moonmoon
noun

- a natural satellite that orbits another natural satellite; a moon of a moon.

- a hypothetical small body orbiting a larger moon, provided the host moon is sufficiently massive and far enough from its parent planet that tidal forces do not destabilise the arrangement.

MNRAS Letters, Smithsonian Magazine

—

WORD ORIGIN

The word moon traces back to Old English mōna, from Proto-Germanic *mēnô, which in turn derives from the Proto-Indo-European root *mēnsis — the same root that gave us month, menstrual, and the Latin luna (via a parallel branch).

Across virtually every Indo-European language, the moon and the measurement of time are inseparable: the moon was humanity's first calendar.

The reduplication moonmoon — simply stacking the word on itself — is a modern linguistic device for expressing recursive nesting. In grammar, reduplication (repeating a word or syllable) often signals repetition or intensification: night-night, bye-bye, so-so. Here, doubling "moon" elegantly mirrors the idea itself: a moon behind a moon, a layer folded inside a layer.

The term entered popular consciousness in October 2018 when New Scientist first used it to describe the theoretical objects investigated by astronomers Juna Kollmeier and Sean Raymond in their arXiv paper "Can Moons Have Moons?" — prompted, charmingly, by a question from Kollmeier's four-year-old son. The internet immediately adopted moonmoon over the scientists' own preferred term submoon, largely because it was more fun to say — and because "Moon Moon" was already a beloved meme.

—
THE MOON'S MOON

In 2014, a four-year-old boy asked his mother whether a moon could have its own moon. His mother, Juna Kollmeier, happened to be an astrophysicist at the Carnegie Institution in Washington. She didn't know the answer.

That gap stayed with her. Four years later, she sat down with astronomer Sean Raymond to do the maths properly — and the result was a short paper titled "Can Moons Have Moons?" posted to arXiv in October 2018. The internet, delighted, immediately named the concept moonmoon.

The science is surprisingly subtle. For a moonmoon to hold a stable orbit, several things have to be true at once. The host moon must be large — at least 1,000 kilometres across. The moonmoon itself must be small, probably no more than 10 kilometres in radius. And there has to be enough distance between the moon and its parent planet that the planet's tidal forces don't sweep the moonmoon away or pull it apart.

It is a very narrow window. In our own Solar System, only four moons theoretically pass the test: Earth's Moon, Saturn's Titan and Lapetus, and Jupiter's Callisto. Yet none of them appear to have a moonmoon. Nobody knows quite why.

Helga & Paul Smith

—
SYNONYMS

child moon, daughter moon, fourth-order celestial body, grandmoon, mini-moon, moooon, moooon, moon-moon, moon of a moon, moonette, moonette, moonito, moonito, MOONMOON, moonlet (of a moon), moonmoonlet, nested satellite, orbital nestling, recursive moon, satellite-of-a-satellite, secondary satellite, sub-satellite body, submoon, subsatellite, tertiary body

—
SMUGGLE OWAD into a conversation today, say something like:

"The maths says it's possible that our Moon could host a MOONMOON. We just haven't found one yet."

—
Why Support OWAD?

On evenings and weekends, I research and write your daily OWAD newsletter together with Helga (my lovely wife and business partner) and our eagle-eyed daughter Jennifer. It remains FREE, AD-FREE, and ALIVE thanks to voluntary donations from appreciative readers.

If you aren’t already, please consider supporting us — even a small donation, equivalent to just 1-cup-of-coffee a month, would help us in covering expenses for mailing, site-hosting, maintenance, and service.

Just head over to DonorBox:
Please help keep OWAD alive

or

Bank transfer:
Paul Smith
IBAN: DE75 7316 0000 0002 5477 40

Important: please state as ’Verwendungszweck’: “OWAD donation” and the email address used to subscribe to OWAD.

Thanks so much,

Paul & Helga Smith