. Compression planets is a dynamic process that continues today, albeit very slowly.
How was the equator measured before the satellites?
Before the satellite era, surveyors used the triangulation method. They built chains of triangles on the ground, measured bases using Invar rulers and calculated distances through angles. Accuracy was impressive for the time, but inferior to modern laser rangefinders.
The influence of rotation on the shape of the planet
The shape of the Earth is the result of the struggle between two forces: gravity, which tends to compress the mass into a ball, and centrifugal force, which arises from rotation. The Earth's rotation speed at the equator is about 1670 km/h. This is enough to create a noticeable centrifugal force.
If the Earth rotated faster, the equatorial bulge would be larger. There are planets, for example Jupiter or Saturn, which rotate much faster than the Earth and have a noticeably more oblate shape. Saturn, for example, is so flattened that the difference between its equatorial and polar radii is about 10%.
For the Earth, this deformation is approximately 1/298 of the radius. This is not much, but enough to change the length of the equator. In addition, the distribution of masses inside the planet (mountains, ocean basins, mantle density) also makes its own adjustments to the shape geoid.
⚠️ Attention: When calculating the trajectories of launching space rockets from the equator, the additional speed of the Earth’s rotation is taken into account. Launching from the equator gives a speed advantage, which saves fuel.
Practical value of equator length
Knowing the exact length of the equator is necessary not only for academic science. In aviation and shipping, distances are often measured in nautical miles. One nautical mile was historically defined as one minute of meridian arc. However, due to the oblateness of the Earth, the length of a minute of arc varies from the equator to the poles.
The modern international nautical mile is taken to be exactly 1852 meters. This is the average value. Pilots and captains use complex algorithms to recalculate coordinates to accurately determine the distance to their destination. An error of a few kilometers over a distance of 40,000 km may seem insignificant (less than 0.1%), but in poor visibility conditions it can be critical.
- ✈️ Aviation: Calculation of fuel reserves for transcontinental flights.
- ⚓ Shipping: Charting a course in the open ocean, where there are no visual references.
- 🛰️ Satellites: Launching satellites into geostationary orbit, which is located strictly above the equator.
- 🗺️ Cartography: Create accurate map projections for navigation applications.
Also, the length of the equator divides the globe into the Northern and Southern Hemispheres. This is an important climatic and geographical milestone. In countries located on the equator, day is always equal to night, and the sun passes through the zenith twice a year.
☑️ What you need for a trip around the world
Interesting facts about the equator
The equator passes through 13 countries, including Brazil, Congo, Indonesia and Kenya. In some places, such as Ecuador (the country's name translates to "equator"), there are memorials and museums where tourists can place one foot in the Northern Hemisphere and the other in the Southern Hemisphere.
There is an interesting physical effect associated with the rotation of the Earth - the Coriolis effect. At the equator it is practically absent, so the water in the funnel flows strictly vertically, without swirling like a vortex, as happens in the Northern or Southern Hemispheres. This is often demonstrated to tourists as the "magic of the equator."
In addition, at the equator, the linear speed of the Earth's rotation is maximum. Theoretically, if you launch a rocket from the equator to the east, it will receive an additional boost of speed from the rotation of the planet. This makes equatorial spaceports such as Kuru in French Guiana, strategically important sites.
The exact length of the equator (40,075 km) is not just a reference figure, but a fundamental parameter necessary for the operation of the entire modern navigation infrastructure of the planet.
Why does the length of the equator change over time?
The length of the equator can change slightly due to tectonic processes, melting glaciers (isostatic uplift of the crust) and even large earthquakes, which can shift the axis of rotation or change the shape of the planet by fractions of a millimeter. The redistribution of water masses in the oceans also affects.
Is it possible to see the curvature of the Earth from above?
Yes, the curvature of the horizon becomes noticeable already at an altitude of about 10-15 kilometers (cruising altitude of the aircraft). However, to see the Earth as a whole ball, you need to rise to a height of hundreds of kilometers, where the orbital stations are located.
Where is the Earth's center of mass?
The Earth's center of mass does not coincide with its geometric center due to the uneven distribution of rock density. It is displaced approximately 2 kilometers towards the Pacific Ocean. This also affects calculations of satellite orbits.
Which country is closest to the center of the Earth?
Because the Earth is flattened at the poles, points at the equator are further from the center of the planet than points at the poles. Therefore, the “summit” furthest from the center of the Earth is not Everest, but the Chimborazo volcano in Ecuador, located at the equator.