Missing Link: The physics behind the blood moon
The blood moon in the Earth's shadow is created by Rayleigh scattering and refraction of sunlight in the Earth's atmosphere – the physics of the spectacle.
Composite image of the different phases of the lunar eclipse in January 2018
(Image: P. Horálek/ESO)
A total lunar eclipse – like the one on Sunday evening – is one of the most fascinating astronomical events that can be observed with the naked eye. When the full moon slips into the shadow of the earth, it does not simply disappear, but transforms into a reddish shimmering disk – known as the "blood moon". This phenomenon, which was often interpreted as an omen in earlier times, has a purely physical cause that is closely linked to the Earth's atmosphere.
Total lunar eclipse: dancing in the umbra
A lunar eclipse only occurs at full moon when the sun, earth and moon are exactly in line. The moon crosses the shadow that the earth casts into space. This shadow consists of two parts: the penumbra and the umbra. While the moon is only slightly darkened in the penumbra, the real spectacle begins as soon as it enters the umbra completely. Seen from the surface of the moon, the sun would now be completely covered by the earth's disk. One would assume that the moon is now completely dark – but it is not.
(Image:Â Peter Rein)
Atmosphere as a lens and filter
The reason for the reddish glow is the Earth's atmosphere. It acts like a gigantic, spherical lens that refracts the sunlight that passes by the Earth and directs it into the dark core shadow. Without the atmosphere, there would be no blood moon; the moon would be almost invisible during totality.
But why is the light red? This is where Rayleigh scattering comes into play. It is named after John William Strutt, the third Baron Rayleigh. It is the same effect that gives us a blue sky during the day and a red sunset in the evening. Sunlight is a mixture of all spectral colors with different wavelengths. When this light hits the molecules in the earth's atmosphere, short-wave blue light is scattered much more effectively in all directions than long-wave red light.
Rayleigh scattering describes the elastic scattering of electromagnetic waves by particles whose diameter is significantly smaller than the wavelength λ, for example, when light is scattered by individual molecules. In the Earth's atmosphere, this mainly occurs on nitrogen and oxygen molecules.
The incident light causes the electrons in the molecules to vibrate. This excitation causes the molecule to behave as if it were a small dipole itself, oscillating in the exact rhythm of the light. This dipole in turn emits light from – at the same wavelength as the incident light.
Rayleigh scattering has a particularly strong effect on short-wave, i.e. blue, light: the intensity of the scattered radiation is inversely proportional to the fourth power of the wavelength:
Where I is the intensity of the scattered light, I0 is the original intensity, λ is the scattered wavelength and λ0 is the reference wavelength.
The formula for the scattering intensity as a function of the scattering angle θ is somewhat more complicated; r is the distance from the scattering source, α is the polarizability:
The dependence of the scattering intensity on λ-4 means that blue light is scattered much more strongly than red light:
As the sunlight makes its way through the Earth's atmosphere at the day-night edge of the Earth, the shorter wavelength blue component is therefore almost completely filtered out and scattered away. What remains is primarily the red light component, which continues its path relatively undisturbed. This deep red light is refracted by the atmosphere as if through a lens and directed precisely into the umbra cone, where it hits the surface of the moon and is reflected back to Earth from there.
Theoretically, this red glow is always present when the moon is close to the horizon. Under normal conditions, however, the white-yellow light reflected by the sun is so clearly predominant that a red component is completely outshone. This is why it is practically imperceptible to Earth observers when the full moon is undarkened.
Mie scattering, named after the German physicist Gustav Mie, plays a clearly subordinate role in the blood moon. It describes the scattering of electromagnetic waves by particles in the order of magnitude of the wavelength, provided that the surface of the particles generates an electromagnetic field that leads to diffraction of the wave. However, Mie scattering can influence the intensity and nuance of the reddish light in the blood moon because it occurs with larger aerosols and dust particles and leads to an overall whiter scattering.
An equally impressive natural spectacle could hypothetically be photographed from the moon itself, because for an observer there, the earth would be a pitch-black disc surrounded by a bright red ring during the total lunar eclipse.
A question of brightness: 19 f-stops difference
Despite the impressive sight, the blood moon is extremely dark. The brightness of the moon during totality is around 600,000 times less than that of a normal full moon. For photographers, this means that you have to increase the exposure by around 19 f-stops to get a correctly exposed image.
This extreme darkness is also the reason the red glow disappears abruptly as soon as the moon leaves the umbra. Even a tiny crescent edge of the moon, which is again directly illuminated by the sun, outshines the faint, reddish residual light so strongly that it becomes invisible to our eyes.
The exact color and brightness of the blood moon can vary – from a bright copper red to a deep, almost black dark red. This depends on the state of the Earth's atmosphere: if the layers of air along the terminator are very dusty or enriched with many aerosols due to volcanic eruptions, less light reaches the core shadow and the moon appears darker. A clean atmosphere, on the other hand, ensures a brighter and more intensely colored eclipse.
Ultimately, the blood moon is therefore not an ominous sign but a wonderful lesson in optics – a demonstration of how the atmosphere of our own planet paints the light of the sun. It transforms the night sky into a work of art.
We hope you enjoy taking photos and keep our fingers crossed that you have a cloudless sky. If you don't have any luck with the blood moon this time, you can try again in Germany on New Year's Eve 2028.
(vza)
