Curious Kids: Why is the Moon Called the Moon?

There are many moons in the galaxy but only ours is called the moon.
There are many moons in the galaxy but only ours is called the moon. (Image credit: Shutterstock)

This article was originally published at The Conversation. The publication contributed the article to's Expert Voices: Op-Ed & Insights.

Toby Brown, Postdoctoral Fellow in Astrophysics, McMaster University

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The origin of the moon’s name is a very interesting story. Humans have been aware of this great white orb in the sky ever since they first roamed the Earth.

The moon’s appearance changes as the months pass. It moves through phases from full moon to new moon, and back again.

Where did the word moon come from?

The Earth has just one moon. It is best known as the moon in the English-speaking world because people in ancient times used the moon to measure the passing of the months.

The word moon can be traced to the word mōna, an Old English word from medieval times. Mōna shares its origins with the Latin words metri, which means to measure, and mensis, which means month.

So, we see that the moon is called the moon because it is used to measure the months.

Galileo’s discovery

So, why do the moons around other planets have names, while ours is just the moon?

When the moon was named, people only knew about our moon. That all changed in 1610 when an Italian astronomer called Galileo Galilei discovered what we now know are the four largest moons of Jupiter.

A statue of Galileo Galilei in Florence, Italy. He was a 16th century Italian scientists who is known as the father of modern astronomy.

A statue of Galileo Galilei in Florence, Italy. He was a 16th century Italian scientists who is known as the father of modern astronomy. (Image credit: Shutterstock)

Other astronomers across Europe discovered five moons around Saturn during the 1600s. These objects became known as moons because they were close to their planets, just as our own moon is close to the Earth.

It’s fair to say that other moons are named after our own moon.

The newly discovered moons were each given beautiful names to identify them among the growing number of planets and moons astronomers were finding in the solar system.

Many of these names came from Greek myths. The four large moons Galilei discovered around Jupiter were named Io, Europa, Ganymede and Callisto.

Astronomers continue to find new moons orbiting planets in our solar system. In October 2019, they announced they had newly discovered 20 moons around Saturn.

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This article is republished from The Conversation under a Creative Commons license. Read the original article.

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Toby Brown
Postdoctoral fellow, Astrophysics, McMaster University

Toby Brown is the Plaskett Fellow at the Herzberg Astronomy & Astrophysics Research Centre (National Research Council of Canada) and principal investigator of VERTICO, an ongoing ALMA Large Program that is studying environmental effects on galaxy evolution by mapping molecular gas at high resolution in 51 Virgo Cluster galaxies. He is an astronomer who focuses on the question, how do galaxies evolve? Beyond this research, much of Toby's time is spent talking about science through radio, writing, and award-winning public talks.

Toby holds a PhD in astrophysics from Swinburne University of Technology in Melbourne, Australia.

  • shyvauller
    We could still go by Latin ... "Luna"
  • rod
    I enjoy observations of the Moon using my telescopes (mostly First Quarter, Last Quarter or waning/waxing phases). When I go out and view the Moon - I do not think about the etymology of the word like this article. I just call it Moon and have some fun viewing :)