Leonardo Solved Earthshine in a Notebook No One Read
I was trying to be the kind of person who goes jogging after work.
The park was quiet, the air surprisingly clean, and somewhere between the second and third lap I did what I always do when I'm outside at night — I looked up.
There was a crescent moon. Just to its left, Venus, faint but steady. The kind of sky that makes you slow down without deciding to.
I kept looking at the crescent. Not the bright edge — the other side. The part that technically shouldn't be visible. A faint outline, barely there, like the moon was retaining its contour even in the dark. I'd seen it before and never thought much about it. That night, something made me stop.
I finished my run. Then I looked it up.
The phenomenon has a name: earthshine. Sunlight hits the Earth, reflects back toward the Moon, and lights the lunar surface just enough to reveal that ghostly outline. Simple enough, once you know it.
What caught me was who got there first. Not an astronomer. Not a physicist. A painter.
And then the part almost none of the popular retellings mention: it changed nothing. He worked it out, wrote it down, drew the diagram — and the notebook stayed shut for the better part of a century while other men, who had never read a word of it, figured the same thing out and got it into print.
A crescent moon over city lights, the dark side of the disk still faintly holding its shape — the detail that started all of this.
What You're Actually Looking At
Earthshine is the faint illumination visible on the night side of a thin crescent Moon. The bright crescent is lit directly by the Sun. The rest of the lunar disk — the part sitting in the Moon's own shadow — still shows a ghostly outline because it is receiving light from a second source: the Earth.
The path is Sun to Earth, Earth to Moon, Moon back to whoever is looking up. Both bodies are reflectors; neither makes its own light. The glow exists because Earth is, in effect, a very large lamp hanging in the Moon's sky.
The traditional names for it are the ashen light, or the old Moon in the new Moon's arms. The conditions are narrow: the Moon has to be a thin crescent so the bright limb doesn't swamp the faint glow, the sky has to be reasonably dark, and — this is the part that matters — you have to be looking for it. Everyone has seen it. Almost nobody has noticed it.
By NASA's reckoning, Earth seen from the lunar surface is roughly fifty times brighter than a full Moon seen from here. The night side of the Moon is not dark. It sits in Earth-light.
A Notebook He Never Published
Sometime between 1506 and 1510, Leonardo da Vinci wrote down the explanation. It sits in the manuscript now called the Codex Leicester — a set of roughly seventy pages on water, optics, geology, and astronomy, written in his usual mirror script.
On folio 2r he took the ashen light head-on. He drew the crescent with the full disk faintly present behind it, then drew and labelled the Sun and the Earth and traced the path of light between them. The British Library, describing that folio, calls it the earliest known attempted explanation of the lumen cinereum. His hypothesis was that the glow on the dark part of the Moon is sunlight reflected from the Earth's oceans.
He was not entirely right. He believed the Moon had seas and an atmosphere of its own, and he put the weight of the reflection on water. But the structural claim — that Earthlight causes the glow, that our planet throws sunlight onto the Moon's night side the way the Moon throws it onto ours — is correct, and no earlier record of anyone making it has been found.
Then he closed the notebook. He never prepared it for publication, and after his death it went to his heir and stayed in a private house.
The Timeline Almost Everyone Gets Wrong
Search for this story and you will keep meeting the same sentence: Leonardo figured it out a hundred years before Kepler proved it. It is repeated by news outlets, science blogs, and observatory newsletters. It is also not what happened.
Kepler did not prove earthshine. In his Astronomiae Pars Optica of 1604, he pointed his readers toward an explanation that Michael Maestlin — his own former teacher at Tübingen — had already put into print in 1596. Maestlin, not Kepler, holds the credit for the first published account.
And Maestlin was not alone in arriving at it independently. Sometime between 1578 and 1583, the Venetian friar Paolo Sarpi worked out much the same thing in his private philosophical notes — reasoning, as Leonardo had, that the sea's polished surface was doing the reflecting. Sarpi did not publish either.
The dating matters because of when Leonardo's manuscripts actually became available. They stayed in his heir's keeping and only began circulating in the 1580s, with a printed selection appearing in 1597. Which means Sarpi could not have read him, and Maestlin almost certainly did not. Each of them got there alone.
Leonardo, c.1506–1510, unpublished. Sarpi, c.1578–1583, unpublished. Maestlin, 1596, the first in print. Kepler, 1604, citing Maestlin. Leonardo's priority is real — and it was worth nothing to anyone, because the sentence never left the page it was written on.
That is a stranger fact than the version people repeat, and I think a more interesting one. The popular telling makes Leonardo a prophet whose insight rippled forward. The record makes him something else: a man who was right, alone, in private, for eighty years, while the world worked it out again from scratch without him.
Why Earth Wasn't Supposed to Shine
To see how large a step this was, it helps to know what the step was leaving behind.
In the early 1500s the standard educated European picture was geocentric — Earth fixed at the centre, everything else turning around it in spheres, a framework inherited from Ptolemy and hardened by centuries of scholastic teaching. Inside that picture the heavens were a different kind of thing from the Earth: perfect, unchanging, made of another substance. Earth was the heavy sediment at the bottom of the universe.
Which left no reason at all to think of Earth as something that shines. Lamps were up there. Earth was the floor.
Saying the Moon's dark side glows because Earth is reflecting sunlight onto it means treating Earth as a globe in space that behaves optically the way the Moon does — a world among worlds rather than the fixed exception. That move belongs more naturally to the planetary thinking that came after Leonardo than to the cosmology he was working inside. He never proposed heliocentrism, and nothing in the notebooks says he rejected the geocentric model outright. But the earthshine argument quietly assumes a premise the official cosmology did not grant him.
Copernicus published in 1543, twenty-four years after Leonardo died, and there is no evidence he read those notebooks — nobody could have. Whatever links the two men is a resemblance in how they had to think, not a line of influence.
Why a Painter Saw It
The connection between his painting and his optics is not a stretch. His notebooks circle back constantly to how light scatters, reflects, and softens — on skin, through haze, off water — and that study fed straight into his technique. Chiaroscuro, modelling form out of the contrast between lit and unlit. Sfumato, the graduated tone that makes edges dissolve in his late work.
Now look at what earthshine is as a problem. It is a region that ought to be dark and isn't. It is a question about why shadow is not uniform, why the boundary between lit and unlit is never absolute. That is the exact perceptual problem a painter spends decades training his eye on.
Whether the brush produced the insight is not something the surviving evidence can settle, and I would not push it further than that. But it is worth noticing that the man who solved it had spent his working life staring at soft edges — and that everyone else was looking at a sky they had been taught was already explained.
The same eye that modelled a face out of graduated light also drew the geometry of earthshine in a notebook — two versions of one long obsession with how light behaves at an edge.
What Measurement Corrected — and Why the Wrong Answer Was So Reasonable
Modern observation confirmed the geometry and corrected the reflector. Earth's brightness is dominated by cloud tops and atmospheric scattering; the oceans contribute far less than Leonardo assumed. NASA's own summary of the Apollo-era observations puts it plainly: looking back at Earth, the astronauts saw dark seas and bright cloud. The main mirror in Earth's sky is weather.
What interests me is that both Leonardo and Sarpi, working three-quarters of a century apart and with no contact, made the identical error — and for the identical reason. Both reasoned that the sea reflects best because its surface is smooth.
That intuition is the everyday one, and it is wrong in a specific way. A smooth surface reflects specularly: it throws light back at a matched angle, in one narrow beam. You get a hard glint and nothing anywhere else. A rough or particulate surface reflects diffusely: it scatters incoming light back over a wide spread of angles, so it looks bright from almost anywhere you stand. For lighting up a body a quarter of a million miles away that is sitting nowhere near the glint angle, diffuse beats specular, and it isn't close. Calm water sends a brilliant streak toward one observer and very little toward everyone else. Cloud tops send light everywhere at once.
So the polished sea, the thing that looks most like a mirror, turns out to be the worse reflector for this job — and the shapeless white stuff drifting above it turns out to be the better one.
| Aspect of the Explanation | Leonardo's Account (c.1506–1510) | Modern Understanding |
|---|---|---|
| Cause of the glow | Sunlight reflected from Earth onto the Moon's night side | Correct |
| Primary reflector on Earth | The oceans, because their surface is smooth | Cloud and atmospheric scattering; oceans contribute much less |
| Moon's own surface | Has seas and an atmosphere | No seas; only a near-vacuum exosphere, reflection off bare regolith |
| Variation with Earth's weather | Implied, not stated | Measured photometrically; earthshine tracks cloud cover |
| Brightness of Earth from the Moon | Very bright (assumed, from the ocean model) | About fifty times a full Moon seen from Earth (NASA) |
Right frame, wrong furniture. That pattern shows up constantly in early reasoning about things nobody could yet measure — and it is usually a sign that someone was thinking rather than repeating.
The Other Record He Didn't Leave
The Codex Leicester tells us what Leonardo thought about the Moon. Across thousands of surviving pages, it and its companions tell us almost nothing about his life. The same instinct that left the earthshine diagram unpublished seems to have governed everything else.
Gian Giacomo Caprotti, called Salaì, arrived in the household around 1490, aged about ten, and stayed more than twenty-five years. The early notes call him a thief and a liar and itemise the money and clothes he took. Leonardo kept him anyway, dressed him expensively, and painted him. Vasari, writing later, described a graceful curly-haired youth in whom Leonardo took great delight, and art historians have long connected Salaì to the androgynous young men in the late paintings, Saint John the Baptist among them. Whether that was employer and difficult servant, guardian and adopted son, teacher and favourite, or something else, the documents do not settle.
Francesco Melzi joined the workshop around 1506–1508 and went with Leonardo to France under Francis I. The will, notarised on 23 April 1519, left the books and notebooks to Melzi — which is how the Codex Leicester survived at all. Melzi kept them at his villa near Milan for decades. After his death around 1570 his son had no idea what they were, and they scattered into other hands.
The one piece of direct documentary evidence about Leonardo's sexuality is an anonymous denunciation dropped into the tamburo at the Palazzo della Signoria on 9 April 1476 — he was twenty-three, six days short of twenty-four, still working under Verrocchio. It named him among four men in connection with Jacopo Saltarelli, a teenaged apprentice goldsmith. The charge was dismissed for want of a signature and witnesses; a second attempt weeks later also failed.
Historians treat that file as the main contemporary indication and as nothing like proof. The general position across the specialist literature is cautious in a consistent way: same-sex attraction is regarded as probable, specific relationships are undocumented, and several biographers have suggested the 1476 episode may be exactly why — that a man who had been anonymously denounced at twenty-three learned early what a written record can cost. Others argue the question simply cannot be answered from what survives. Most agree the bonds with Salaì and Melzi were central to his life, and disagree about their nature.
He died at the Château du Clos Lucé near Amboise on 2 May 1519, at sixty-seven, with Melzi and others present. Vasari later wrote that Francis I cradled his head at the end — a famous image that modern historians read as embellishment.
He never married. No love letters, no documented attachments to women, in a century where marriage was simply what one did. The silence itself was conspicuous.
Whether he was gay in the sense we mean now, nobody can say. The category did not exist to be occupied. What is clear is that he carried something carefully, and left no record of it.
Which is, when you set the two halves of his life side by side, the same act twice. He solved the ashen light and did not publish it. He lived a life and did not write it down. In both cases the world eventually arrived at its own version without him, and in both cases we are reading around an absence he left deliberately.
Maybe that's why he stopped at the dark side of the moon. Everyone else saw the crescent and moved on. He stayed. He looked at the part that wasn't supposed to be visible — and then said very little about what he found.
He chose discretion. Not invisibility. It's hard not to read the paintings as the place the rest of it went: the soft features of Saint John the Baptist, the androgynous angels, the restrained tension in every male figure he ever painted. He never wrote about desire in his notebooks. He may simply have put it somewhere less searchable.
That's what stays with me. Not the genius. The quiet.
And a small, uncomfortable thought I keep turning over on the nights I look up: being right first is not the same as being the one who mattered. Maestlin got it into print and history moved. Leonardo got it into a drawer. The next time I see that faint outline inside the crescent, I don't think I'll be thinking about a man who was ahead of his time. I'll be thinking about how much of what he knew went into the ground with him.
Sources & References
- The British Library, via Google Arts & Culture — Codex Leicester folio 2r, the lumen cinereum and its dating: artsandculture.google.com
- Molaro & Selvelli, On the Earthshine depicted in Galileo's watercolors of the Moon — Sarpi's notes, Maestlin's 1596 publication, Kepler's 1604 citation, and the circulation history of Leonardo's manuscripts: arxiv.org
- Dictionary of Scientific Biography, Mästlin entry — earthshine explained correctly in print for the first time by Mästlin: encyclopedia.com
- NASA Science feature, The Da Vinci Glow — Earth roughly fifty times brighter than a full Moon, clouds rather than oceans as the primary reflector, Apollo observations: phys.org
- Wikipedia — Personal life of Leonardo da Vinci and Jacopo Saltarelli, used as an entry point to the primary documents rather than as authority
- Giorgio Vasari, Lives of the Artists (1550/1568) — contemporary accounts of Leonardo, Salaì, and Melzi
Related reading: Nancy Grace Roman: The Story Behind the Telescope Name — another case where the work arrived long before the credit did.
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