# What is the circumference of the Earth? Earth Observation - Credit: Alex Gerst, NASA/ISS

Before answering the question to know how long is the circumference of the Earth, let’s look at some definitions and background information.

## Circumference of the Earth in history

The measurement of the Earth’s circumference has a long and storied history, dating back to ancient civilizations. The earliest known measurements of the Earth’s circumference were made by the ancient Egyptians and the ancient Greeks.

The ancient Greeks, particularly Eratosthenes, a Greek librarian in Alexandria, Egypt, made more accurate measurements using a technique called “stereographic projection.” By measuring the angle of the sun at different locations, he was able to calculate the circumference of the Earth to be 40,250 to 45,900 kilometers, which was only slightly off from the Egyptian measurement.

In the second century, a philosopher named Claudius Ptolemaeus, or Ptolemy, living in Alexandria, revised previous calculations and defined the Earth’s circumference at 28,985 kilometers.

In the 16th century, the famous explorer Ferdinand Magellan circumnavigated the globe and proved that the Earth was round. His measurements, however, were not accurate enough to determine the Earth’s circumference with any significant precision.

It wasn’t until the 18th century that the Earth’s circumference was accurately measured. In 1735, the French Academy of Sciences commissioned the French mathematician and astronomer Jean Picard to make a precise measurement of the Earth’s circumference. Using a combination of triangulation and trigonometry, Picard was able to calculate the Earth’s circumference to be 40,016 kilometers.

In modern times, the circumference of the Earth has been measured using satellite technology, with a precision of a few centimeters. The currently accepted value for the Earth’s circumference is 40,075.017 kilometers at equator, with a margin of error of less than 1 millimeter.

## How to measure the circumference of the Earth

Measuring the circumference of the Earth is a task that has been of great interest to scientists and explorers throughout history. The ancient Egyptians, Greeks, and Romans all made attempts to measure the Earth’s circumference, but it wasn’t until the 18th century that a precise measurement was finally made. Today, modern techniques have made it possible to measure the Earth’s circumference with a high degree of accuracy.

One of the most widely used modern techniques for measuring the Earth’s circumference is satellite geodesy. This technique utilizes the Global Positioning System (GPS) network of satellites to precisely measure the distance between points on the Earth’s surface. By measuring the distance between several points located along the Earth’s equator, scientists can calculate the Earth’s circumference.

Another method for measuring the circumference of the Earth is through the use of laser ranging. This technique utilizes a laser beam to measure the distance between two points on the Earth’s surface. By measuring the distance between two points located along the Earth’s equator, scientists can again calculate the Earth’s circumference.

A third method for measuring the Earth’s circumference is through the use of radar. This technique utilizes radar to measure the distance between two points on the Earth’s surface. By measuring the distance between two points located along the Earth’s equator, scientists can again calculate the Earth’s circumference.

Another method that is used is called VLBI (Very Long Baseline Interferometry) which is a technique that utilizes radio telescopes to measure the distance between two points on the Earth’s surface. By measuring the distance between two points located along the Earth’s equator, scientists can calculate the Earth’s circumference.

Finally, a new method that is still being researched and developed is the use of quantum sensors. This technique utilizes the properties of quantum mechanics to measure the distance between two points on the Earth’s surface. By measuring the distance between two points located along the Earth’s equator, scientists can again calculate the Earth’s circumference.

## The real shape of the Earth

It’s also worth noting that the Earth is not a perfect sphere, but an oblate spheroid, meaning it is slightly flattened at the poles and bulging at the equator. This shape is due to the Earth’s rotation, which causes the equator to bulge outwards, and the poles to flatten inwards. This means that the Earth’s circumference at the equator is slightly larger than the circumference at the poles.

The shape of the Earth has been the subject of much debate and discussion throughout history. The ancient Greeks believed the Earth was a perfect sphere, while the ancient Egyptians believed it was a flat disk. It wasn’t until the 3rd century BCE that the Greek mathematician and astronomer Eratosthenes proposed that the Earth was an oblate spheroid, which means it is slightly flattened at the poles and bulging at the equator.

The idea that the Earth is an oblate spheroid is supported by a number of different lines of evidence. One of the most important pieces of evidence is the fact that the Earth’s gravity is not the same at all points on the surface. The gravity is stronger at the poles and weaker at the equator, which is consistent with the idea that the Earth is slightly flattened at the poles and bulging at the equator.

Another piece of evidence is the fact that the Earth’s rotation causes it to bulge at the equator and flatten at the poles. As the Earth rotates, the centrifugal force generated by the rotation causes the equator to bulge and the poles to flatten. This effect is known as the centrifugal force and it was first observed by French mathematician and astronomer Jean Richer in 1672.

Additionally, measurements of the Earth’s gravity field using satellite-based techniques like the Gravity Recovery and Climate Experiment (GRACE) and the European Space Agency’s GOCE (Gravity field and steady-state Ocean Circulation Explorer) have provided detailed information about the Earth’s gravity field and its variations over the surface of the Earth. These measurements show that the Earth’s gravity field is not symmetrical and that it is consistent with the idea that the Earth is an oblate spheroid.

## Finally, the circumference of the Earth is …

• Earth circumference at the equator: 40,075.017 km (24,901.461 miles).
• Circumference around the Earth at poles: 40,007.863 km  (24,859.734 miles).