juliensimon/nasa-apod huggingface dataset:
Northern Pluto (2016-02-27)

Gaze across the frozen canyons of northern Pluto in this contrast enhanced color scene, imaged last July by the New Horizons spacecraft. Currently known as Lowell Regio, the region has been informally named for Percival Lowell, founder of the Lowell Observatory. Also famous for his speculation that there were canals on Mars, in 1906 Lowell started the search that ultimately led to Pluto's discovery. Pluto's North Pole itself is above and left of center in the the frame. The pale bluish floor of the broad canyon on the left is about 70 kilometers (45 miles) wide, running vertically toward the south. Higher elevations take on a yellowish hue. New Horizon's measurements have determined that in addition to nitrogen ice, methane ice is abundant across northern Pluto's Lowell Regio.
IC 1848: The Soul Nebula (2016-02-28)

Stars are forming in the Soul of the Queen of Aethopia. More specifically, a large star forming region called the Soul Nebula can be found in the direction of the constellation Cassiopeia, who Greek mythology credits as the vain wife of a King who long ago ruled lands surrounding the upper Nile river. The Soul Nebula houses several open clusters of stars, a large radio source known as W5, and huge evacuated bubbles formed by the winds of young massive stars. Located about 6,500 light years away, the Soul Nebula spans about 100 light years and is usually imaged next to its celestial neighbor the Heart Nebula (IC 1805). The featured image appears mostly red due to the emission of a specific color of light emitted by excited hydrogen gas.   Follow APOD on: Facebook,  Google Plus,  Twitter, or Instagram

Image credit: Roberto Colombari
Julius Caesar and Leap Days (2016-02-29)

Today, February 29th, is a leap day - a relatively rare occurrence. In 46 BC, Julius Caesar, featured here in a self-decreed minted coin, created a calendar system that added one leap day every four years. Acting on advice by Alexandrian astronomer Sosigenes, Caesar did this to make up for the fact that the Earth's year is slightly more than 365 days. In modern terms, the time it takes for the Earth to circle the Sun is slightly more than the time it takes for the Earth to rotate 365 times (with respect to the Sun -- actually we now know this takes about 365.24219 rotations). So, if calendar years contained 365 days they would drift from the actual year by about 1 day every 4 years. Eventually July (named posthumously for Julius Caesar himself) would occur during the northern hemisphere winter! By adopting a leap year with an extra day every four years, the calendar year would drift much less. This Julian Calendar system was used until the year 1582 when Pope Gregory XIII provided further fine-tuning when he added that leap days should not occur in years ending in "00", unless divisible by 400. This Gregorian Calendar system is the one in common use today.
NGC 3310: A Starburst Spiral Galaxy (2016-03-01)

The party is still going on in spiral galaxy NGC 3310.  Roughly 100 million years ago, NGC 3310 likely collided with a smaller galaxy causing the large spiral galaxy to light up with a tremendous burst of star formation.  The changing gravity during the collision created density waves that compressed existing clouds of gas and triggered the star-forming party.  The featured image from the Gemini North Telescope shows the galaxy in great detail, color-coded so that pink highlights gas while white and blue highlight stars.  Some of the star clusters in the galaxy are quite young, indicating that starburst galaxies may remain in star-burst mode for quite some time.  NGC 3310 spans about 50,000 light years, lies about 50 million light years away, and is visible with a small telescope towards the constellation of Ursa Major.

Image credit: AAO ITSO Office, 
Gemini Obs./AURA & 
T. A. Rector 
(U. Alaska Anchorage)
Unusual Clouds over Hong Kong (2016-03-02)

What's that in the sky? Earlier this month, in the sky high above Hong Kong, China, not just one unusual type of cloud appeared -- but two.  In the foreground was a long lenticular cloud, a cloud that forms near mountains from uprising air and might appear to some as an alien spaceship. Higher in the sky, and further in the background, was a colorful iridescent cloud. Iridescent clouds are composed of water droplets of similar size that diffract different colors of sunlight by different amounts.  Furthest in the background is the Sun, blocked from direct view by the opaque lenticular, but providing the light for the colors of the iridescent. Either type of cloud is unusual to see in Hong Kong, and unfortunately, after only a few minutes, both were gone.

Image credit: Alfred Lee
Moons and Jupiter (2016-03-03)

Some of the Solar System's largest moons rose together on February 23. On that night, a twilight pairing of a waning gibbous Moon and Jupiter was captured in this sharp telescopic field of view. The composite of short and long exposures reveals the familiar face of our fair planet's own large natural satellite, along with a line up of the ruling gas giant's four Galilean moons. Left to right, the tiny pinpricks of light are Callisto, Io, Ganymede, [Jupiter], and Europa. Closer and brighter, our own natural satellite appears to loom large. But Callisto, Io, and Ganymede are actually larger than Earth's Moon, while water world Europa is only slightly smaller.  In fact, of the Solar System's six largest planetary satellites, only Saturn's moon Titan is missing from the scene. (Editor's note: Composite corrected for orientation and field of view posted on March 7.)

Image credit: Phillip A Cruden
Sculptor Galaxy NGC 134 (2016-03-04)

NGC 134 is probably not the best known spiral galaxy in the constellation Sculptor. Still, the tantalizing island universe is a clearly a telescopic treasure in southern skies. It shares a bright core, clumpy dust lanes, and loosely wrapped spiral arms with spiky foreground stars of the Milky Way and the more diminutive galaxy NGC 131 in this sharp cosmic vista. From a distance of about 60 million light-years, NGC 134 is seen tilted nearly edge-on. It spans some 150,000 light-years, making it even larger than our own Milky Way galaxy. NGC 134's warped disk and faint extensions give the appearance of past gravitational interactions with neighboring galaxies. Like the much closer and brighter Sculptor galaxy NGC 253, tendrils of dust appear to rise from a galactic disk sprinkled with blue star clusters and pinkish star forming regions.

Image credit: CHART32 Team
Cities at Night (2016-03-05)

Looking toward the south from an altitude of 400 kilometers, this stunning snapshot from orbit finds bright lights of Tokyo and cities across central and southern Japan, planet Earth shining upward through broken clouds. The spacefaring perspective was captured last July by astronaut Scott Kelly during his stay on board the International Space Station. Thin stripes of airglow follow the curve of the planet's dark limb, while beyond lie stars of the constellation Centaurus and the southern sky. Their solar panels extended, a docked Soyuz (bottom) and Progress spacecraft are posed in the foreground.  Kelly returned to planet Earth this week after his one-year mission in space.
A Solar Prominence Eruption from SDO (2016-03-06)

One of the most spectacular solar sights is an erupting prominence. In 2011, NASA's Sun-orbiting Solar Dynamic Observatory spacecraft imaged an impressively large prominence erupting from the surface. The dramatic explosion was captured in ultraviolet light in the above time lapse video covering 90 minutes, where a new frame was taken every 24 seconds.  The scale of the prominence is huge -- the entire Earth would easily fit under the flowing  curtain of hot gas.  A solar prominence is channeled and sometimes held above the Sun's surface by the Sun's magnetic field. A quiescent prominence typically lasts about a month, and may erupt in a Coronal Mass Ejection (CME) expelling hot gas into the Solar System.  The energy mechanism that creates a solar prominence is still a topic of research.  As the Sun passes Solar Maximum, solar activity like eruptive prominences are expected to become less common over the next few years.
Mystery Feature Now Disappears in Titan Lake (2016-03-07)

What is that changing object in a cold hydrocarbon sea of Titan? Radar images from the robotic Cassini spacecraft orbiting Saturn have been recording the surface of the cloud-engulfed moon Titan for years. When imaging the flat -- and hence radar dark -- surface of the methane and ethane lake called Ligeia Mare, an object appeared in 2013 July just was not there in 2007. Subsequent observations in 2014 August found the object remained -- but had changed. In an image released last week, the mystery object seems to have disappeared in 2015 January.  The featured false-color image shows how the 20-km long object has come, changed, and gone. Current origin speculative explanations include waves, bubbling foam and floating solids, but still no one is sure. Future observations, in particular Cassini's final close flyby of Titan in 2017 April, may either resolve the enigma or open up more speculation.   Astrophysicists: Browse 1,200+ codes in the Astrophysics Source Code Library
Solar Eclipse Shoes in the Classroom (2016-03-08)

The total solar eclipse of March 8/9 will be the only total eclipse in 2016. Crossing the international date line, the New Moon's dark shadow traces a limited, narrow path for viewing the total phase, making landfall in Indonesia and mostly tracking across the Pacific Ocean. A much larger region will be witness to a partially eclipsed Sun though, during morning hours on March 9 for southeast Asia and northeast Australia, and before sunset March 8 for Hawaii and Alaska. Safely viewing the eclipse can actually be very easy. One technique is demonstrated in this shoe group portrait from a classroom in Rosenfeld, Germany, taken during March 2015's solar eclipse. With blinds closed to darken the room, each threaded hole in the window blind creates a pinhole camera, projecting multiple images of the eclipsed sun that march across the floor. Other viewing alternatives include eclipse glasses and a comfortable chair, but be sure to wear a fashionable eclipse shirt.  NASA Television: Live Eclipse Coverage begins at 8pm ET.

Image credit: Progymnasium Rosenfeld
Edge-On Galaxy NGC 5866 (2016-03-09)

Why is this galaxy so thin?  Many disk galaxies are actually just as thin as NGC 5866, pictured above, but are not seen edge-on from our vantage point.  One galaxy that is situated edge-on is our own Milky Way Galaxy.  Classified as a lenticular galaxy, NGC 5866 has numerous and complex dust lanes appearing dark and red, while many of the bright stars in the disk give it a more blue underlying hue.  The blue disk of young stars can be seen extending past the dust in the extremely thin galactic plane, while the bulge in the disk center appears tinged more orange from the older and redder stars that likely exist there. Although similar in mass to our Milky Way Galaxy, light takes about 60,000 years to cross NGC 5866, about 30 percent less than light takes to cross our own Galaxy. In general, many disk galaxies are very thin because the gas that formed them collided with itself as it rotated about the gravitational center. Galaxy NGC 5866 lies about 50 million light years distant toward the constellation of the Dragon (Draco).   Follow APOD on: Facebook,  Google Plus,  Twitter, or Instagram

Image credit: Hunter Wilson
Dark Sun over Ternate (2016-03-10)

A dark Sun hangs in the clearing sky over a volcanic planet in this morning sea and skycape. It was taken during this week's total solar eclipse, a dramatic snapshot from along the narrow path of totality in the dark shadow of a New Moon. Earth's Indonesian isle of Ternate, North Maluku lies in the foreground. The sky is still bright near the eastern horizon though, beyond the region's flattened volcanic peaks and outside the Moon's umbral shadow. In fact, near the equator the dark lunar umbra is rushing eastward across Earth's surface at about 1,700 kilometers (1,100 miles) per hour.  Shining through the thin clouds, around the Sun's silhouette is the alluring glow of the solar corona, only easily seen during totality. An inspiring sight for eclipse watchers, this solar corona is the tenuous, hot outer atmosphere of the Sun.

Image credit: Babak Tafreshi
Lunar Shadow Transit (2016-03-11)

This snapshot from deep space captures planet Earth on March 9. The shadow of its large moon is falling on the planet's sunlit hemisphere. Tracking toward the east (left to right) across the ocean-covered world the moon shadow moved quickly in the direction of the planet's rotation. Of course, denizens of Earth located close to the shadow track centerline saw this lunar shadow transit as a brief, total eclipse of the Sun. From a spacebased perspective between Earth and Sun, the view of this shadow transit was provided by the Deep Space Climate Observatory (DSCOVR) spacecraft's Earth Polychromatic Imaging Camera (EPIC).
The Flash Spectrum of the Sun (2016-03-12)

In a flash, the visible spectrum of the Sun changed from absorption to emission on March 9 during the total solar eclipse. That fleeting moment, at the beginning the total eclipse phase, is captured by telephoto lens and diffraction grating in this image from clearing skies over Ternate, Indonesia.  At left, the overwhelming light from the Sun is just blocked by the lunar disk. The normally dominant absorption spectrum of the solar photosphere is hidden. What remains, spread by the diffraction grating into the spectrum of colors to the right of the eclipsed Sun, are individual eclipse images. The images appear at each wavelength of light emitted by atoms along the thin visible arc of the solar chromosphere and in an enormous prominence extending beyond the Sun's upper limb. The brightest images, or strongest chromospheric emission lines, are due to Hydrogen atoms that produce the red hydrogen alpha emission at the far right and blue hydrogen beta emission to the left. In between, the bright yellow emission image is caused by atoms of Helium, an element only first discovered in the flash spectrum of the Sun.

Image credit: Len Fulham
Neon Saturn (2016-03-13)

If seen in the right light, Saturn glows like a neon sign.  Although Saturn has comparatively little of the element neon, a composite image false-colored in three bands of infrared light highlights features of the giant ringed planet like a glowing sign.  At the most blue band of the infrared light featured, false-colored blue in the above image, Saturn itself appears dark but Saturn's thin rings brightly reflect light from our Sun.   Conversely, Saturn's B ring is so thick that little reflected light makes it through, creating a dark band between Saturn's A and C rings.  At the most red band of the infrared, false-colored red above, Saturn emits a surprisingly detailed thermal glow, indicating planet-wide bands, huge hurricane-like storms, and a strange hexagon-shaped cloud system around the North Pole.  In the middle infrared band, false-colored green, the sunlit side of Saturn's atmosphere reflects brightly.   The above image was obtained in 2007 by the robotic Cassini spacecraft orbiting about 1.6 million kilometers out from Saturn.   Follow APOD on: Facebook,  Google Plus,  Twitter, or Instagram
Dark Nebulas across Taurus (2016-03-14)

Sometimes even the dark dust of interstellar space has a serene beauty. One such place occurs toward the constellation of Taurus.  The filaments featured here can be found on the sky between the Pleiades star cluster and the California Nebula. This dust is not known not for its bright glow but for its absorption and opaqueness. Several bright stars are visible with their blue light seen reflecting off the brown dust.  Other stars appear unusually red as their light barely peaks through a column of dark dust, with red the color that remains after the blue is scattered away. Yet other stars are behind dust pillars so thick they are not visible here.  Although appearing serene, the scene is actually an ongoing loop of tumult and rebirth. This is because massive enough knots of gas and dust will gravitationally collapse to form new stars -- stars that both create new dust in their atmospheres and destroy old dust with their energetic light and winds.

Image credit: Oliver Czernetz -
Data: 
Digitized Sky Survey (POSS-II)
Cheering a Total Solar Eclipse (2016-03-15)

What would you do if you saw the Sun disappear?  Quite possibly: cheer.  That's what many exuberant sky watchers did across Indonesia during a total eclipse of the Sun last week. There and then, the land and sky went dark during the day as our Sun disappeared for a few minutes behind our Moon.  Many people watching knew they were witnessing a rare event, and their joyous exclamations can be heard on the featured video. What a far cry this reaction is from centuries ago, when more typical eclipse reactions derived from fear and worry.  The video shows first shows a Sun only partly eclipsed by the Moon as totality approached.  From many locations, foreground clouds on our Earth either obscured the view or made the view more interesting.  The total eclipse was only visible from a narrow swath of Earth that included several Indonesian islands. At the same time, in the opposite direction, NASA's EPIC camera aboard NOAA's DSCOVR satellite captured the shadow of the Moon moving across the Earth.   APOD is also available in: Arabic, Bahasa Indonesian, Catalan, Chinese, Chinese, Croatian, Czech, Dutch, Farsi, Farsi, Galego, German, French, Hebrew, Japanese, Korean, Montenegrin, Polish, Romanian, Russian, Serbian, Slovenian, Spanish, and Turkish

Image credit: Associated Press, 
Exploratorium
A Phoenix Aurora over Iceland (2016-03-16)

All of the other aurora watchers had gone home. By 3:30 am in Iceland, on a quiet night last September, much of that night's auroras had died down. Suddenly though, a new burst of particles streamed down from space, lighting up the Earth's atmosphere once again. This time, unexpectedly, pareidoliacally, they created an amazing shape reminiscent of a giant phoenix. With camera equipment at the ready, two quick sky images were taken, followed immediately by a third of the land.  The mountain in the background is Helgafell, while the small foreground river is called Kald�, both located about 30 kilometers north of Iceland's capital Reykjavik. Seasoned skywatchers will note that just above the mountain, toward the left, is the constellation of Orion, while the Pleiades star cluster is also visible just above the frame center.  The new aurora lasted only a minute and would be gone forever -- possibly dismissed as an embellished aberration -- were it not captured in the featured, digitally-composed, image mosaic.

Image credit: Hallgrimur P. Helgason;
 Rollover Annotation:  Judy Schmidt
Close Comet and Large Magellanic Cloud (2016-03-17)

Sporting a surprisingly bright, lovely green coma Comet 252P/Linear poses next to the Large Magellanic Cloud in this southern skyscape. The stack of telephoto exposures was captured on March 16 from Penwortham, South Australia. Recognized as a Jupiter family periodic comet, 252P/Linear will come close to our fair planet on March 21, passing a mere 5.3 million kilometers away. That's about 14 times the Earth-Moon distance. In fact, it is one of two comets that will make remarkably close approaches in the next few days as a much fainter Comet Pan-STARRS (P/2016 BA14) comes within 3.5 million kilometers (9 times the Earth-Moon distance) on March 22. The two have extremely similar orbits, suggesting they may have originally been part of the same comet. Sweeping quickly across the sky because of their proximity to Earth, both comets will soon move into northern skies.

Image credit: Justin Tilbrook
The W in Cassiopeia (2016-03-18)

A familiar, zigzag, W pattern in northern constellation Cassiopeia is traced by five bright stars in this colorful and broad mosaic. Stretching about 15 degrees across rich starfields, the celestial scene includes dark clouds, bright nebulae, and star clusters along the Milky Way. In yellow-orange hues Cassiopeia's alpha star Shedar is a standout though. The yellowish giant star is cooler than the Sun, over 40 times the solar diameter, and so luminous it shines brightly in Earth's night from 230 light-years away. A massive, rapidly rotating star at the center of the W, bright Gamma Cas is about 550 light-years distant. Bluish Gamma Cas is much hotter than the Sun. Its intense, invisible ultraviolet radiation ionizes hydrogen atoms in nearby interstellar clouds to produce visible red H-alpha emission as the atoms recombine with electrons. Of course, night skygazers in the Alpha Centauri star system would also see the recognizable outline traced by Cassiopeia's bright stars. But from their perspective a mere 4.3 light-years away they would see our Sun as a sixth bright star in Cassiopeia, extending the zigzag pattern just beyond the left edge of this frame.

Image credit: Rogelio Bernal
Andreo
3D Ahuna Mons (2016-03-19)

Get out your red/blue glasses and gaze across Ceres at mysterious mountain Ahuna Mons. Shown in a 3D anaglyph perspective view, the mosaicked image data was captured in December of 2015, taken from the Dawn spacecraft's low-altitude mapping orbit about 385 kilometers above the surface of the dwarf planet. A remarkable dome-shaped feature on Ceres, with steep, smooth sides Ahuna Mons is about 20 kilometers (12 miles) in diameter at its base, rising on average 4 kilometers to a flattened summit. Similar in size to mountains found on planet Earth, no other Cerean surface feature is so tall and well-defined. It is not known what process shaped the lonely Ahuna Mons, or if the bright material streaking its steepest side is the same material responsible for Ceres' famous bright spots.
A Picturesque Equinox Sunset (2016-03-20)

What's that at the end of the road? The Sun. Many towns have roads that run east - west, and on two days each year, the Sun rises and sets right down the middle. Today is one of those days: an equinox. Not only is today a day of equal night ("aequus"-"nox") and day time, but also a day when the sun rises precisely to the east and sets due west. Featured here is a picturesque road in northwest Illinois, USA that runs approximately east -west. The image was taken one year ago today, during the March Equinox of 2015, and shows the Sun down the road at sunset. In many cultures, this March equinox is taken to be the first day of a season, typically spring in Earth's northern hemisphere, and autumn in the south. Does your favorite street run east - west? Tonight at sunset, with a quick glance, you can actually find out.   Quiz (really hard): What road is pictured?

Image credit: Roland Christen
Alaskan Moondogs (2016-03-21)

What's happened to the sky?  Moonlight illuminates a snowy scene in this night land and skyscape made on 2013 January from Lower Miller Creek, Alaska, USA. Overexposed near the mountainous western horizon is the first quarter Moon itself, surrounded by an icy halo and flanked left and right by moondogs. Sometimes called mock moons, a more scientific name for the luminous apparitions is paraselenae (plural). Analogous to a sundog or parhelion, a paraselene is produced by moonlight refracted through thin, hexagonal, plate-shaped ice crystals. As determined by the crystal geometry, paraselenae are seen at an angle of 22 degrees or more from the Moon. Compared to the bright lunar disk, paraselenae are faint and easier to spot when the Moon is low.   Follow APOD on: Facebook,  Google Plus,  Instagram, or Twitter

Image credit: Sebastian Saarloos