Why the Moon Turns Red
A blood moon is every sunrise and sunset on Earth at once, projected onto the Moon.
The Moon inside our shadow
Junior level β plain language, no maths
Some nights the full Moon does something unsettling: instead of going black, it turns a deep, dusty red. People call it a blood moon, and it happens during a lunar eclipse - when the Earth passes exactly between the Sun and the Moon and drops its shadow onto it.
Here is the strange part. If the Earth simply blocked the sunlight, the Moon would vanish into blackness. It doesn't, because our planet has something a bare rock would not have: air. Sunlight skimming the edge of the Earth passes through a long sliver of atmosphere, and two things happen to it on the way. The air bends it inwards, curling light into a shadow that should have been empty. And the air strips out the blue, scattering it sideways across the sky and letting the red and orange carry straight on.
So the light landing on an eclipsed Moon has already been filtered by our atmosphere. It is, quite literally, the light of every sunrise and every sunset happening on Earth at that moment, gathered up and thrown 384,000 km onto a rock. Stand on the Moon during a lunar eclipse and you would see a black Earth ringed by a thin, blazing circle of red: all the world's dawns and dusks at once.
That is also why no two blood moons look alike. When the air is clean the Moon glows bright copper. When a big volcano has thrown dust and sulphur high into the atmosphere, more light is absorbed on the way through and the Moon can turn a murky brown - or almost disappear.
Things worth knowing
- The red light on an eclipsed Moon is sunlight filtered through Earth atmosphere - every sunrise and sunset on the planet at the same moment.
- After the 1991 Pinatubo eruption the next lunar eclipse was so dark that the Moon was hard to find at all: volcanic dust had swallowed the light.
- A lunar eclipse is completely safe to watch with bare eyes, unlike a solar one - you are looking at a dim Moon, not at the Sun.
Umbra, penumbra and the Danjon scale
Student level β the core equations
Earth casts two nested shadows. The penumbra is the outer cone, where the Sun is only partly blocked; a Moon inside it dims so slightly that most people never notice. The umbra is the inner cone, where the Earth blocks the Sun completely. That is where the colour happens.
At the Moon's distance the umbra is about 2.6 times wider than the Moon itself, which is why totality can last well over an hour. How deeply the Moon enters depends on how closely the line-up falls, and astronomers quote that as the umbral magnitude: 1.0 or more is total, 0.96 is a deep partial with one stubborn bright sliver left, and below 0 the Moon never leaves the penumbra at all.
Inside the umbra the only illumination is refracted, Rayleigh-filtered sunlight. Earth's atmosphere works as a lens, bending grazing rays by up to about a degree, and as a filter: scattering goes as \(1/\lambda^4\), so blue is scattered roughly ten times more strongly than red. What survives the trip is the reddened remainder, thousands of times fainter than an ordinary full Moon.
Because that light has to survive our air, the Moon's appearance becomes a live readout of atmospheric conditions. Observers score it on the Danjon scale, from L=4 for a bright coppery-orange eclipse down to L=0 for one so dark the Moon nearly vanishes. Volcanic aerosols and heavy cloud along the terminator push the score down.
Key Formulas
| Rayleigh scattering | \(I_{\text{scattered}} \propto \dfrac{1}{\lambda^{4}}\) | blue leaves, red survives |
|---|---|---|
| Umbral magnitude | \(U = \dfrac{R_u + r_m - d}{2 r_m}\) | total when U \ge 1 |
| Umbra at the Moon | \(R_u \approx 1.3^\circ \approx 2.6\, r_m\) | |
Things worth knowing
- Earth umbra at the Moon distance is about 2.6 lunar diameters across, so a central total eclipse can hold the Moon fully shadowed for over 100 minutes.
- Rayleigh scattering scales as 1/Ξ»β΄, so blue light is scattered about ten times more than red - the leftover light is red by elimination.
- The Danjon scale runs from L=0 (Moon almost invisible) to L=4 (bright copper-orange with a bluish rim).
Refraction, extinction and the variable colour
Scholar level β full mathematical depth
The umbra is not dark because Earth's atmosphere refracts sunlight into it. Rays passing at grazing incidence are bent by roughly \(0.5^\circ\) to \(1^\circ\), enough to fill the geometric shadow at lunar distance with transmitted light. The illumination reaching an eclipsed Moon comes almost entirely from Earth's limb, integrated around the whole terminator, so its spectrum is the solar spectrum seen through a very long atmospheric path.
That path is what reddens it. Molecular Rayleigh scattering removes short wavelengths with a \(\lambda^{-4}\) dependence, while ozone absorption in the Chappuis band takes a bite out of 500-700 nm. That second effect is why deep eclipses often show a distinctly turquoise fringe just inside the umbral edge, where the light has taken the longest path through the ozone layer, before the interior settles into red.
Total brightness is highly variable: geometric depth in the umbra, aerosol loading and even cloud cover along the terminator all matter. A totally eclipsed Moon is typically \(10^{4}\) to \(10^{5}\) times fainter than an uneclipsed full Moon - a spread of several magnitudes that the Danjon scale captures only crudely. After major stratospheric injections, El ChichΓ³n in 1982 and Pinatubo in 1991, eclipses were recorded at L=0, effectively invisible to the unaided eye.
The geometry repeats on the saros, 6585.32 days, because that interval is very nearly a whole number of synodic, draconic and anomalistic months at the same time. The leftover third of a day turns the Earth by 120Β°, so each repeat is best seen a third of the way around the globe from the last, and a given saros comes home every three cycles - 54 years.
Key Formulas
| Refraction at the limb | \(\Delta\theta \approx 0.5^\circ - 1^\circ\) | fills the geometric umbra |
|---|---|---|
| Rayleigh cross-section | \(\sigma \propto \lambda^{-4}\) | |
| Brightness in totality | \(L/L_{\text{full}} \sim 10^{-4} - 10^{-5}\) | |
| Saros | \(223\ \text{synodic} \approx 6585.32\ \text{d}\) | |
Things worth knowing
- Light reaching the eclipsed Moon comes from Earth limb integrated around the entire terminator: an observer standing there would see a black Earth ringed by fire.
- Ozone absorption in the 500-700 nm Chappuis band gives many eclipses a turquoise fringe just inside the umbral edge.
- The saros is 6585.32 days; the extra third of a day shifts each repetition 120Β° in longitude, so the same saros returns to your part of the world every 54 years.