APOD: Firefly Milky Way over Russia (2021 Sep 06)

Comments and questions about the APOD on the main view screen.
User avatar
APOD Robot
Otto Posterman
Posts: 4401
Joined: Fri Dec 04, 2009 3:27 am

APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by APOD Robot » Mon Sep 06, 2021 4:05 am

Image Firefly Milky Way over Russia

Explanation: It started with a pine tree. The idea was to photograph a statuesque pine in front of the central band of our Milky Way Galaxy. And the plan, carried out two months ago, was successful -- they both appear prominently. But the resulting 3-frame panorama captured much more. Colorful stars, for example, dot the distant background, with bright Altair visible on the upper left. The planet Saturn, a bit closer, was captured just over the horizon on the far left. Just beyond the Earth's atmosphere, seen in the upper right, an Earth-orbiting satellite was caught leaving a streak during the 25-second exposure. The Earth's atmosphere itself was surprisingly visible -- as green airglow across the image top. Finally, just by chance, there was a firefly. Do you see it? Near the image bottom, the firefly blinked in yellow several times as it fluttered before the rolling hills above Milogradovka River in Primorsky Krai, Russia.

<< Previous APOD This Day in APOD Next APOD >>

RocketRon

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by RocketRon » Mon Sep 06, 2021 4:30 am

Starry starry night.....

A lot going on in that photo. Thanks.

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18338
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by neufer » Mon Sep 06, 2021 12:48 pm

How many fireflies would it take to match the brightness of the Sun?
[Ergo: A firefly 34 meters away is as bright as Sirius.]

https://www.youtube.com/watch?v=erXrnvyuhJs
Last edited by neufer on Mon Sep 06, 2021 1:08 pm, edited 1 time in total.
Art Neuendorffer

User avatar
orin stepanek
Plutopian
Posts: 6791
Joined: Wed Jul 27, 2005 3:41 pm
Location: Nebraska

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by orin stepanek » Mon Sep 06, 2021 12:59 pm

Glow little glowworm; glimmer, glimmer!
FireFlyMilkyWay_Komlev_960_annotated.jpg
I like both pictures; with or without the anotations!
FireFlyMilkyWay_Komlev_1446.jpg
I dont know if I can get a wallpaper from this; not
wide enough! :cry: Beautiful photo though!
meb0p9b84q001.jpg
Kitty liles the photos also! :wink:
You do not have the required permissions to view the files attached to this post.
Last edited by orin stepanek on Mon Sep 06, 2021 6:20 pm, edited 1 time in total.
Orin

Smile today; tomorrow's another day!

User avatar
Fred the Cat
Theoretic Apothekitty
Posts: 702
Joined: Mon Feb 22, 2016 4:09 pm
AKA: Ron
Location: Eagle, Idaho

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by Fred the Cat » Mon Sep 06, 2021 2:39 pm

In such an idyllic setting one might like to call them fairyflies but the name's been taken. :thumb_up:
Freddy's Felicity "Only ascertain as a cat box survivor"

User avatar
johnnydeep
Commander
Posts: 802
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by johnnydeep » Mon Sep 06, 2021 3:54 pm

I wasn't sure what caused airglow, and as with most things, "it's complicated". From https://en.wikipedia.org/wiki/Airglow#Description
Airglow is caused by various processes in the upper atmosphere of Earth, such as the recombination of atoms which were photoionized by the Sun during the day, luminescence caused by cosmic rays striking the upper atmosphere, and chemiluminescence caused mainly by oxygen and nitrogen reacting with hydroxyl free radicals at heights of a few hundred kilometres. It is not noticeable during the daytime due to the glare and scattering of sunlight.

Even at the best ground-based observatories, airglow limits the photosensitivity of optical telescopes. Partly for this reason, space telescopes like Hubble can observe much fainter objects than current ground-based telescopes at visible wavelengths.

Airglow at night may be bright enough for a ground observer to notice and appears generally bluish. Although airglow emission is fairly uniform across the atmosphere, it appears brightest at about 10° above the observer's horizon, since the lower one looks, the greater the mass of atmosphere one is looking through. Very low down, however, atmospheric extinction reduces the apparent brightness of the airglow.

One airglow mechanism is when an atom of nitrogen combines with an atom of oxygen to form a molecule of nitric oxide (NO). In the process, a photon is emitted. This photon may have any of several different wavelengths characteristic of nitric oxide molecules. The free atoms are available for this process, because molecules of nitrogen (N2) and oxygen (O2) are dissociated by solar energy in the upper reaches of the atmosphere and may encounter each other to form NO. Other chemicals that can create air glow in the atmosphere are hydroxyl (OH),[3][4][5] atomic oxygen (O), sodium (Na), and lithium (Li).[6]
So, what caused the particular green airglow seen here, and why is it green? Still not sure...
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18338
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by neufer » Mon Sep 06, 2021 4:05 pm

johnnydeep wrote:
Mon Sep 06, 2021 3:54 pm

So, what caused the particular green airglow seen here, and why is it green? Still not sure...
Art Neuendorffer

User avatar
johnnydeep
Commander
Posts: 802
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by johnnydeep » Mon Sep 06, 2021 6:15 pm

neufer wrote:
Mon Sep 06, 2021 4:05 pm
johnnydeep wrote:
Mon Sep 06, 2021 3:54 pm

So, what caused the particular green airglow seen here, and why is it green? Still not sure...
Thanks. That'll do:
Airglow’s subtle radiance arises from excitation of a different kind [than that of auroras]. Ultraviolet light from the daytime sun ionizes or knocks electrons off of oxygen and nitrogen atoms and molecules; at night the electrons recombine with their host atoms, releasing energy as light of different colors including green, red, yellow and blue. The brightest emission, the one responsible for creating the green streaks and bands visible from the ground and orbit, stems from excited oxygen atoms beaming light at 557.7 nanometers, smack in the middle of the yellow-green parcel of spectrum where our eyes are most sensitive.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

VictorBorun
Science Officer
Posts: 311
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by VictorBorun » Mon Sep 06, 2021 10:03 pm

johnnydeep wrote:
Mon Sep 06, 2021 6:15 pm
That'll do:
Airglow’s subtle radiance arises from excitation of a different kind [than that of auroras]. Ultraviolet light from the daytime sun ionizes or knocks electrons off of oxygen and nitrogen atoms and molecules; at night the electrons recombine with their host atoms, releasing energy as light of different colors including green, red, yellow and blue. The brightest emission, the one responsible for creating the green streaks and bands visible from the ground and orbit, stems from excited oxygen atoms beaming light at 557.7 nanometers, smack in the middle of the yellow-green ██ parcel of spectrum where our eyes are most sensitive.
Where is that spooky nebulium ██ glow at 500.7 nm and 495.9 nm ?
Is not our Sun good enough for pumping oxigen to O²⁺, or OIII ?

User avatar
johnnydeep
Commander
Posts: 802
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by johnnydeep » Tue Sep 07, 2021 12:56 pm

VictorBorun wrote:
Mon Sep 06, 2021 10:03 pm
johnnydeep wrote:
Mon Sep 06, 2021 6:15 pm
That'll do:
Airglow’s subtle radiance arises from excitation of a different kind [than that of auroras]. Ultraviolet light from the daytime sun ionizes or knocks electrons off of oxygen and nitrogen atoms and molecules; at night the electrons recombine with their host atoms, releasing energy as light of different colors including green, red, yellow and blue. The brightest emission, the one responsible for creating the green streaks and bands visible from the ground and orbit, stems from excited oxygen atoms beaming light at 557.7 nanometers, smack in the middle of the yellow-green ██ parcel of spectrum where our eyes are most sensitive.
Where is that spooky nebulium ██ glow at 500.7 nm and 495.9 nm ?
Is not our Sun good enough for pumping oxigen to O²⁺, or OIII ?
Don't know. I guess either emission at those wavelengths is happen but just not brightly enough to see here, or maybe some mechanism prevents those excited states from occurring in the first place.

And for those wondering, like I was, about "nebulium", here's https://en.wikipedia.org/wiki/Nebulium :
In the early days of telescopic astronomy, the word nebula was used to describe any fuzzy patch of light that did not look like a star. Many of these, such as the Andromeda Nebula, had spectra that looked like stellar spectra, and these turned out to be galaxies. Others, such as the Cat's Eye Nebula, had very different spectra. When William Huggins looked at the Cat's Eye, he found no continuous spectrum like that seen in the Sun, but just a few strong emission lines. The two green lines at 495.9 nm and 500.7 nm were the strongest.[1] These lines did not correspond to any known elements on Earth. The fact that helium had been identified by the emission lines in the Sun in 1868, and had then also been found on Earth in 1895, encouraged astronomers to suggest that the lines were due to a new element. The name nebulium (occasionally nebulum or nephelium) was first mentioned by Margaret Lindsay Huggins in a short communication in 1898, although it is stated that her husband occasionally used the term before.[2]
And from https://en.wikipedia.org/wiki/Doubly_ionized_oxygen :
In astronomy and atomic physics, doubly ionized oxygen is the ion O2+ (O III in spectroscopic notation). Its emission forbidden lines in the visible spectrum fall primarily at the wavelength 500.7 nm, and secondarily at 495.9 nm. Before spectra of oxygen ions became known, these lines once led to a spurious identification of the substance as a new chemical element. Concentrated levels of O III are found in diffuse and planetary nebulae. Consequently, narrow band-pass filters that isolate the 500.7 nm and 495.9 nm wavelengths of light, that correspond to green-turquoise-cyan spectral colors, are useful in observing these objects, causing them to appear at higher contrast against the filtered and consequently blacker background of space (and possibly light-polluted terrestrial atmosphere) where the frequencies of [O III] are much less pronounced.

These emission lines were first discovered in the spectra of planetary nebulae in the 1860s. At that time, they were thought to be due to a new element which was named nebulium. In 1927, Ira Sprague Bowen published the current explanation identifying their source as doubly ionized oxygen.[1]
PS - leave it to Victor to find a way to write O²⁺ without using the [ sup ] [ / sup ] tags like I did in the wikipedia quote :ssmile:
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18338
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by neufer » Tue Sep 07, 2021 6:46 pm

johnnydeep wrote:
Tue Sep 07, 2021 12:56 pm
VictorBorun wrote:
Mon Sep 06, 2021 10:03 pm

Where is that spooky nebulium ██ glow at 500.7 nm and 495.9 nm ?
Is not our Sun good enough for pumping oxigen to O²⁺, or OIII ?
Don't know. I guess either emission at those wavelengths is happen but just not brightly enough to see here, or maybe some mechanism prevents those excited states from occurring in the first place.
A Planetary Nebula:
  • 1) is driven by a very hot UV emitting star
    2) ionizing a multi Earth-mass O, N & S ultra low-density plasma
    3) of 100 to 10,000 particles/cm3 & long collision times (>1000 seconds).
The near-Earth environ has none of this :!:

Even much of the weak UV from our Sun is blocked by the Earth's hydrogen geocorona:
https://en.wikipedia.org/wiki/Geocoron wrote:
:arrow: This ultraviolet picture was taken in 1972 with a camera operated by Apollo 16 astronauts on the Moon.

The geocorona is the luminous part of the outermost region of the Earth's atmosphere, the exosphere. It is seen primarily via far-ultraviolet light (Lyman-alpha) from the Sun that is scattered from neutral hydrogen. It extends to at minimum 15.5 Earth radii and probably up to about 100 Earth radii. The geocorona has been studied from outer space by the Astrid satellites and the Galileo spacecraft (among others), using its ultraviolet spectrometer during an Earth flyby.>>
https://en.wikipedia.org/wiki/Forbidden_mechanism#In_astrophysics_and_atomic_physics wrote:
<<Forbidden emission lines have been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. In space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon. Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.>>
Art Neuendorffer

User avatar
johnnydeep
Commander
Posts: 802
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by johnnydeep » Tue Sep 07, 2021 7:31 pm

neufer wrote:
Tue Sep 07, 2021 6:46 pm
johnnydeep wrote:
Tue Sep 07, 2021 12:56 pm
VictorBorun wrote:
Mon Sep 06, 2021 10:03 pm

Where is that spooky nebulium ██ glow at 500.7 nm and 495.9 nm ?
Is not our Sun good enough for pumping oxigen to O²⁺, or OIII ?
Don't know. I guess either emission at those wavelengths is happen but just not brightly enough to see here, or maybe some mechanism prevents those excited states from occurring in the first place.
A Planetary Nebula:
  • 1) is driven by a very hot UV emitting star
    2) ionizing a multi Earth-mass O, N & S ultra low-density plasma
    3) of 100 to 10,000 particles/cm3 & long collision times (>1000 seconds).
The near-Earth environ has none of this :!:

Even much of the weak UV from our Sun is blocked by the Earth's hydrogen geocorona:
https://en.wikipedia.org/wiki/Geocoron wrote:
:arrow: This ultraviolet picture was taken in 1972 with a camera operated by Apollo 16 astronauts on the Moon.

The geocorona is the luminous part of the outermost region of the Earth's atmosphere, the exosphere. It is seen primarily via far-ultraviolet light (Lyman-alpha) from the Sun that is scattered from neutral hydrogen. It extends to at minimum 15.5 Earth radii and probably up to about 100 Earth radii. The geocorona has been studied from outer space by the Astrid satellites and the Galileo spacecraft (among others), using its ultraviolet spectrometer during an Earth flyby.>>
Huh, so the Moon probably orbits (at about 60 Earth radii) within the geocorona !
neufer wrote:
Tue Sep 07, 2021 6:46 pm
<<Forbidden emission lines have been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. In space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon. Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.>>
Still not sure what a forbidden emission line is. Is it simply caused by a electron transitioning from one shell to a lower one that doesn't usually happen, but can still happen under extreme or rare conditions?
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

User avatar
neufer
Vacationer at Tralfamadore
Posts: 18338
Joined: Mon Jan 21, 2008 1:57 pm
Location: Alexandria, Virginia

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by neufer » Tue Sep 07, 2021 9:11 pm

johnnydeep wrote:
Tue Sep 07, 2021 7:31 pm

Huh, so the Moon probably orbits (at about 60 Earth radii) within the geocorona !
  • The very tenuous outer limits of the geocorona, yes.
johnnydeep wrote:
Tue Sep 07, 2021 7:31 pm
neufer wrote:
Tue Sep 07, 2021 6:46 pm

<<Forbidden emission lines have been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. In space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon. Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.>>
Still not sure what a forbidden emission line is. Is it simply caused by a electron transitioning from one shell to a lower one that doesn't usually happen, but can still happen under extreme or rare conditions?
Is it caused by a electron dropping into a low "shell" from which it can escape only by either:
  • 1) slowly emitting a photon by a non-electric dipole transition (e.g., magnetic dipole, electric quadrupole...)
    2) or by colliding in-elastically with another atom/molecule.
At the level airglow occurs, atomic/molecular collisions still take place tens of thousands of times a second so there is no time for a forbidden emission. In planetary nebula, however, atomic/molecular collisions require thousands of seconds such that slow forbidden emissions are the preferred mode of escape.

Delayed forbidden emissions in gases/plasmas are similar to phosphorescence in solids & liquids.
https://en.wikipedia.org/wiki/Phosphorescence wrote: <<Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and re-emits it for a much longer time after the radiation source is removed.

In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for glow-in-the-dark materials which are "charged" by exposure to light. Unlike the relatively swift reactions in fluorescence, such as those seen in laser mediums like the common ruby, phosphorescent materials "store" absorbed energy for a longer time, as the processes required to reemit energy occur less often.

When the stored energy becomes locked in by the spin of the atomic electrons, a triplet state can occur, slowing the emission of light, sometimes by several orders of magnitude. Because the atoms usually begin in a singlet state of spin, favoring fluorescence, these types of phosphors typically produce both types of emission during illumination, and then a dimmer afterglow of strictly phosphorescent light typically lasting less than a second after the illumination is switched off. Conversely, when the stored energy is due to persistent phosphorescence, an entirely different process occurs without a fluorescence precursor. When electrons become trapped within a defect in the atomic or molecular lattice, light is prevented from reemitting until the electron can escape. To escape, the electron needs a boost of thermal energy to help spring it out of the trap and back into orbit around the atom. Only then can the atom emit a photon. Thus, persistent phosphorescence is highly dependent on the temperature of the material.>>
Art Neuendorffer

User avatar
johnnydeep
Commander
Posts: 802
Joined: Sun Feb 20, 2011 8:57 pm

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by johnnydeep » Wed Sep 08, 2021 3:28 pm

neufer wrote:
Tue Sep 07, 2021 9:11 pm
johnnydeep wrote:
Tue Sep 07, 2021 7:31 pm

Huh, so the Moon probably orbits (at about 60 Earth radii) within the geocorona !
  • The very tenuous outer limits of the geocorona, yes.
johnnydeep wrote:
Tue Sep 07, 2021 7:31 pm
neufer wrote:
Tue Sep 07, 2021 6:46 pm

<<Forbidden emission lines have been observed in extremely low-density gases and plasmas, either in outer space or in the extreme upper atmosphere of the Earth. In space environments, densities may be only a few atoms per cubic centimetre, making atomic collisions unlikely. Under such conditions, once an atom or molecule has been excited for any reason into a meta-stable state, then it is almost certain to decay by emitting a forbidden-line photon. Since meta-stable states are rather common, forbidden transitions account for a significant percentage of the photons emitted by the ultra-low density gas in space.>>
Still not sure what a forbidden emission line is. Is it simply caused by a electron transitioning from one shell to a lower one that doesn't usually happen, but can still happen under extreme or rare conditions?
Is it caused by a electron dropping into a low "shell" from which it can escape only by either:
  • 1) slowly emitting a photon by a non-electric dipole transition (e.g., magnetic dipole, electric quadrupole...)
    2) or by colliding in-elastically with another atom/molecule.
At the level airglow occurs, atomic/molecular collisions still take place tens of thousands of times a second so there is no time for a forbidden emission. In planetary nebula, however, atomic/molecular collisions require thousands of seconds such that slow forbidden emissions are the preferred mode of escape.

Delayed forbidden emissions in gases/plasmas are similar to phosphorescence in solids & liquids.
https://en.wikipedia.org/wiki/Phosphorescence wrote: <<Phosphorescence is a type of photoluminescence related to fluorescence. When exposed to light (radiation) of a shorter wavelength, a phosphorescent substance will glow, absorbing the light and reemitting it at a longer wavelength. Unlike fluorescence, a phosphorescent material does not immediately reemit the radiation it absorbs. Instead, a phosphorescent material absorbs some of the radiation energy and re-emits it for a much longer time after the radiation source is removed.

In simple terms, phosphorescence is a process in which energy absorbed by a substance is released relatively slowly in the form of light. This is in some cases the mechanism used for glow-in-the-dark materials which are "charged" by exposure to light. Unlike the relatively swift reactions in fluorescence, such as those seen in laser mediums like the common ruby, phosphorescent materials "store" absorbed energy for a longer time, as the processes required to reemit energy occur less often.

When the stored energy becomes locked in by the spin of the atomic electrons, a triplet state can occur, slowing the emission of light, sometimes by several orders of magnitude. Because the atoms usually begin in a singlet state of spin, favoring fluorescence, these types of phosphors typically produce both types of emission during illumination, and then a dimmer afterglow of strictly phosphorescent light typically lasting less than a second after the illumination is switched off. Conversely, when the stored energy is due to persistent phosphorescence, an entirely different process occurs without a fluorescence precursor. When electrons become trapped within a defect in the atomic or molecular lattice, light is prevented from reemitting until the electron can escape. To escape, the electron needs a boost of thermal energy to help spring it out of the trap and back into orbit around the atom. Only then can the atom emit a photon. Thus, persistent phosphorescence is highly dependent on the temperature of the material.>>
Thanks. I understood at least a bit of that.
--
"To B̬̻̋̚o̞̮̚̚l̘̲̀᷾d̫͓᷅ͩḷ̯᷁ͮȳ͙᷊͠ Go......Beyond The F͇̤i̙̖e̤̟l̡͓d͈̹s̙͚ We Know."

VictorBorun
Science Officer
Posts: 311
Joined: Fri Oct 16, 2020 10:25 pm

Re: APOD: Firefly Milky Way over Russia (2021 Sep 06)

Post by VictorBorun » Thu Sep 09, 2021 6:25 pm

neufer wrote:
Tue Sep 07, 2021 6:46 pm
Even much of the weak UV from our Sun is blocked by the Earth's hydrogen geocorona:
https://en.wikipedia.org/wiki/Geocoron wrote: :arrow: This ultraviolet picture was taken in 1972 with a camera operated by Apollo 16 astronauts on the Moon.
The geocorona is the luminous part of the outermost region of the Earth's atmosphere, the exosphere. It is seen primarily via far-ultraviolet light (Lyman-alpha) from the Sun that is scattered from neutral hydrogen. It extends to at minimum 15.5 Earth radii and probably up to about 100 Earth radii. The geocorona has been studied from outer space by the Astrid satellites and the Galileo spacecraft (among others), using its ultraviolet spectrometer during an Earth flyby.>>
wow.
At first I mistook part of an artifact X-shape bright reflection in the camera for a conic boundary of Earth's shadow.
Then I managed to made out more plausible almost cylinder boundary of Earth's shadow.