The Leonardo da Vinci Discovery Nobody Talks About
I was trying to be the kind of person who goes jogging after work.
The park was quiet, the air clean enough to actually notice, 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 holding its shape even in the dark. I'd seen it before and never thought much about it. Tonight, 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 up the lunar surface just enough to reveal that ghostly outline. Simple enough, once you know it. But what caught me wasn't the science.
It was who got there first.
Not an astronomer. Not a physicist. A painter.
What Earthshine Actually Is
The question the introduction raises — who got there first — depends on understanding what earthshine is and why it's not obvious. So here is the physical picture.
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 that is technically in the Moon's own shadow — still shows a ghostly outline because it is receiving light from a second source: the Earth itself.
The path of that light is Sun → Earth → Moon → Earth observer. Sunlight hits Earth's dayside, bounces off toward the Moon, illuminates the lunar surface, and then reflects back toward whoever happens to be looking up from the right place at the right time. Both Earth and Moon are reflecting bodies; neither generates its own light. The glow exists because Earth is, in effect, a very large mirror in the Moon's sky.
The traditional names for the effect include "ashen light" and "the old Moon in the new Moon's arms." The geometry required to see it is narrow: the Moon needs to be a thin crescent so the bright limb doesn't overpower the faint glow, the sky needs to be reasonably dark, and the observer needs to be looking for it.
Earth seen from the Moon can be tens of times brighter than a full Moon seen from Earth — which is why the reflection is strong enough to light the lunar surface visibly at all. The night side of the Moon is not truly dark. It is in Earth-light.
What Leonardo Wrote — and Drew — in the Codex Leicester
Around 1506–1510, Leonardo da Vinci wrote the physical explanation of earthshine that modern astronomy would eventually confirm. He did it in a manuscript now called the Codex Leicester, a collection of scientific notes covering water, optics, and astronomical observations, written in his characteristic mirror script.
On folios dealing with the Moon, Leonardo addressed the ashen light directly. He drew a crescent Moon with the full disk faintly visible, then drew and labeled the Earth and Sun to show the path of light: from the Sun to Earth, from Earth to the dark part of the Moon, then back to the observer. The diagram is explicit about the geometry. This is not vague speculation; it is a worked physical argument, illustrated.
His written explanation, as noted by Google Arts & Culture in their description of folio 2r, is that the glow of the dark part of the Moon during the crescent phase is a reflection of light from the Earth — making it the earliest known explanation of the ashen light. Leonardo's own framing was that "our" world illuminates the Moon in the same way the Moon illuminates us, a symmetry of reflection that the geocentric cosmology of his time gave people no particular reason to think about.
He was not entirely right about the mechanism. Leonardo thought the Moon had oceans and an atmosphere, and he placed particular emphasis on water as the main reflective element. That part was wrong. But the core conceptual step — that Earthlight causes the glow, that Earth reflects sunlight onto the Moon's night side — is correct, and it is the step no one before him appears to have recorded.
Leonardo's notes in the Codex Leicester date to roughly 1506–1510. Copernicus published the heliocentric model in 1543. The explanation of earthshine preceded the explanation of the solar system by about 35 years — and was written by someone who was not an astronomer.
The Cosmological Problem Leonardo Was Working Against
In the early 1500s, the standard educated European view of the cosmos was geocentric: Earth fixed at the center, the Sun and Moon and planets orbiting it in spheres, a framework derived from Ptolemy and reinforced by medieval scholastic tradition. Within that picture, the heavens were considered qualitatively different from Earth — perfect, unchanging, made of a special substance — while Earth was the heavy, corrupt center of things.
That cosmological separation had a specific implication: there was no obvious reason to think of Earth as a reflective body, as something that shines in space and can light up other worlds. Earth was the bottom of the universe, not a planet among planets.
Leonardo's explanation of earthshine required cutting across that assumption. To say that the Moon's dark side glows because Earth is reflecting sunlight onto it is to treat Earth as a globe in space — a world that behaves optically the same way the Moon does. That conceptual move is more naturally at home in the heliocentric and planetary thinking that came after him than in the geocentric framework he was working within. He did not propose heliocentrism; there is no evidence in his notebooks that he rejected the geocentric model explicitly. But the earthshine argument implicitly required treating Earth as a luminous body in space, not as an inert center.
Copernicus, writing his heliocentric model decades later, does not appear to have been directly influenced by Leonardo's unpublished notebooks. The connection between the two is thematic rather than historical: both required thinking of Earth as a world among worlds, not as a fixed special case.
Why a Painter Noticed What Astronomers Hadn't Explained
The connection between Leonardo's painting practice and his earthshine analysis is not forced. His notebooks return repeatedly to how light scatters, reflects, and diffuses — on surfaces, through the atmosphere, in water — and this analysis fed directly into his painting technique. He is associated with chiaroscuro, the modeling of form through contrasts of light and shadow, and with sfumato, the soft graduation of tone that characterizes his late work.
The earthshine problem is, at its core, a question about a region that should be dark but isn't — a question about why shadow is not uniform, why a lit area and an unlit area share a boundary that isn't absolute. That is exactly the kind of perceptual problem that chiaroscuro training develops sensitivity to. Whether Leonardo's artistic habits directly produced his astronomical insight is a question the data doesn't answer; but the structural similarity between the two problems is real.
NASA and other public astronomy sources now use the term "Da Vinci Glow" to refer to the earthlit portion of the crescent Moon, describing Leonardo as having "solved the ancient riddle of earthshine" roughly 500 years ago. The term is modern and commemorative — Leonardo did not use it. But its adoption by major scientific outreach organizations signals that the historical credit is, by now, settled.
What Modern Science Confirmed — and What It Corrected
Space-age measurements have confirmed the geometry Leonardo described and added precision about which parts of Earth do the reflecting. His emphasis on oceans as the primary source turned out to be wrong: Earth's reflectivity is dominated by clouds and atmospheric scattering, with oceans and land contributing less than Leonardo assumed. The main mirror in Earth's sky is weather, not water.
Photometric measurements from Earth and from spacecraft have since quantified earthshine brightness and tracked how it changes with cloud cover. The variation is real and measurable: when Earth's cloud cover increases, earthshine brightens; when it decreases, earthshine dims. That relationship between Earth's weather and the Moon's faint glow is precisely what Leonardo had speculated about qualitatively — that Earth's reflective capacity drives the phenomenon — even though he had the wrong reflector in mind.
Apollo missions and lunar orbiters photographed Earth from the Moon's surface and orbit, providing direct visual confirmation of how bright Earth appears in the lunar sky. Those images are consistent with the idea that Earthlight illuminates the Moon's night side by a large factor compared with a full Moon seen from Earth.
| Aspect of the Explanation | Leonardo's Account (~1506–1510) | Modern Understanding |
|---|---|---|
| Cause of the glow | Sunlight reflected from Earth onto the Moon's night side | Confirmed correct |
| Primary reflector on Earth | Oceans (water) | Clouds and atmospheric scattering; oceans secondary |
| Moon's surface properties | Moon has oceans and atmosphere | Moon has neither; reflection is from bare regolith |
| Variability with weather | Implied qualitatively | Confirmed and quantified by photometric measurements |
The core of the argument survived. The specific mechanism he proposed for why Earth reflects so well did not. That pattern — correct structural insight, incorrect supporting detail — describes a lot of early scientific reasoning about phenomena that couldn't yet be measured directly.
Salaì, Melzi, and the Life the Notebooks Don't Record
The Codex Leicester records what Leonardo thought about the Moon. His notebooks as a whole record almost nothing about his personal life. That gap between what he wrote and what he apparently experienced is part of what makes him difficult to place as a person.
Gian Giacomo Caprotti, known as Salaì, entered Leonardo's household around 1490 as a boy of about ten and remained with him for roughly 25 to 28 years. Early notes describe him as a thief, a liar, stubborn and gluttonous — Leonardo recorded specific incidents of stolen money and clothes — and yet Leonardo kept him. Giorgio Vasari later described Salaì as a "graceful and beautiful youth with fine curly hair." Art historians widely associate him with the models for Saint John the Baptist and Bacchus, works that share a youthful, androgynous quality. Whether the relationship was that of an employer and a difficult servant, an adoptive father and a son, a teacher and a favorite pupil, or something else is a question the surviving documents don't settle. There is no explicit record of an erotic relationship.
Francesco Melzi joined Leonardo's workshop probably around 1506–1508 and accompanied him to France when Leonardo moved there under Francis I's patronage. Leonardo's will, notarized on 23 April 1519, left "all and each of the books which the aforesaid testator has at present" to Melzi — meaning the notebooks, including what would become the Codex Leicester, passed into Melzi's keeping. Melzi preserved them at his villa near Milan for decades after Leonardo's death on 2 May 1519. After Melzi's own death in 1570, his son did not recognize the notebooks' importance, and they scattered into various collections.
The only direct documentary evidence about Leonardo's sexuality is the sodomy accusation filed on 9 April 1476, when he was about 24 and still working in Verrocchio's workshop. An anonymous denunciation named him among four men accused in connection with Jacopo Saltarelli, a 17-year-old goldsmith. The charges were dismissed for lack of witnesses. A second filing appears to have been made some weeks later; no further action was taken, and the case was eventually dropped. The record is cited by scholars as the main primary-source indicator that contemporaries associated Leonardo with same-sex activity, but it does not establish what happened.
The scholarly consensus, as summarized across multiple academic treatments, is cautious: the balance of evidence makes same-sex attraction "very probable," but direct proof of specific relationships is absent. Some historians argue that the 1476 trial and its associated danger may have contributed to a pattern of discretion — or possibly celibacy — that persisted through his life. Others suggest the question is not fully answerable given what survives. Most specialists agree that his relationships with Salaì and Melzi were emotionally central, and differ on how explicitly erotic those relationships were.
He died at the Château du Clos Lucé near Amboise on 2 May 1519, aged 67, with Melzi and other assistants present. Vasari later wrote that Francis I held Leonardo's head as he died — an image that became famous — but modern historians treat it as a romanticized account rather than a reliably documented fact.
He was accused. Twice, in 1476. Anonymous charges of sodomy, filed against a 24-year-old painter still working in Verrocchio's workshop. The case was dismissed — no witnesses, no evidence. But the record remained.
He never married. No documented relationships with women. No love letters. In an era when marriage was a social obligation, that absence alone was considered unusual.
Whether he was gay in the way we define the word today, nobody can say with certainty. The 15th century didn't have that category. What we do know is that he carried something — quietly, carefully — through his entire life.
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.
He chose discretion. Not invisibility. The paintings still carry it — 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. But he painted it.
Five hundred years later, his name is known in every corner of the world. Painter. Philosopher. Inventor. Scientist. A man so complete that even a child growing up anywhere on Earth learns his name and thinks — what would it be like to have even a fraction of that mind.
And yet he lived quietly. By necessity, perhaps. By choice, perhaps. We don't know which.
That's what stays with me. Not the genius. The quiet.
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Frequently Asked Questions
When can you actually see earthshine with the naked eye?
Earthshine is most visible when the Moon is a thin crescent — typically within a few days of the new Moon, either just after sunset or just before sunrise. The geometry needs to be right: the bright crescent should be present but not so wide that its glare overwhelms the faint glow on the rest of the disk. A dark sky with no competing light sources helps considerably. Under good conditions, the faint outline of the full lunar disk is visible without any optical aid.
Did Leonardo da Vinci publish his earthshine explanation anywhere?
No. The Codex Leicester, like most of Leonardo's scientific notebooks, was not published during his lifetime. It remained in manuscript form, passed through various collections after Melzi's death in 1570, and was not widely recognized or studied until centuries later. This is why the explanation had no direct influence on Copernicus or other astronomers of the 16th century — the notebooks were effectively unknown to the broader scientific community of that era.
What does the "Da Vinci Glow" term mean and who uses it?
The "Da Vinci Glow" is a modern commemorative name for the earthlit portion of the crescent Moon, used by NASA and other public astronomy sources to credit Leonardo's historical explanation. Leonardo himself never used the term. It began appearing in popular astronomy writing in the late 20th and early 21st century as writers and outreach communicators sought to highlight that the physical explanation of earthshine — which most people encounter without a name — had been worked out by a Renaissance painter roughly 500 years ago.
What happened to the Codex Leicester after Leonardo's death?
Leonardo's will left his manuscripts and books to Francesco Melzi, who preserved them at his villa near Milan for several decades. After Melzi's death in 1570, his son did not recognize the papers' significance, and they were gradually scattered — sold to or acquired by collectors, notably the sculptor Pompeo Leoni. The Codex Leicester itself passed through multiple private owners over the centuries. It is currently owned by Bill Gates, who purchased it in 1994, and pages from it have been exhibited publicly on various occasions.
Sources & References
- Google Arts & Culture — Codex Leicester, folio 2r description: https://artsandculture.google.com
- NASA public astronomy outreach — "Da Vinci Glow" usage and earthshine description: https://www.nasa.gov
- Giorgio Vasari, Lives of the Artists — biographical accounts of Leonardo, Salaì, and Melzi
- Nicolaus Copernicus, De revolutionibus orbium coelestium (1543)
- Louvre Museum — provenance records for Saint John the Baptist and the 1518 royal acquisition
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