juliensimon/nasa-apod huggingface dataset:
Northern Lights (2004-07-30)

While enjoying the spaceweather on a gorgeous summer evening in mid-July, astronomer Philippe Moussette captured this colorful fish-eye lens view looking north from the Observatoire Mont Cosmos, Quebec, Canada, planet Earth. In the foreground, lights along the northern horizon give an orange cast to the low clouds. But far above the clouds, at altitudes of 100 kilometers or more, are alluring green and purple hues of the aurora borealis or northern lights, a glow powered by energetic particles at the edge of space. In the background are familiar stars of the northern sky. In particular, that famous celestial kitchen utensil, the Big Dipper (left), and the W-shaped constellation Cassiopeia (right) are easy to spot. Then, just follow the pointer stars of the Big Dipper to Polaris, perhaps the most famous northern light of all.

Image credit: Philippe
Moussette
(Obs. Mont
Cosmos)
Tonight: A Blue Moon (2004-07-31)

How often does a full moon occur twice in a single month?  Exactly once in a Blue Moon.  In fact, the modern usage of the term "Blue Moon" refers to the second Full Moon in a single month.  Tonight's Blue Moon will be the first since November 2001.  A Blue Moon typically occurs every few years.  The reason for the rarity of the Blue Moon is that the 29.53 days between full moons is just slightly shorter than the number of days in the average month.  Don't, however, expect the moon to look blue tonight!  The term "Blue Moon" has recently been traced to an error in a magazine article in 1946.  It is possible for the Moon to appear tinged by a blue hue, sometimes caused by fine dirt circulating in the Earth's atmosphere, possibly from a volcanic explosion.  The above picture was taken not during a full moon but through a morning sky that appeared dark blue.  The bright crescent is the only part directly exposed to sunlight - the rest of the Moon glows from sunlight reflected from the Earth.  In this dramatic photo, however, the planet Jupiter is also visible along with its four largest moons.

Image credit: Vic Winter (ICSTARS)
A Force from Empty Space: The Casimir Effect (2004-08-01)

This tiny ball provides evidence that the universe will expand forever.  Measuring slightly over one tenth of a millimeter, the ball moves toward a smooth plate in response to energy fluctuations in the vacuum of empty space.  The attraction is known as the Casimir Effect, named for its discoverer, who, 50 years ago, was trying to understand why fluids like mayonnaise move so slowly.  Today, evidence is accumulating that most of the energy density in the universe is in an unknown form dubbed dark energy.  The form and genesis of dark energy is almost completely unknown, but postulated as related to vacuum fluctuations similar to the Casimir Effect but generated somehow by space itself.  This vast and mysterious dark energy appears to gravitationally repel all matter and hence will likely cause the universe to expand forever.  Understanding vacuum fluctuations is on the forefront of research not only to better understand our universe but also for stopping micro-mechanical machine parts from sticking together.

Image credit: Umar Mohideen 
(U. California at Riverside)
Spicules: Jets on the Sun (2004-08-02)

Imagine a pipe as wide as a state and as long as half the Earth.  Now imagine that this pipe is filled with hot gas moving 50,000 kilometers per hour.  Further imagine that this pipe is not made of metal but a transparent magnetic field.  You are envisioning just one of thousands of young spicules on the active Sun.  Pictured above is perhaps the highest resolution image yet of these enigmatic solar flux tubes.  Spicules dot the above frame of solar active region 10380 that crossed the Sun in June, but are particularly evident as a carpet of dark tubes on the right.  Time-sequenced images have recently shown that spicules last about five minutes, starting out as tall tubes of rapidly rising gas but eventually fading as the gas peaks and falls back down to the Sun.  These images also indicate, for the first time, that the ultimate cause of spicules is sound-like waves that flow over the Sun's surface but leak into the Sun's atmosphere.
Shadow of a Martian Robot (2004-08-03)

What if you saw your shadow on Mars and it wasn't human?  Then you might be the Opportunity rover currently exploring Mars.  Opportunity and sister robot Spirit have been probing the red planet since January, finding evidence of ancient water, and sending breathtaking images across the inner Solar System.  Pictured above, Opportunity looks opposite the Sun into Endurance Crater and sees its own shadow.  Two wheels are visible on the lower left and right, while the floor and walls of the unusual crater are visible in the background.  Opportunity is cautiously edging its way into this enigmatic crater, hoping to find new clues into the wet ancient past of our Solar System's second most habitable planet.
Solar Arcs and Halos (2004-08-04)

Have you ever seen a bright halo around the Sun?  Unusual halos and arcs were so bright one recent afternoon in Trier, Germany that even casual people on the street noticed them.  The fantastic sky display is pictured above and included a 22 degree halo arc, a complete parhelic circle, a circumhorizon arc and even an infralateral arc.  A computer simulation has been run that mimics the above rare display.  A cloud partially blocked the usually more intense direct glare of the Sun. Sunlight refracting through falling and fluttering hexagonal ice crystals creates such displays.  Such atmospheric ice crystals also cause sundogs and Moon halos.

Image credit: Eva Seidenfaden
Emission Nebula IC 1396 (2004-08-05)

Sprawling across hundreds of light-years, emission nebula IC 1396 mixes glowing cosmic gas and dark dust clouds. Stars are forming in this area, only about 3,000 light-years from Earth. This particularly colorful view of the region is a composite of digital images recorded through narrow band filters. The filters actually block out most of the light ... but narrowly transmit wavelengths characteristic of specific glowing atoms in the nebula. In fact, the color scheme used here makes it easy to trace some of the elements which contribute to the emission from IC 1396. Emission from sulfur atoms is shown in red, hydrogen atoms green, and oxygen in blue. The beautiful and useful result is still very different from what the eye might see. IC 1396 lies in the high and far off constellation of Cepheus.

Image credit: Richard Crisp
The Giant and the Glory (2004-08-06)

On a flight from Vienna to Brussels, astronomer Franz Kerschbaum looked out the window and photographed this beautiful atmospheric phenomenon, the glory, shining in the direction directly opposite the Sun.  Before airplanes, the glory, known to some as the heiligenschein or the Specter of the Brocken, was occasionally 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.  Of course, the giant turns out to be the observer, and in the modern version a silhouette of a plane frequently occupies the glory's center. The cause of the glory is relatively complex.  Briefly, small droplets of water reflect, refract, and diffract sunlight backwards towards the Sun. The phenomenon has a counterpart in astronomy, where looking out from planet Earth in the direction opposite the Sun yields a bright spot called the gegenschein.

Image credit: Franz Kerschbaum
(Univ. Vienna)
Giant Cluster Bends, Breaks Images (2004-08-07)

What are those strange blue objects?  Many are images of a single, unusual, beaded, blue, ring-like galaxy which just happens to line-up behind a giant  cluster of galaxies.  Cluster galaxies here appear yellow and -- together with the cluster's dark matter -- act as a gravitational lens.  A gravitational lens can create several images of background galaxies, analogous to the many points of light one would see while looking through a wine glass at a distant street light.  The distinctive shape of this background galaxy -- which is probably just forming -- has allowed astronomers to deduce that it has separate images at 4, 8, 9 and 10 o'clock, from the center of the cluster.  Possibly even the blue smudge just left of center is yet another image!  This spectacular photo from the Hubble Space Telescope was taken in October 1994.
Contemplating the Sky (2004-08-08)

Have you contemplated your sky recently?  This week will be a good one for midnight meditators at many northerly locations as meteors from the Perseid meteor shower will frequently streak through.  The Perseid meteor shower has slowly been building to a crescendo and should peak on the nights of August 11 and 12. Pictured above on 2002 August 1, a group of celestial sightseers near Quebec, Canada are treated to a dark and wondrous night sky that contained bright stars, green auroras, the band of our Milky Way galaxy, a majestic Moon rising, the International Space Station slowly gliding by, and the occasional flash of a Perseid meteor.  Although no meteors were caught in this frame, the Big Dipper remained quite prominent.

Image credit: Sebastien Giguere
(Astrolab du Parc du Mont-Megantic)
The Dark River to Antares (2004-08-09)

Connecting the Pipe Nebula to the bright star Antares is a flowing dark cloud nicknamed the Dark River.  The murkiness of the Dark River is caused by absorption of background starlight by dust, although the nebula contains mostly hydrogen and molecular gas.  Antares, the brightest star in the frame, is embedded in the colorful Rho Ophiuchi nebula clouds.  The Dark River, pictured above across the upper left, spans over 20 times the angular diameter of the Moon and lies about 500 light years distant.  Other types of nebula visible here include red emission nebula and the blue reflection nebula.

Image credit: StarryScapes
The Double Haze above Titan (2004-08-10)

Most moons have no haze layer at all - why does Titan have two?  Images from the Cassini spacecraft that  slipped into orbit around Saturn last month confirm that the Solar System's most mysterious moon is surrounded not only by a thick atmosphere but also by two distinct spheres of haze.  These layers are visible as purple in the above false-color ultraviolet image.  Titan's opaque atmosphere is similar to Earth's atmosphere in that it is composed mostly of nitrogen.  As energetic sunlight strikes high level atmospheric nitrogen and methane, trace amounts of organic compounds such as ethane and carbon dioxide appear to form.  These and other complex organic molecules likely populate the detached haze layer.  In December 2004, Cassini will launch the Huygens probe to land on Titan.
A Perseid Meteor (2004-08-11)

The ongoing Perseid Meteor Shower should be at its strongest tonight and tomorrow night. Although meteors should be visible all night long, the best time to watch will be between 2:00 AM and dawn each night. In dark, moonless, predawn skies you may see dozens of meteors per hour. Sky enthusiasts in Europe and Asia might see an unusual burst of meteors near 2100 hours UT. Grains of cosmic sand and gravel shed from Comet Swift-Tuttle will streak across the sky as they vaporize during entry into Earth's atmosphere. Tracing the meteor trails backwards, experienced skygazers will find they converge on the constellation Perseus, thus this annual meteor shower's name. Pictured above is a Perseid meteor from 2002 over a rock formation in the US Southwest desert. Shadowing and blurring are caused by the long 10-minute exposure. The brightest Perseids can be seen from anywhere on Earth by monitoring the continuously returning images from the Night Sky Live cameras.

Image credit: Wally Pacholka (Astropics)
The Spectrum of a Meteor (2004-08-12)

Chasing the brief flash of a meteor trail across the sky with a very large telescope is a nearly impossible task. But on May 12, 2002, astronomers got lucky, as a bright meteor chanced across the narrow slit of their spectrograph at the Paranal Observatory. At the time, the spectrograph was being used to study the light from a supernova, separating and recording the many near-infrared emission lines produced by atoms in the distant stellar explosion. Below this artistic montage of a meteor streak and Very Large Telescope units at Paranal, panel a shows the near-infrared sky background spectrum and the May 12 meteor combined. Panel b shows the emission spectrum of the meteor alone, after subtracting away the background contributions. The meteor emission is due to colliding oxygen and nitrogen atoms and molecules in the superheated air along the glowing trail at an altitude of about 100 kilometers.

Image credit: P. Jenniskens
(SETI Inst.),
E. Jehin (ESO) 
et al.,
FORS1/VLT, 
ESO
Perseid Fireball Over Japan (2004-08-13)

joying the bright Moon's absence from early morning skies, observers around the world reported lovely displays during this year's Perseid meteor shower. As anticipated, peak rates were about one meteor per minute. Though most Perseids were faint, this bright and colorful fireball meteor flashed through skies over Japan on August 12 at 0317 JST. Ending at the upper right, the meteor's trail points down and to the left, back to the shower's radiant point between the constellations of Perseus and Cassiopeia, seen here just above the tower structure in the foreground. The Pleiades star cluster is also visible well below the meteor's trail. Perseid shower meteors can be traced to particles of dust from the tail of comet Swift-Tuttle. The comet dust impacts the atmosphere at speeds of around 60 kilometers per second. While this annual shower's peak has come and gone, Perseid meteors should still be visible over the next few nights, but at a greatly reduced rate.

Image credit: Katsuhiro Mouri &
Shuji Kobayashi
(Nagoya City
Science Museum /
Planetarium)
Messenger Launch (2004-08-14)

Streaking into the early morning sky on August 3rd, a Delta II rocket launches NASA's Messenger spacecraft on an interplanetary voyage to Mercury. Scheduled to become the first probe to orbit Mercury, Messenger will begin by looping through the inner Solar System in a series of close flybys of planet Earth and Venus. The flybys are designed as trajectory changing gravity assist encounters to ultimately achieve the goal of orbiting Mercury in 2011. Prior to entering orbit, Messenger will also flyby Mercury in 2008 and 2009 as the first spacecraft to visit the Solar System's innermost planet since Mariner 10 in the mid 1970s. This dramatic view of the Messenger launch was recorded from a pier in Jetty Park at the north end of Cocoa Beach about 2.5 miles from the Cape Canaveral launch site. So what's that erratic blue streak on the right? It's the reflection from a camera blurred in the time exposure.

Image credit: Ben Cooper
Hoag's Object: A Strange Ring Galaxy (2004-08-15)

Is this one galaxy or two?  This question came to light in 1950 when astronomer Art Hoag  chanced upon this unusual extragalactic object.  On the outside is a ring dominated by bright blue stars, while near the center lies a ball of much redder stars that are likely much older.  Between the two is a gap that appears almost completely dark.  How Hoag's Object formed remains unknown, although similar objects have now been identified and collectively labeled as a form of ring galaxy.  Genesis hypotheses include a galaxy collision billions of years ago and perturbative gravitational interactions involving an unusually shaped core.  The above photo taken by the Hubble Space Telescope in July 2001 reveals unprecedented details of Hoag's Object and may yield a better understanding.   Hoag's Object spans about 100,000 light years and lies about 600 million light years away toward the constellation of  Serpens.  Coincidentally, visible in the gap (at about one o'clock) is yet another ring galaxy that likely lies far in the distance.
Close-Up of the Lagoon (2004-08-16)

Stars are battling gas and dust in the Lagoon Nebula but the photographers are winning. Also known as M8, this photogenic nebula is visible even without binoculars towards the constellation of Sagittarius. The energetic processes of star formation create not only the colors but the chaos.  The red-glowing gas results from high-energy starlight striking interstellar hydrogen gas.  The dark dust filaments that lace M8 were created in the atmospheres of cool giant stars and in the debris from supernovae explosions.  This spectacular portion of the Lagoon Nebula taken by the CFHT was created from light emitted by hydrogen (shown in red) and light emitted by oxygen (shown in green).  The light from M8 we see today left about 5000 years ago. Light takes about 50 years to cross this section of M8.

Image credit: Canada-France-Hawaii Telescope
The Unusual Blueberries at Bylot Rock (2004-08-17)

Why aren't these Martian rocks round?  Small rocks dubbed blueberries have been found by the Opportunity rover all over Meridiani Planum on Mars, but the ones perched on Bylot rock have unusually non-spherical shapes.  The strangely shaped blueberry rocks are shown above in an image taken by Opportunity's microscopic imager on August 9.  Dark sand also covers much of Bylot rock.  One hypothesis for the lack of blueberry roundness here is a coating relatively resistant to erosion.  The average diameter of a blueberry is about 4 millimeters.  The study of these and other rock features is allowing a better understanding of the wet past of ancient Mars.
Lightning on Earth (2004-08-18)

Nobody knows what causes lightning.  It is known that charges slowly separate in some clouds causing rapid electrical discharges (lightning), but how electrical charges get separated in clouds remains a topic of much research.  Nevertheless, lightning bolts are common in clouds during rainstorms, and on average 6000 lightning bolts occur between clouds and the Earth every minute.  Above, several lightning strokes were photographed under a starry sky behind Kitt Peak National Observatory near Tucson, Arizona.  Lightning has also been found on the planets Jupiter,  Saturn, and Uranus.  NASA launched the TRMM mission in 1997 that continues to measure rainfall and lightning on planet Earth.

Image credit: Elizabeth Warner
(U. Maryland)
Windblown N44F (2004-08-19)

A fast and powerful wind from a hot young star has created this stunning bubble-shaped nebula poised on the end of a bright filament of hydrogen gas. Cataloged as N44F, the cosmic windblown bubble is seen at the left of this Hubble Space Telescope image. N44F lies along the northern outskirts of the N44 complex of emission nebulae in the Large Magellanic Cloud, a mere 160,000 light-years away. The bright, blue, hot star itself is just below the center of the bubble. Peering into the bubble's interior, the Hubble image reveals dramatic structures, including pillars of dust, aligned toward N44F's hot central star. Reminiscent of dust pillars in stellar nurseries within our Milky Way galaxy, they likely contain young stars at their tips. Expanding into the surrounding gas and dust at about 12 kilometers per second, N44F is around 35 light-years across.
Raining Perseids (2004-08-20)

Comet dust rained down on planet Earth last week, streaking through dark skies in the annual Perseid meteor shower. So, while enjoying the anticipated space weather, astronomer Fred Bruenjes recorded a series of many 30 second long exposures spanning about six hours on the night of August 11/12 using a wide angle lens. Combining those frames which captured meteor flashes, he produced this dramatic view of the Perseids of summer. Although the comet dust particles are traveling parallel to each other, the resulting shower meteors clearly seem to radiate from a single point on the sky in the eponymous constellation Perseus. The radiant effect is due to perspective, as the parallel tracks appear to converge at a distance. Bruenjes notes that there are 51 Perseid meteors in the composite image, including one seen nearly head-on.

Image credit: Fred Bruenjes
Solar Sail (2004-08-21)

Could solar sailing become a future Olympic sport? Nearly 400 years ago astronomer Johannes Kepler observed comet tails blown by a solar breeze and suggested that vessels might likewise navigate through space using appropriately fashioned sails. It is now widely recognized that sunlight does indeed produce a force which moves comet tails and a large, reflective sail could be a practical means of propelling a spacecraft. In fact, the illustration above represents a concept explored by NASA for an interstellar probe pushed along by sunlight reflected from an ultrathin sail. Nearly half a kilometer wide, the delicate solar sail would be unfurled in space. Continuous pressure from sunlight would ultimately accelerate the craft to speeds about five times higher than possible with conventional rockets. While not quite ready for the Olympics, NASA has recently tested solar sail technologies on earth and the Japanese Space Agency ISAS has deployed solar sail material in space. The Planetary Society in collaboration with the Space Research Institute (IKI) in Moscow and partners is preparing to launch Cosmos 1, a solar sail powered spacecraft.
Earth at Night (2004-08-22)

This is what the Earth looks like at night.  Can you find your favorite country or city?  Surprisingly, city lights make this task quite possible.  Human-made lights highlight particularly developed or populated areas of the Earth's surface, including the seaboards of Europe, the eastern  United States, and Japan.  Many large cities are located near rivers or oceans so that they can exchange goods cheaply by boat.  Particularly dark areas include the central parts of South America, Africa, Asia, and Australia.  The above image is actually a composite of hundreds of pictures made by the orbiting DMSP satellites. (Editor's note: This image has become an email-attachment phenomenon! It has also generated many print requests. Unfortunately, we do not sell prints. However, a high-resolution digital version of the image is available (click here or here).
Looking Out Over Mars (2004-08-23)

What would it be like to climb a hill and look out over Mars?  That opportunity was afforded the Spirit rover earlier this month as it rolled to a high perch in the Columbia Hills.  Peering out, the rolling robot spied the interior plains and distant rim of Gusev Crater, beyond an outcrop of rocks called Longhorn.  Spirit continues to find evidence that many rock shapes have been altered by ancient water.  Both Spirit and her sister robot Opportunity have completed their primary three-month mission but remain in good enough condition to continue to explore Mars.
Supply Ship Approaches the Space Station (2004-08-24)

The crew on board the International Space Station sometimes needs supplies.  As the US Space Shuttle fleet prepares to return to flight, supplies usually now come from a robot Progress supply vessel launched from Kazakhstan.  Pictured above, a Progress ship approaches the ISS on May 27, delivering over 2,500 kilograms of food, water, fuel and other important items.  The supply ship soon docked with the Zvezda Service Module while orbiting the Earth over 300 kilometers over central Asia.
Zodiacal Light and the False Dawn (2004-08-25)

Tomorrow's picture: Cas A  < | Archive | Index | Search | Calendar | Glossary | Education | About APOD | >  Authors & editors: Robert Nemiroff (MTU) & Jerry Bonnell (USRA) NASA Web Site Statements, Warnings, and Disclaimers NASA Official: Jay Norris. Specific rights apply. A service of: LHEA at NASA / GSFC & Michigan Tech. U.

Image credit: Stefan Seip