Diameter of the Earth. Image credit: NASA
Diameter of the Earth. Image credit: NASA

Astronomy, Guide to Space

Diameter of Earth

14 Jun , 2008 by

Since the Earth is not a perfect sphere, there are three numbers to know when answering questions about the diameter of Earth. The rotation of the planet has slightly flattened it out, so it has a larger diameter at the equator than at the poles. The equatorial diameter of Earth is 12,756 km, its polar diameter is 12,713 km, and its average diameter, which is referred to in common usage, is 12,742 km. For our friends who are not using the metric system, that translates to 7,926 miles.

The planet’s diameter is only one of any number of interesting facts about our home planet. Here are a few other facts that may not be common knowledge among your friends:

Plate tectonics cause earthquakes and form mountains, but did you know that they are one of the reasons that our planet is livable? Organisms in the ocean die and fall to the ocean floor. Their remains are rich in carbon. This carbon rich material is recycled into the planet’s inner regions by plate tectonics. If it was not for this recycling action, the greenhouse effect of the carbon rich material would increase the temperature of our planet until it was no longer viable for most of the life forms it currently supports.

Our planet’s atmosphere extends to about 10,000 km above the surface. It is thickest within the first 50 km and becomes very negligible at the outer reaches. The outermost layer is called the exosphere and starts about 500 km above the surface. Some where close to the 10,000 km mark free moving particles begin to escape Earth’s gravity and are blown away by the solar wind. Even though it extends to 10,000 km, 75% of the Earth’s atmosphere is contained within the 11 km closest to the planet’s surface.

Earth may only have one moon, but it has two quasi-satellites that few people know about. The asteroids Cruithne 3753 and 2002 AA29 have orbits that are synchronized with Earth’s but they are not actually orbiting the Earth.

The diameter of Earth is a great physical characteristic to know, but do not stop your research there. Look over NASA’s website for more statistics like it and to find some facts that others may not know. Part of the fun of looking things up is finding something to stump your friends with.

You can read more articles from Universe Today about the Earth. For example, did know know that the Earth has an inner, inner core? And here’s a nice photograph of the Earth and Moon, seen from Mars.

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.


Jerry Coffey is a Registered Nurse and father of 5. He enjoys skydiving, astronomy, and time with his children.

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