juliensimon/nasa-apod huggingface dataset:
Saturn: Shadows of a Seasonal Sundial (2011-10-12)

Saturn's rings form one of the larger sundials known. This sundial, however, determines only the season of Saturn, not the time of day. In 2009, during Saturn's last equinox, Saturn's thin rings threw almost no shadows onto Saturn, since the ring plane pointed directly toward the Sun. As Saturn continued in its orbit around the Sun, however, the ring shadows become increasingly wider and cast further south.  These shadows are not easily visible from the Earth because from our vantage point near the Sun, the rings always block the shadows. The above image was taken in August by the robotic Cassini spacecraft currently orbiting Saturn. The rings themselves appear as a vertical bar on the image right. The Sun, far to the upper right, shines through the rings and casts captivatingly complex shadows on south Saturn, on the image left. Cassini has been exploring Saturn, its rings, and its moons since 2004, and is expected to continue until at least the maximum elongation of Saturn's shadows occurs in 2017.
The Color of IC 1795 (2011-10-13)

This sharp cosmic portrait features glowing gas and obscuring dust clouds in IC 1795, a star forming region in the northern constellation Cassiopeia. Also cataloged as NGC 896, the nebula's remarkable details, shown in its dominant red color, were captured using a sensitive camera, and long exposures that include image data from a narrowband filter. The narrow filter transmits only H-alpha light, the red light of hydrogen atoms. Ionized by ultraviolet light from energetic young stars, a hydrogen atom emits the characteristic H-alpha light as its single electron is recaptured and transitions to lower energy states. Not far on the sky from the famous Double Star Cluster in Perseus, IC 1795 is itself located next to IC 1805, the Heart Nebula, as part of a complex of star forming regions that lie at the edge of a large molecular cloud. Located just over 6,000 light-years away, the larger star forming complex sprawls along the Perseus spiral arm of our Milky Way Galaxy. At that distance, this picture would span about 70 light-years across IC 1795.

Image credit: Fera
Photography
MAGIC Star Trails (2011-10-14)

Colorful star trails arc across the night in this surreal timelapse skyscape from the Roque de los Muchachos Observatory on the Canary island of La Palma. A reflection of the Earth's daily rotation on its axis, the star trails are also reflected in one of a pair of 17 meter diameter, multi-mirrored MAGIC telescopes. The MAGIC (Major Atmospheric Gamma Imaging Cherenkov) telescope itself is intended to detect gamma rays - photons with over 100 billion times the energy of visible light. As the high energy gamma rays impact the upper atmosphere they produce air showers of high-energy particles. A fast camera monitoring the multi-mirrored surface records in detail brief flashes of optical light, called Cherenkov light, created by the air shower particles. Astronomers can then ultimately relate the optical flashes to cosmic sources of extreme gamma-rays.

Image credit: Babak Tafreshi
NGC 4565: Galaxy on Edge (2011-10-15)

Magnificent spiral galaxy NGC 4565 is viewed edge-on from planet Earth. Also known as the Needle Galaxy for its narrow profile, bright NGC 4565 is a stop on many telescopic tours of the northern sky, in the faint but well-groomed constellation Coma Berenices. This sharp, colorful image reveals the galaxy's bulging central core cut by obscuring dust lanes that lace NGC 4565's thin galactic plane. An assortment of other background galaxies is included in the pretty field of view, with neighboring galaxy NGC 4562 at the lower right. NGC 4565 itself lies about 40 million light-years distant and spans some 100,000 light-years.  Easily spotted with small telescopes, sky enthusiasts consider NGC 4565 to be a prominent celestial masterpiece Messier missed.

Image credit: Bob Franke
A Picturesque Venus Transit (2011-10-16)

The rare transit of Venus across the face of the Sun in 2004 was one of the better-photographed events in sky history.  Both scientific and artistic images flooded in from the areas that could see the transit: Europe and much of Asia, Africa, and North America.  Scientifically, solar photographers confirmed that the black drop effect is really better related to the viewing clarity of the camera or telescope than the atmosphere of Venus. Artistically, images might be divided into several categories.  One type captures the transit in front of a highly detailed Sun.  Another category captures a double coincidence such as both Venus and an airplane simultaneously silhouetted, or Venus and the International Space Station in low Earth orbit. A third image type involves a fortuitous arrangement of interesting looking clouds, as shown by example in the above image taken from North Carolina, USA. The next transit of Venus across the Sun will be in 2012 June.

Image credit: David Cortner
MACS 1206: A Galaxy Cluster Gravitational Lens (2011-10-17)

It is difficult to hide a galaxy behind a cluster of galaxies. The closer cluster's gravity will act like a huge lens, pulling images of the distant galaxy around the sides and greatly distorting them. This is just the case observed in the above recently released image from the CLASH survey with the Hubble Space Telescope. The cluster MACS J1206.2-0847 is composed of many galaxies and is lensing the image of a yellow-red background galaxy into the huge arc on the right. Careful inspection of the image will reveal at least several other lensed background galaxies -- many appearing as elongated wisps. The foreground cluster can only create such smooth arcs if most of its mass is smoothly distributed dark matter -- and therefore not concentrated in the cluster galaxies visible. Analyzing the positions of these gravitational arcs also gives astronomers a method to estimate the dark matter distribution in galaxy clusters, and infer from that when these huge conglomerations of galaxies began to form.   APOD Retrospective: The best of Clusters of Galaxies
Movie: Approaching Light Speed (2011-10-18)

What would it look like to travel near the speed of light? Strange visual effects would appear as documented in the above relativistically-accurate animation. First of all, relativistic aberration would cause objects to appear to bunch up in front you. Next, the Doppler shift would cause the colors of forward objects to shift toward the blue, while things behind you would shift toward the red. Similarly, the world in front of you would seem to move unusually fast, while the world behind you would appear to slow down. Objects to the sides will appear rotated, possibly enabling surfaces normally hidden from you to become visible. Of course, since constant motion is relative, the same effects would occur were you to remain stationary and the entire world advanced toward you.

Image credit: Antony Searle & 
Craig Savage 
(ANU)
Draconid Meteors Over Spain (2011-10-19)

What are those streaks in the sky? They're meteors from the Draconids meteor shower that peaked earlier this month. The above composite image captured numerous meteor streaks over 90 minutes above the Celtic ruins of Capote in Badajoz province, Spain. The particles that caused these meteors were typically the size of a pebble and were expelled long ago from the nucleus of comet 21P/Giacobini-Zinner.  Most of the above meteors can be traced back to a single radiant emanating from the constellation of the Dragon (Draco).  Reports from this year's meteor shower indicate that the Draconids were unusually good this year with activity was concentrated around 8 pm UT on October 8.  The most intense Draconid meteor showers in recent history occurred in 1933 and 1946 when thousands of meteors per hour were recorded as the Earth plowed through particularly dense streams of comet debris. Although the Draconids occur every October, it is usually difficult to know just how active each year's meteor shower will be.   APOD Challenge: How many meteors can you find on the above image?

Image credit: Juan Carlos Casado
Tails of Comet Garradd (2011-10-20)

A good target for binoculars and small telescopes, Comet Garradd (C/2009 P1) now shines in planet Earth's evening skies, a steady performer but just below naked-eye visibility. Telescopic images like this composite from October 15 can find the comet with a lovely green coma, sporting multiple tails, and lingering against a background of faint stars. The field of view spans over 1 degree or about 2 full moons within the southern boundaries of the constellation Hercules. Now around 16 light minutes (2 astronomical units) away, P1 Garradd is an intrinsically large comet, but will never make a very close approach to Earth or the Sun while sweeping through the inner solar system. As a result, the comet will likely stay a sight for telescopic eyes only, moving slowly through the sky and remaining in Hercules during the coming months.

Image credit: Gregg Ruppel
Clouds of Perseus (2011-10-21)

Cosmic clouds of gas and dust drift across this magnificent panorama, spanning some 17 degrees near the southern boundary of the heroic constellation Perseus. The collaborative skyscape begins with bluish stars of Perseus at the left, but the eye is drawn to the striking, red NGC 1499. Also known as the California Nebula, its characteristic glow of atomic hydrogen gas is powered by ultraviolet light from luminous blue star Xi Persei immediately to the nebula's right. Farther along, intriguing young star cluster IC 348 and neighboring Flying Ghost Nebula are right of center. Connected by dark and dusty tendrils on the outskirts of a giant molecular cloud, another active star forming region, NGC 1333, lies near the upper right edge of the wide field of view. Shining faintly, dust clouds strewn throughout the scene are hovering hundreds of light-years above the galactic plane and reflect starlight from the Milky Way.

Image credit: Bob Caton
Jupiter Near Opposition (2011-10-22)

On October 29 (UT), Jupiter, the solar system's largest planet, will be at opposition, opposite the Sun in planet Earth's sky, shining brightly and rising as the Sun sets. That configuration results in Jupiter's almost annual closest approach to planet Earth, so near opposition the gas giant offers earthbound telescopes stunning views of its stormy, banded atmosphere and large Galilean moons. This sharp snapshot of Jupiter was captured on October 13 with the 1 meter telescope at the Pic Du Midi mountain top observatory in the French Pyrenees. North is up in the image that shows off oval shaped vortices and planet girdling dark belts and light zones. Also seen in remarkable detail, Jupiter's icy Ganymede, the solar system's largest moon, is emerging from behind the planet (top) while volcanic Io enters the frame near the lower left edge.

Image credit: Jean-Luc Dauvergne
Jupiter's Clouds from New Horizons (2011-10-23)

The New Horizons spacecraft took some stunning images of Jupiter on its way out to Pluto. Famous for its Great Red Spot, Jupiter is also known for its regular, equatorial cloud bands, visible through even modest sized telescopes. The above image, horizontally compressed, was taken in 2007 near Jupiter's terminator and shows the Jovian giant's wide diversity of cloud patterns.  On the far left are clouds closest to Jupiter's South Pole.  Here turbulent whirlpools and swirls are seen in a dark region, dubbed a belt, that rings the planet.  Even light colored regions, called zones, show tremendous structure, complete with complex wave patterns.   The energy that drives these waves surely comes from below.  New Horizons is the fastest space probe ever launched, has now passed the orbits of Saturn and Uranus and is on track to reach Pluto in 2015.
HH-222: The Waterfall Nebula (2011-10-24)

What created the Waterfall Nebula? No one knows.  The structure seen in the region of NGC 1999 in the Great  Orion Molecular Cloud complex is one of the more mysterious structures yet found on the sky. Designated HH-222, the elongated gaseous stream stretches about ten light years and emits an unusual array of colors. One hypothesis is that the gas filament results from the wind from a young star impacting a nearby molecular cloud. That would not explain, however, why the Waterfall and fainter streams all appear to converge on a bright but unusual non thermal radio source located toward the upper left of the curving structure. Another hypothesis is that the unusual radio source originates from a binary system containing a hot white dwarf, neutron star, or black hole, and that the Waterfall is just a jet from this energetic system. Such systems, though, are typically strong X-rays emitters, and no X-rays have been detected. For now, this case remains unsolved.  Perhaps well-chosen future observations and clever deductive reasoning will unlock the true origin of this enigmatic wisp in the future.   Astronomers: What other "mysterious structures" appear on the sky?
IC 1805: The Heart Nebula in HDR (2011-10-25)

What powers the Heart Nebula?  The large emission nebula dubbed IC 1805 looks, in whole, like a human heart.  The nebula glows brightly in red light emitted by its most prominent element: hydrogen.  The red glow and the larger shape are all created by a small group of stars near the nebula's center.  A close up in high dynamic range (HDR) spanning about 30 light years contains many of these stars is shown above. This open cluster of stars contains a few bright stars nearly 50 times the mass of our Sun, many dim stars only a fraction of the mass of our Sun, and an absent microquasar that was expelled millions of years ago.  The Heart Nebula is located about 7,500 light years away toward the constellation of Cassiopeia.

Image credit: Daniel Verloop  (Beursacademie)
In, Through, and Beyond Saturn's Rings (2011-10-26)

A fourth moon is visible on the above image if you look hard enough. First -- and farthest in the background -- is Titan, the largest moon of Saturn and one of the larger moons in the Solar System.  The dark feature across the top of this perpetually cloudy world is the north polar hood. The next most obvious moon is bright Dione, visible in the foreground, complete with craters and long ice cliffs. Jutting in from the left are several of Saturn's expansive rings, including Saturn's A ring featuring the dark Encke Gap. On the far right, just outside the rings, is Pandora, a moon only 80-kilometers across that helps shepherd Saturn's F ring. The fourth moon?  If you look closely in the Encke Gap you'll find a speck that is actually Pan. Although one of Saturn's smallest moons at 35-kilometers across, Pan is massive enough to help keep the Encke gap relatively free of ring particles.
Young Suns of NGC 7129 (2011-10-27)

Young suns still lie within dusty NGC 7129, some 3,000 light-years away toward the royal constellation Cepheus. While these stars are at a relatively tender age, only a few million years old, it is likely that our own Sun formed in a similar stellar nursery some five billion years ago. Most noticeable in the sharp image are the lovely bluish dust clouds that reflect the youthful starlight. But the compact, deep red crescent shapes are also markers of energetic, young stellar objects. Known as Herbig-Haro objects, their shape and color is characteristic of glowing hydrogen gas shocked by jets streaming away from newborn stars. Paler, extended filaments of reddish emission mingling with the bluish clouds are caused by dust grains effectively converting the invisible ultraviolet starlight to visible red light through photoluminesence. Ultimately the natal gas and dust in the region will be dispersed, the stars drifting apart as the loose cluster orbits the center of the Galaxy. At the estimated distance of NGC 7129, this telescopic view spans about 40 light-years.

Image credit: Panther Observatory
October Skylights (2011-10-28)

As northern hemisphere nights grow longer, October is a good month for spotting auroras, or even other eerie apparitions after dark. And this week the night sky did not disappoint. On October 24th a solar coronal mass ejection impacted planet Earth's magnetosphere triggering far ranging auroral displays. On that night, this dramatic silhouette against deep red and beautiful green curtains of shimmering light was captured near Whitby, Ontario, Canada. But auroras were reported even farther south, in US states like Alabama, Kansas, and Oklahoma at latitudes rarely haunted by the northern lights. Well above 100 kilometers, at the highest altitudes infused by the auroral glow, the red color comes from the excitation of oxygen atoms.

Image credit: Malcolm Park
Spiral Galaxy NGC 3370 from Hubble (2011-10-29)

Is this what our own Milky Way Galaxy looks like from far away? Similar in size and grand design to our home Galaxy (although without the central bar), spiral galaxy NGC 3370 lies about 100 million light-years away toward the constellation of the Lion (Leo).  Recorded above in exquisite detail by the Hubble Space Telescope's Advanced Camera for Surveys, the big, beautiful face-on spiral is not only photogenic, but has proven sharp enough to study individual stars known as Cepheids. These pulsating stars have been used to accurately determine NGC 3370's distance. NGC 3370 was chosen for this study because in 1994 the  spiral galaxy was also home to a well studied stellar explosion -- a Type Ia supernova.  Combining the known distance to this standard candle supernova, based on the Cepheid measurements, with observations of supernovas at even greater distances, has helped to reveal the size and expansion rate of the entire Universe itself.
White Rock Fingers on Mars (2011-10-30)

What caused this unusual light rock formation on Mars?  Intrigued by the possibility that they could be salt deposits left over as an ancient lakebed dried-up, detailed studies of these fingers now indicate a more mundane possibility: volcanic ash.  Studying the exact color of the formation indicated the possible volcanic origin.  The light material appears to have eroded away from surrounding area, indicating a very low-density substance. The stark contrast between the rocks and the surrounding sand is compounded by the unusual darkness of the sand.  The above picture was taken with the Thermal Emission Imaging System on the Mars Odyssey spacecraft currently orbiting Mars.  The image spans about 10 kilometers inside a larger crater.
Ghost of the Cepheus Flare (2011-10-31)

Spooky shapes seem to haunt this starry expanse, drifting through the night in the royal constellation Cepheus. Of course, the shapes are cosmic dust clouds faintly visible in dimly reflected starlight. Far from your own neighborhood on planet Earth, they lurk at the edge of the Cepheus Flare molecular cloud complex some 1,200 light-years away. Over 2 light-years across the ghostly nebula and relatively isolated Bok globule, also known as vdB 141 or Sh2-136, is near the center of the field. The core of the dark cloud on the right is collapsing and is likely a binary star system in the early stages of formation.  Even so, if the spooky shapes could talk, they might well wish you a happy Halloween.

Image credit: Adam Block
Hammer Versus Feather on the Moon (2011-11-01)

If you drop a hammer and a feather together, which reaches the ground first? On the Earth, it's the hammer, but is the reason only because of air resistance?  Scientists even before Galileo have pondered and tested this simple experiment and felt that without air resistance, all objects would fall the same way. Galileo tested this principle himself and noted that two heavy balls of different masses reached the ground simultaneously, although many historians are skeptical that he did this experiment from Italy's Leaning Tower of Pisa as folklore suggests. A good place free of air resistance to test this equivalence principle is Earth's Moon, and so in 1971, Apollo 15 astronaut David Scott dropped both a hammer and a feather together toward the surface of the Moon.  Sure enough, just as scientists including Galileo and Einstein would have predicted, they reached the lunar surface at the same time. The demonstrated equivalence principle states that the acceleration an object feels due to gravity does not depend on its mass, density, composition, color, shape, or anything else. The equivalence principle is so important to modern physics that its depth and reach are still being debated and tested even today.
NGC 7380: The Wizard Nebula (2011-11-02)

What powers are being wielded in the Wizard Nebula? Gravitation strong enough to form stars, and stellar winds and radiations powerful enough to create and dissolve towers of gas. Located only 8,000 light years away, the Wizard nebula, pictured above, surrounds developing open star cluster NGC 7380. Visually, the interplay of stars, gas, and dust has created a shape that appears to some like a fictional medieval sorcerer.   The active star forming region spans 100 about light years, making it appear larger than the angular extent of the Moon. The Wizard Nebula can be located with a small telescope toward the constellation of the King of Aethiopia (Cepheus). Although the nebula may last only a few million years, some of the stars being formed may outlive our Sun.   Imagine: What shape(s) do you see in the nebula?

Image credit: Rolf Geissinger
IC 59 and IC 63 in Cassiopeia (2011-11-03)

These bright rims and flowing shapes suggest to some melting ice cream on a cosmic scale. Looking toward the constellation Cassiopeia, the colorful (zoomable) skyscape features the swept back, comet-shaped clouds IC 59 (left) and IC 63. About 600 light-years distant, the clouds aren't actually melting, but they are slowly dissipating under the influence of ionizing ultraviolet radiation from hot,luminous star gamma Cas. Gamma Cas is physically located only 3 to 4 light-years from the nebulae, just off the upper right edge of the frame. In fact, slightly closer to gamma Cas, IC 63 is dominated by red H-alpha light emitted as the ionized hydrogen atoms recombine with electrons. Farther from the star, IC 59 shows proportionally less H-alpha emission but more of the characteristic blue tint of dust reflected star light. The field of view spans about 1 degree or 10 light-years at the estimated distance of gamma Cas and friends.

Image credit: Ken Crawford
Edge-on NGC 3628 (2011-11-04)

Sharp telescopic views of magnificent edge-on spiral galaxy NGC 3628 show a puffy galactic disk divided by dark dust lanes. The tantalizing scene puts many astronomers in mind of its popular moniker, The Hamburger Galaxy. About 100,000 light-years across and 35 million light-years away in the constellation Leo, NGC 3628 shares its neighborhood in the local Universe with two other large spirals, a grouping otherwise known as the Leo Triplet. Gravitational interactions with its cosmic neighbors are likely responsible for the extended flare and warp of this spiral's disk, populated by the galaxy's young blue star clusters and tell tale pinkish star forming regions. Also a result of past close encounters, a faint tidal tail of material is just visible extending upward and left in this deep galaxy portrait.

Image credit: Stephen Leshin