What is the Diameter of Earth?

Our beautiful, precious, life-supporting Earth as seen on July 6, 2015 from a distance of one million miles by a NASA scientific camera aboard the Deep Space Climate Observatory spacecraft. Credits: NASA

For those people who have had the privilege of jet-setting or traveling the globe, its pretty obvious that the world is a pretty big place. When you consider how long it took for human beings to settle every corner of it (~85,000 years, give or take a decade) and how long it took us to explored and map it all out, terms like “small world” cease to have any meaning.

But to complicate matters a little, the diameter of Earth – i.e. how big it is from one end to the other – varies depending on where you are measuring from. Since the Earth is not a perfect sphere, it has a different diameter when measured around the equator than it does when measured from the poles. So what is the Earth’s diameter, measured one way and then the other?

Oblate Spheroid:

Thanks to improvements made in the field of astronomy by the 17th and 18th centuries  – as well as geodesy, a branch of mathematics dealing with the measurement of the Earth – scientists have learned that the Earth is not a perfect sphere. In truth, it is what is known as an “oblate spheroid”, which is a sphere that experiences flattening at the poles.

Data from the Earth2014 global relief model, with distances in distance from the geocentre denoted by color. Credit: Geodesy2000
Data from the Earth2014 global relief model, with distances in distance from the geocentre denoted by color. Credit: Geodesy2000

According to the 2004 Working Group of the International Earth Rotation and Reference Systems Service (IERS), Earth experiences a flattening of 0.0033528 at the poles. This flattening is due to Earth’s rotational velocity – a rapid 1,674.4 km/h (1,040.4 mph) – which causes the planet to bulge at the equator.

Equatorial vs Polar Diameter:

Because of this, the diameter of the Earth at the equator is about 43 kilometers (27 mi) larger than the pole-to-pole diameter. As a result, the latest measurements indicate that the Earth has an equatorial diameter of 12,756 km (7926 mi), and a polar diameter of 12713.6 km (7899.86 mi).

In short, objects located along the equator are about 21 km further away from the center of the Earth (geocenter) than objects located at the poles. Naturally, there are some deviations in the local topography where objects located away from the equator are closer or father away from the center of the Earth than others in the same region.

The most notable exceptions are the Mariana Trench – the deepest place on Earth, at 10,911 m (35,797 ft) below local sea level – and Mt. Everest, which is 8,848 meters (29,029 ft) above local sea level. However, these two geological features represent a very minor variation when compared to Earth’s overall shape – 0.17% and 0.14% respectively.

Meanwhile, the highest point on Earth is Mt. Chiborazo. The peak of this mountain reaches an attitude of 6,263.47 meters (20,549.54 ft) above sea level. But because it is located just 1° and 28 minutes south of the equator (at the highest point of the planet’s bulge), it receives a natural boost of about 21 km.

Mean Diameter:

Because of the discrepancy between Earth’s polar and equatorial diameter, astronomers and scientists often employ averages. This is what is known as its “mean diameter”, which in Earth’s case is the sum of its polar and equatorial diameters, which is then divided in half. From this, we get a mean diameter of 12,742 km (7917.5 mi).

The difference in Earth’s diameter has often been important when it comes to planning space launches, the orbits of satellites, and when circumnavigating the globe. Given that it takes less time to pass over the Arctic or Antarctica than it does to swing around the equator, sometimes this is the preferred path.

We have written many interesting articles about the Earth and mountains here at Universe Today. Here’s Planet Earth, The Rotation of the Earth, What is the Highest Point on Earth?, and Mountains: How Are They Formed?

Here’s how the diameter of the Earth was first measured, thousands of years ago. And here’s NASA’s Earth Observatory.

We did an episode of Astronomy Cast just on the Earth. Give it a listen, Episode 51: Earth.


Are We Living in a New Geologic Epoch?


Have humans changed our planet Earth so much in the past 200 years that we are now living in a new geological age? A group of geologists believes this is the case. They have formally proposed designating a new geologic epoch, the Anthropocene, which would encompass the past 200 years or so of geologic history. The action is appropriate, they say, because during the past 2 centuries, human activity has caused most of the major changes in Earth’s topography and climate.

Like rings in a tree, each layer in Earth’s geologic record reflects the conditions of the time it was deposited and offers a glimpse into Earth’s past. In this geologic history that is written in the rocks and soil of our planet, researchers have differentiated the layers into classifications of time called eons, eras, periods, epochs, and ages that reflect characteristic conditions. For example, the Carboniferous period, which lasted from 360 million to 300 million years ago, is known for the vast deposits of coal that formed from jungles and swamps. Even some of the longer stretches have been named based on biology, such as the Paleozoic (“old life”) and the Cenozoic (“recent life”).

Earth has been has always been subject to the same kinds of physical forces–wind, waves, sunlight–throughout the planet’s existence. But the life that has arisen on the planet has had a much more varied impact such as the rise of plants that has shaped the planet in dramatic ways. But in the past 200 years, ever since the human population has reached 1 billion, our influences have affected the composition of Earth’s strata, altering the physical and chemical nature of ocean sediments, ice cores and surface deposits. Some of these influences are the use of fossil fuels and the growth of large cities.

British Geologist Jan Zalasiewicz and several colleagues argue that the International Commission on Stratigraphy should officially mark the end of the current epoch. That would be the Holocene (“entirely recent”), which started after the end of the last ice age, about 10,000 years ago. The new epoch would be the Anthropocene.

The evidence the geologists cite include the dramatic increase in lead concentration in the soil and water since about 1800 and the increase of carbon dioxide in the atmosphere. They claim that human processes now vastly outpace the equivalent natural forces. “A reasonable case can be made for the Anthropocene as a valid formal unit,” Zalasiewicz says.

The argument has merit, says American geologist Richard Alley. “In land, water, air, ice, and ecosystems, the human impact is clear, large, and growing,” he says. “A geologist from the far distant future almost surely would draw a new line, and begin using a new name, where and when our impacts show up.”

Original News Source: AAAS ScienceNow