juliensimon/nasa-apod huggingface dataset:
An Airplane Glory (2016-12-22)

Looking out the window of an airplane, you might be lucky enough to see "the glory" in the direction directly opposite the Sun.  Before airplanes, the phenomenon, known to some as the heiligenschein or the Specter of the Brocken, was sometimes seen from mountaintops.  There, when conditions were right, one could look away from the Sun and see what appeared to be the shadow of a giant surrounded by a bright halo.  The giant turns out to be the observer, as in the modern version a silhouette of an airplane frequently occupies the glory's center.  This bright  glory was photographed two weeks ago over Michigan from an airplane on approach to O'Hare International Airport.  The cause of the glory is still being researched  and is relatively complex.  Surely, small droplets of water in some way reflect, refract, and diffract sunlight backwards towards the Sun.  The phenomenon has similar counterparts in other branches of science including astronomy, where looking out from the Earth  in the direction opposite the Sun yields a bright spot called the gegenschein.

Image credit: Shane Larson 
(Adler Planetarium, 
CIERA-Northwestern)
Once Upon a Solstice Eve (2016-12-23)

Once upon a solstice eve a little prince gazed across a frozen little planet at the edge of a large galaxy. The little planet was planet Earth of course, seen in this horizon to horizon, nadir to zenith projection, a digitally stitched mosaic from the shores of the Sec reservoir in the Czech Republic. So the large galaxy must be the Milky Way, and the brightest beacon on the planet's horizon Venus, visible around the globe as this season's brilliant evening star. Celestial treasures in surrounding dark skies include the Pleiades star cluster, and the North America nebula found along a dusty galactic rift. Embracing Venus, Zodiacal light traces a faint band across the night, but the more colorful pillars of light shine above streets a little closer to home.

Image credit: Petr Horálek
Fox Fur, Unicorn, and Christmas Tree (2016-12-24)

Clouds of glowing hydrogen gas fill this colorful skyscape in the faint but fanciful constellation Monoceros, the Unicorn. A star forming region cataloged as NGC 2264, the complex jumble of cosmic gas and dust is about 2,700 light-years distant and mixes reddish emission nebulae excited by energetic light from newborn stars with dark interstellar dust clouds. Where the otherwise obscuring dust clouds lie close to the hot, young stars they also reflect starlight, forming blue reflection nebulae. The tall, telescopic mosaic image stands up about 3/4 degree or nearly 1.5 full moons, covering 40 light-years at the distance of NGC 2264. Its cast of cosmic characters includes the the Fox Fur Nebula, whose dusty, convoluted pelt lies just left of center, bright variable star S Monocerotis immersed in the blue-tinted haze right of the Fox Fur, and the Cone Nebula pointing down from the top of the frame. Of course, the stars of NGC 2264 are also known as the Christmas Tree star cluster. The triangular tree shape traced by the stars has its apex at the Cone Nebula. The tree's broader base is centered near S Monocerotis.

Image credit: Michael Miller
The Magnificent Horsehead Nebula (2016-12-25)

Sculpted by stellar winds and radiation, a magnificent interstellar dust cloud by chance has assumed this recognizable shape.  Fittingly named the Horsehead Nebula, it is some 1,500 light-years distant, embedded in the vast Orion cloud complex. About five light-years "tall", the dark cloud is cataloged as Barnard 33 and is visible only because its obscuring dust is silhouetted against the glowing red emission nebula IC 434.  Stars are forming within the dark cloud. Contrasting blue reflection nebula NGC 2023, surrounding a hot, young star, is at the lower left.  The gorgeous color image combines both narrowband and broadband images recorded using three different telescopes.    Free Present: APOD 2017 Calendar: NASA Images

Image credit: Marco Burali, Tiziano Capecchi, Marco Mancini 
(Osservatorio MTM)
NGC 6357: Stellar Wonderland (2016-12-26)

For reasons unknown, NGC 6357 is forming some of the most massive stars ever discovered. This complex wonderland of star formation consists of numerous filaments of dust and gas surrounding huge cavities of massive star clusters.  The intricate patterns are caused by complex interactions between interstellar winds, radiation pressures, magnetic fields, and gravity.  The featured image includes not only visible light taken by the UKIRT Telescope in Hawaii (blue) as part of the SuperCosmos Sky Surveys, but infrared light from NASA's orbiting Spitzer Space Telescope (orange) and X-ray light from NASA's orbiting Chandra X-ray Observatory (pink).  NGC 6357 spans about 100 light years and lies about 5,500 light years away toward the constellation of the Scorpion.  Within 10 million years, the most massive stars currently seen in NGC 6357 will have exploded.
M31: The Andromeda Galaxy (2016-12-27)

What is the nearest major galaxy to our own Milky Way Galaxy?  Andromeda.  In fact, our Galaxy is thought to look much like Andromeda. Together these two galaxies dominate the Local Group of galaxies. The diffuse light from Andromeda is caused by the hundreds of billions of stars that compose it. The several distinct stars that surround Andromeda's image are actually stars in our Galaxy that are well in front of the background object. Andromeda is frequently referred to as M31 since it is the 31st object on Messier's list of diffuse sky objects. M31 is so distant it takes about two million years for light to reach us from there. Although visible without aid, the featured image of M31 is a digital mosaic of several frames taken with a small telescope. Much about M31 remains unknown, including exactly how many billions of years it will before it  collides with our home galaxy.   News: Vera Rubin, Dark Matter Pioneer, 1928 - 2016

Image credit: Farmakopoulos Antonis
Curiosity Surveys Lower Mount Sharp on Mars (2016-12-28)

If you could stand on Mars -- what might you see?  If you were the Curiosity rover, then just last month you would have contemplated the featured image -- a breathtaking panorama of the lower portion of Mount Sharp. The colors have been adjusted to mimic lighting familiar to Earthlings.  Surveyed here was a rocky plain before increasingly high rolling hills. The rounded hills in the middle distance, called the Sulfate Unit, are Curiosity's highest currently planned destination. One reason these hills are interesting is because sulfates are an energy source for some micro-organisms.  The immediate path forward, though, was toward the southeast on the left part of the image.
Shell Game in the LMC (2016-12-29)

An alluring sight in southern skies, the Large Magellanic Cloud (LMC) is seen here through narrowband filters. The filters are designed to transmit only light emitted by ionized sulfur, hydrogen, and oxygen atoms. Ionized by energetic starlight, the atoms emit their characteristic light as electrons are recaptured and the atom transitions to a lower energy state. As a result, this false color image of the LMC seems covered with shell-shaped clouds of ionized gas surrounding massive, young stars. Sculpted by the strong stellar winds and ultraviolet radiation, the glowing clouds, dominated by emission from hydrogen, are known as H II (ionized hydrogen) regions. Itself composed of many overlapping shells, the Tarantula Nebula is the large star forming region at top center. A satellite of our Milky Way Galaxy, the LMC is about 15,000 light-years across and lies a mere 180,000 light-years away in the constellation Dorado.

Image credit: John Gleason
Lunar Farside (2016-12-30)

Tidally locked in synchronous rotation, the Moon always presents its familiar nearside to denizens of planet Earth. From lunar orbit, the Moon's farside can become familiar, though. In fact this sharp picture, a mosaic from the Lunar Reconnaissance Orbiter's wide angle camera, is centered on the lunar farside. Part of a global mosaic of over 15,000 images acquired between November 2009 and February 2011, the highest resolution version shows features at a scale of 100 meters per pixel. Surprisingly, the rough and battered surface of the farside looks very different from the nearside covered with smooth dark lunar maria.  The likely explanation is that the farside crust is thicker, making it harder for molten material from the interior to flow to the surface and form the smooth maria.
Infrared Trifid (2016-12-31)

The Trifid Nebula, also known as Messier 20, is easy to find with a small telescope, a well known stop in the nebula rich constellation Sagittarius. But where visible light pictures show the nebula divided into three parts by dark, obscuring dust lanes, this penetrating infrared image reveals filaments of glowing dust clouds and newborn stars. The spectacular false-color view is courtesy of the Spitzer Space Telescope. Astronomers have used the Spitzer infrared image data to count newborn and embryonic stars which otherwise can lie hidden in the natal dust and gas clouds of this intriguing stellar nursery. As seen here, the Trifid is about 30 light-years across and lies only 5,500 light-years away.  News: Leap second to be added to 2016.
A Full Sky Aurora Over Norway (2017-01-01)

Higher than the highest building, higher than the highest mountain, higher than the highest airplane, lies the realm of the aurora.  Auroras rarely reach below 60 kilometers, and can range up to 1000 kilometers.  Aurora light results from energetic electrons and protons striking molecules in the Earth's atmosphere.  Frequently, when viewed from space, a complete aurora will appear as a circle around one of the Earth's magnetic poles.  The featured wide-angle image, horizontally compressed, captured an unexpected auroral display that stretched across the sky five years ago over eastern Norway.

Image credit: Sebastian Voltmer
Comet 45P Returns (2017-01-02)

An old comet has returned to the inner Solar System. Not only is Comet 45P/Honda�Mrkos�Pajdu��kov� physically ancient, it was first discovered 13 orbits ago in 1948. Comet 45P spends most of its time out near the orbit of Jupiter and last neared the Sun in 2011.  Over the past few months, however, Comet 45P's new sunward plummet has brightened it considerably.  Two days ago, the comet passed the closest part of its orbit to the Sun.  The comet is currently visible with binoculars over the western horizon just after sunset, not far from the much brighter planet Venus.  Pictured, Comet 45P was captured last week sporting a long ion tail with impressive structure.  Comet 45P will pass relatively close to the Earth early next month.    APOD Lecture: Friday, Jan. 6,  Amateur Astronomers Association of New York City

Image credit: Fritz Helmut Hemmerich
Pandora Close-up at Saturn (2017-01-03)

What do the craters of Saturn's small moon Pandora look like up close?  To help find out, NASA sent the robotic Cassini spacecraft, now orbiting Saturn, past the unusual moon two weeks ago. The highest resolution image of Pandora ever taken was then captured from about 40,000 kilometers out and is featured here.  Structures as small as 300 meters can be discerned on 80-kilometer wide Pandora.  Craters on Pandora appear to be covered over by some sort of material, providing a more smooth appearance than sponge-like Hyperion, another small moon of Saturn.  Curious grooves and ridges also appear to cross the surface of the small moon.  Pandora is partly interesting because, along with its companion moon Prometheus, it helps shepherd the particles of Saturn's F ring into a distinct ring.
Clouds of Andromeda (2017-01-04)

The beautiful Andromeda Galaxy is often imaged by planet Earth-based astronomers. Also known as M31, the nearest large spiral galaxy is a familiar sight with dark dust lanes, bright yellowish core, and spiral arms traced by blue starlight. A mosaic of well-exposed broad and narrow-band image data, this colorful, premier portrait of our neighboring island universe offers strikingly unfamiliar features though, faint reddish clouds of glowing ionized hydrogen gas in the same wide field of view. Still, the ionized hydrogen clouds likely lie in the foreground of the scene, well within our Milky Way Galaxy. They could be associated with the pervasive, dusty interstellar cirrus clouds scattered hundreds of light-years above our own galactic plane. If they were located at the 2.5 million light-year distance of the Andromeda Galaxy they would be enormous, since the Andromeda Galaxy itself is 200,000 or so light-years across.

Image credit: Rogelio Bernal
Andreo
Peculiar Galaxies of Arp 273 (2017-01-05)

The spiky stars in the foreground of this sharp cosmic portrait are well within our own Milky Way Galaxy. The two eye-catching galaxies lie far beyond the Milky Way, at a distance of over 300 million light-years. Their distorted appearance is due to gravitational tides as the pair engage in close encounters. Cataloged as Arp 273 (also as UGC 1810), the galaxies do look peculiar, but interacting galaxies are now understood to be common in the universe. In fact, the nearby large spiral Andromeda Galaxy is known to be some 2 million light-years away and approaching the Milky Way. Arp 273 may offer an analog of their far future encounter. Repeated galaxy encounters on a cosmic timescale can ultimately result in a merger into a single galaxy of stars. From our perspective, the bright cores of the Arp 273 galaxies are separated by only a little over 100,000 light-years.

Image credit: Wolfgang Ries/Stefan Heutz
New York Harbor Moonset (2017-01-06)

Moonset on January 1 is captured in this sea and night sky snapshot from the port city of New York. Its warm moonlight shining through haze and thin clouds, this New Year's Moon was about 3 days old, in a waxing crescent phase. The visible lunar disk is about 10 percent illuminated. Also easy to spot in hazy urban skies, Venus blazes forth over the western horizon, begining the year as Earth's evening star. Like the Moon, Venus goes through a range of phases as seen from planet Earth. As the year began, telescopic views of the brilliant inner planet's disk would show it about 50 percent illuminated, growing into a larger but thinner crescent by early March. New York Harbor's welcoming beacon, the Statue of Liberty, anchors a terrestrial corner of the night's triangle at the far left.

Image credit: Stan Honda
Sharpless 249 and the Jellyfish Nebula (2017-01-07)

Normally faint and elusive, the Jellyfish Nebula is caught in this alluring telescopic mosaic. The scene is anchored below by bright star Eta Geminorum, at the foot of the celestial twin, while the Jellyfish Nebula is the brighter arcing ridge of emission with tentacles dangling below and left of center. In fact, the cosmic jellyfish is part of bubble-shaped supernova remnant IC 443, the expanding debris cloud from a massive star that exploded. Light from the explosion first reached planet Earth over 30,000 years ago. Like its cousin in astrophysical waters the Crab Nebula supernova remnant, the Jellyfish Nebula is known to harbor a neutron star, the remnant of the collapsed stellar core. An emission nebula cataloged as Sharpless 249 fills the field at the upper right. The Jellyfish Nebula is about 5,000 light-years away. At that distance, this narrowband composite image presented in the Hubble Palette would be about 300 light-years across.

Image credit: Eric Coles
IC 4406: A Seemingly Square Nebula (2017-01-08)

How can a round star make a square nebula?  This conundrum comes to light when studying  planetary nebulae like IC 4406.  Evidence indicates that IC 4406 is likely a hollow cylinder, with its square appearance the result of our vantage point in viewing the cylinder from the side.  Were IC 4406 viewed from the top, it would likely look similar to the Ring Nebula.  This representative-color picture is a composite made by combining images taken by the Hubble Space Telescope in 2001 and 2002. Hot gas flows out the ends of the cylinder, while filaments of dark dust and molecular gas lace the bounding walls.  The star primarily responsible for this interstellar sculpture can be found in the planetary nebula's center.  In a few million years, the only thing left visible in IC 4406 will be a fading white dwarf star.
In the Center of Spiral Galaxy NGC 5033 (2017-01-09)

What's happening in the center of spiral NGC 5033? Many things -- some circular, some energetic, and some not well understood. NGC 5033 is known as a Seyfert galaxy because of the great activity seen in its nucleus. Bright stars, dark dust, and interstellar gas all swirl quickly around a galactic center that appears slightly offset from a supermassive black hole. This offset is thought to be the result of NGC 5033 merging with another galaxy sometime in the past billion years.  The featured image was taken by the Hubble Space Telescope in 2005. NGC 5033 spans about 100,000 light years and is so far away that we see it only as it existed about 40 million years ago.
Sentinels of a Northern Sky (2017-01-10)

Who guards the north?  The featured picture was taken last March in Finnish Lapland where weather can include sub-freezing temperatures and driving snow.  Surreal landscapes sometimes result, where white alien-looking sentinels seem to patrol the landscape.  In actuality though, the aliens are snow-covered trees, and the red hut they seem to be guarding is an outhouse.  Far in the distance, behind this uncommon Earthly vista, is a beautiful night sky which includes a green aurora, bright stars, and streaks of orbiting satellites.  Of course, in the spring, the trees thaw and Lapland looks much  different.

Image credit: Pierre Destribats
Mimas, Crater, and Mountain (2017-01-11)

Mimas is an icy, crater-pocked moon of Saturn a mere 400 kilometers (250 miles) in diameter. Its largest crater Herschel is nearly 140 kilometers wide. About a third the diameter of Mimas itself, Herschel crater gives the small moon an ominous appearance, especially for scifi fans of the Death Star battlestation of Star Wars fame. In fact, only a slightly bigger impact than the one that created such a large crater on a small moon could have destroyed Mimas entirely. In this Cassini image from October 2016, the anti-Saturn hemisphere of the synchronously rotating moon is bathed in sunlight, its large crater near the right limb. Casting a long shadow across the crater floor, Herschel's central mountain peak is nearly as tall as Mount Everest on planet Earth.
Edge-On NGC 891 (2017-01-12)

Large spiral galaxy NGC 891 spans about 100 thousand light-years and is seen almost exactly edge-on from our perspective. In fact, about 30 million light-years distant in the constellation Andromeda, NGC 891 looks a lot like our Milky Way. At first glance, it has a flat, thin, galactic disk of stars and a central bulge cut along the middle by regions of dark obscuring dust. But remarkably apparent in NGC 891's edge-on presentation are filaments of dust that extend hundreds of light-years above and below the center line. The dust has likely been blown out of the disk by supernova explosions or intense star formation activity. Fainter galaxies can also be seen near the edge-on disk in this deep portrait of NGC 891. (Editor's Note: The NGC 891 image used in today's APOD posting has been replaced and the credit corrected to indicate the author of the original work.)

Image credit: Adam Block
When Mars met Neptune (2017-01-13)

On January 1, a Mars-assisted viewing opportunity allowed binocular-equipped skygazers to cross an ice giant off their life list. Remarkably, the line-of-sight to the bright Red Planet could guide you to within 0.02 degrees of a faint, pale Neptune in Earth's night skies. Taken within 3 hours of their closest conjunction, these panels capture the odd couple's appearance in skies over Brisbane, Australia. A wide field view includes the new year's slender crescent moon near the western horizon and Venus as the brilliant evening star. Mars and Neptune are indicated at the upper right. The two inset magnified views were taken with the same telephoto lens and so do show the Mars-Neptune conjunction and the apparent size of the crescent moon at the same scale. This week Neptune hangs out near Venus on the western sky.

Image credit: Stephen Mudge
Stardust in the Perseus Molecular Cloud (2017-01-14)

Clouds of stardust drift through this deep skyscape. The cosmic scene spans nearly 2 degrees across the Perseus molecular cloud some 850 light-years away. A triangle of dusty nebulae reflecting light from embedded stars is captured in the telescopic field of view. With a characteristic bluish color reflection nebula NGC 1333 is at left, vdB13 at bottom right, and rare yellowish reflection nebula vdB12 lies at the top. Stars are forming in the Perseus molecular cloud, though most are obscured at visible wavelengths by the pervasive dust. Still, hints of contrasting red emission from Herbig-Haro objects, the jets and shocked glowing gas emanating from recently formed stars, are evident in NGC 1333. At the estimated distance of the molecular cloud, legs of the triangle formed by the reflection nebulae would be about 20 light-years long.

Image credit: Lorand Fenyes
The Matter of the Bullet Cluster (2017-01-15)

What's the matter with the Bullet Cluster? This massive cluster of galaxies (1E 0657-558) creates gravitational lens distortions of background galaxies in a way that has been interpreted as strong evidence for the leading theory: that dark matter exists within. Different recent analyses, though, indicate that a less popular alternative -- modifying gravity-- could explain cluster dynamics without dark matter, and provide a more likely progenitor scenario as well. Currently, the two scientific hypotheses are competing to explain the observations: it's invisible matter versus amended gravity. The duel is dramatic as a clear Bullet-proof example of dark matter would shatter the simplicity of modified gravity theories.  For the near future, the battle over the Bullet cluster is likely to continue as new observations, computer simulations, and analyses are completed.  The featured image is a Hubble/Chandra/Magellan composite with red depicting the X-rays emitted by hot gas, and blue depicting the suggested separated dark matter distribution.
Geostationary Highway through Orion (2017-01-16)

Put a satellite in a circular orbit about 42,000 kilometers from the center of the Earth and it will orbit once in 24 hours. Because that matches Earth's rotation period, it is known as a geosynchronous orbit. If that orbit is also in the plane of the equator, the satellite will hang in the sky over a fixed location in a geostationary orbit. As predicted in the 1940s by futurist Arthur C. Clarke, geostationary orbits are in common use for communication and weather satellites, a scenario now well-known to astroimagers. Deep images of the night sky made with telescopes that follow the stars can also pick up geostationary satellites glinting in sunlight still shining far above the Earth's surface. Because they all move with the Earth's rotation against the background of stars, the satellites leave trails that seem to follow a highway across the celestial landscape.  The phenomenon was captured last month in this video showing several satellites in geostationary orbit crossing the famous Orion Nebula.   Open Science: Browse 1,400+ codes in the Astrophysics Source Code Library

Image credit: James A. DeYoung