A ring around the Moon could warn of rain. A new Moon arriving on Saturday could supposedly spoil the whole month. And a crescent with its horns pointing upwards might be holding the water in.
For generations, people treated the night sky as a weather forecast. The curious part is that some of their observations were useful — even when their explanations were decidedly imaginative.
THE SHORT ANSWER
People watched the Moon because its changing appearance offered visible clues in a world where the weather could determine whether crops survived or a boat returned home.
Lunar weather lore combined observations of the atmosphere with beliefs that the Moon’s phases governed events below. These deserve different verdicts: cloud around the Moon can reveal something about approaching weather, but a calendar rule needs evidence beyond a memorable saying.
There is no single known origin for the whole collection of beliefs.
WHAT WE KNOW
Lunar forecasting appears in ancient literature. In Book I of the Georgics, Virgil linked a misty young Moon with rain and a reddish appearance with wind. He also promised settled conditions through the month if its fourth rising was clear.
That is evidence of an ancient forecasting tradition, rather than proof that its predictions worked.
The Book of Days preserves a particularly vivid later collection under 10 August. Its Suffolk contributor recorded lunar weather tables, warnings about Saturday new Moons, and the idea that an upward-facing crescent retained water.
The account also questioned several of these beliefs. People were arguing about lunar forecasts long before weather apps gave them something else to complain about.
HOW IT BECAME A HABIT
For someone deciding whether to cut hay, begin a journey or leave harbour, even an imperfect warning could feel worth remembering.
A saying offered a portable rule. It could be repeated across a kitchen table or aboard a boat without charts, instruments or a lengthy explanation.
The Moon was particularly suited to this role. It was conspicuous, changed its appearance and provided a recurring calendar in the sky.
It is easy to see how those qualities could encourage people to connect its cycle with other changes around them. That is a plausible explanation for the appeal of lunar forecasting, though it cannot establish where each individual belief began.
THE STORY EVERYONE TELLS
The familiar assumption is that a change of Moon brings a change of weather.
That makes the heavens reassuringly orderly: watch the Moon, learn the rule and prepare accordingly.
But recorded folklore was less consistent. The Book of Days treated the faintly visible “old moon” inside the bright crescent as a favourable sign. An 1888 article in Popular Science Monthly challenged that verdict and associated it with bad weather.
The same sight could acquire opposing forecasts.
Inherited confidence was not the same thing as agreement.
WHAT THE EVIDENCE ACTUALLY SAYS
The evidence label is DOCUMENTED FOLKLORE: the beliefs are recorded, but they do not form one proven forecasting system.
A halo has a physical explanation. Moonlight passing through ice crystals in high cloud can produce a ring. Such clouds may precede an approaching weather front, so the sight can offer a useful warning. It does not guarantee rain.
A smaller coloured ring, or corona, is a different optical effect, produced by diffraction around cloud droplets.
In both cases, the atmosphere changes what we see. Observers can learn something from that appearance without assuming the Moon is arranging the weather.
Rules based on a Saturday new Moon or the precise hour of a lunar phase require a separate demonstration of accuracy. Their presence in an old book establishes that people repeated them; it does not establish that they forecast successfully.
There is a further complication: the Moon really does exert a small influence on the atmosphere. University of Washington researchers reported a slight rainfall variation associated with lunar atmospheric tides in 2016. The effect was too small for people to notice and did not validate traditional monthly forecasts.
A measurable influence is a much narrower claim than knowing whether next Thursday will be wet.
WHY IT STUCK
Weather lore offered something useful even when it failed: a feeling that uncertainty could be read.
The Moon supplied a changing picture, the proverb supplied an interpretation, and a successful prediction supplied a story worth repeating.
That last point is a plausible explanation for survival, rather than a documented origin. A striking success is easier to tell your neighbour about than an uneventful failure.
The enduring fascination lies in that mixture. Some people were watching the clouds carefully. Others were giving the Moon responsibilities it had never agreed to accept.
RELATED CURIOSITIES
Why Do Sailors Say “Red Sky at Night”?
Why Was the Seventh Son of a Seventh Son Supposed to Have Magical Powers?
SOURCES & FURTHER READING
Robert Chambers, The Book of Days — 10 August, “Superstitions and Sayings Regarding the Moon and the Weather”; includes a Suffolk contribution discussing lunar forecasting beliefs.
https://www.thebookofdays.com/months/aug/10.htm
Virgil, Georgics, Book I, especially lines 424–437 — ancient lunar weather predictions; English translation hosted by Theoi Classical Texts Library.
https://www.theoi.com/Text/VirgilGeorgics1.html
Popular Science Monthly, February 1888 — “The Moon and the Weather”; a historical discussion of lunar beliefs and atmospheric observations. Its scientific explanations should be read in their nineteenth-century context.
Met Office — “Optical effects: nature’s light show”; modern explanations of haloes and coronas.
https://weather.metoffice.gov.uk/learn-about/weather/optical-effects
University of Washington, 29 January 2016 — “Moon’s tidal forces affect amount of rainfall on Earth”; research on a small atmospheric tidal effect.
https://www.washington.edu/news/2016/01/29/phases-of-the-moon-affect-amount-of-rainfall/