Η Αστρονομική Εικόνα της Ημέρας από τη NASA
Interplanetary Earth
23/11/2024
In an interplanetary first, on July 19, 2013 Earth was photographed on the same day from two other worlds of the Solar System, innermost planet Mercury and ringed gas giant Saturn. Pictured on the left, Earth is the pale blue dot just below the rings of Saturn, as captured by the robotic Cassini spacecraft then orbiting the outermost gas giant. On that same day people across planet Earth snapped many of their own pictures of Saturn. On the right, the Earth-Moon system is seen against the dark background of space as captured by the sunward MESSENGER spacecraft, then in Mercury orbit. MESSENGER took its image as part of a search for small natural satellites of Mercury, moons that would be expected to be quite dim. In the MESSENGER image, the brighter Earth and Moon are both overexposed and shine brightly with reflected sunlight. Destined not to return to their home world, both Cassini and MESSENGER have since retired from their missions of Solar System exploration.
Copyright: NASA
Προηγούμενες Αστρονομικές Εικόνες της Ημέρας από τη NASA
A SAR Arc from New Zealand
03/01/2024
What is that unusual red halo surrounding this aurora? It is a Stable Auroral Red (SAR) arc. SAR arcs are rare and have only been acknowledged and studied since 1954. The featured wide-angle photograph, capturing nearly an entire SAR arc surrounding more common green and red aurora, was taken earlier this month from Poolburn, New Zealand, during an especially energetic geomagnetic storm. Why SAR arcs form remains a topic of research, but is likely related to Earth's protective magnetic field, a field created by molten iron flowing deep inside the Earth. This magnetic field usually redirects incoming charged particles from the Sun's wind toward the Earth's poles. However, it also traps a ring of ions closer to the equator, where they can gain energy from the magnetosphere during high solar activity. The energetic electrons in this ion ring can collide with and excite oxygen higher in Earth's ionosphere than typical auroras, causing the oxygen to glow red. Ongoing research has uncovered evidence that a red SAR arc can even transform into a purple and green STEVE.
Copyright: Tristian McDonald; Text: Tiffany Lewis (Michigan Tech U.)
Rocket Transits Rippling Moon
02/01/2024
Can a rocket make the Moon ripple? No, but it can make a background moon appear wavy. The rocket, in this case, was a SpaceX Falcon Heavy that blasted off from NASA's Kennedy Space Center last week. In the featured launch picture, the rocket's exhaust plume glows beyond its projection onto the distant, rising, and nearly full moon. Oddly, the Moon's lower edge shows unusual drip-like ripples. The Moon itself, far in the distance, was really unchanged. The physical cause of these apparent ripples was pockets of relatively hot or rarefied air deflecting moonlight less strongly than pockets of relatively cool or compressed air: refraction. Although the shot was planned, the timing of the launch had to be just right for the rocket to be transiting the Moon during this single exposure.
Copyright: Steven Madow
NGC 1232: A Grand Design Spiral Galaxy
01/01/2024
Galaxies are fascinating not only for what is visible, but for what is invisible. Grand spiral galaxy NGC 1232, captured in detail by one of the Very Large Telescopes, is a good example. The visible is dominated by millions of bright stars and dark dust, caught up in a gravitational swirl of spiral arms revolving about the center. Open clusters containing bright blue stars can be seen sprinkled along these spiral arms, while dark lanes of dense interstellar dust can be seen sprinkled between them. Less visible, but detectable, are billions of dim normal stars and vast tracts of interstellar gas, together wielding such high mass that they dominate the dynamics of the inner galaxy. Leading theories indicate that even greater amounts of matter are invisible, in a form we don't yet know. This pervasive dark matter is postulated, in part, to explain the motions of the visible matter in the outer regions of galaxies. Free APOD Lecture: January 9, 2024 to the Amateur Astronomers of Association of New York
Copyright: NASA
Illustris: A Simulation of the Universe
31/12/2023
How did we get here? Click play, sit back, and watch. A computer simulation of the evolution of the universe provides insight into how galaxies formed and perspectives into humanity's place in the universe. The Illustris project exhausted 20 million CPU hours in 2014 following 12 billion resolution elements spanning a cube 35 million light years on a side as it evolved over 13 billion years. The simulation tracks matter into the formation of a wide variety of galaxy types. As the virtual universe evolves, some of the matter expanding with the universe soon gravitationally condenses to form filaments, galaxies, and clusters of galaxies. The featured video takes the perspective of a virtual camera circling part of this changing universe, first showing the evolution of dark matter, then hydrogen gas coded by temperature (0:45), then heavy elements such as helium and carbon (1:30), and then back to dark matter (2:07). On the lower left the time since the Big Bang is listed, while on the lower right the type of matter being shown is listed. Explosions (0:50) depict galaxy-center supermassive black holes expelling bubbles of hot gas. Interesting discrepancies between Illustris and the real universe have been studied, including why the simulation produced an overabundance of old stars.
Copyright: NASA
The Persistence of Moonlight
30/12/2023
Known to some in the northern hemisphere as December's Cold Moon or the Long Night Moon, the last full moon of 2023 is rising in this surreal mountain and skyscape. The Daliesque scene was captured in a single exposure with a camera and long telephoto lens near Monte Grappa, Italy. The full moon is not melting, though. Its stretched and distorted appearance near the horizon is caused as refraction along the line of sight changes and creates shifting images or mirages of the bright lunar disk. The changes in atmospheric refraction correspond to atmospheric layers with sharply different temperatures and densities. Other effects of atmospheric refraction produced by the long sight-line to this full moon rising include the thin red rim seen faintly on the distorted lower edge of the Moon and a thin green rim along the top.
Copyright: Giacomo Venturin
Shakespeare in Space
29/12/2023
In 1986, Voyager 2 became the only spacecraft to explore ice giant planet Uranus close up. Still, this newly released image from the NIRCam (Near-Infrared Camera) on the James Webb Space Telescope offers a detailed look at the distant world. The tilted outer planet rotates on its axis once in about 17 hours. Its north pole is presently pointed near our line of sight, offering direct views of its northern hemisphere and a faint but extensive system of rings. Of the giant planet's 27 known moons, 14 are annotated in the image. Mixed with fuzzy background galaxies, the brighter moons show hints of Webb's characteristic diffraction spikes. And though these worlds of the outer Solar System were unknown in Shakespearean times, all but two of the 27 Uranian moons are named for characters in the English bard's plays.
Copyright: NASA
Jupiter and the Geminid
28/12/2023
For a brief moment, this brilliant fireball meteor outshone Jupiter in planet Earth's night. The serendipitous image was captured while hunting meteors under cold Canadian skies with a camera in timelapse mode on December 14, near the peak of the Geminid meteor shower. The Geminid meteor shower, asteroid 3200 Phaethon's annual gift, always arrives in December. Dust shed along the orbit of the mysterious asteroid causes the meteor streaks, as the vaporizing grains plow through our fair planet's upper atmosphere at 22 kilometers per second. Of course Geminid shower meteors appear to radiate from a point in the constellation of the Twins. That's below and left of this frame. With bright Jupiter on the right, also in the December night skyview are the Pleiades and Hyades star clusters.
Copyright: Gaurav Singh
Rainbow Aurora over Icelandic Waterfall
27/12/2023
Yes, but can your aurora do this? First, yes, auroras can look like rainbows even though they are completely different phenomena. Auroras are caused by Sun-created particles being channeled into Earth's atmosphere by Earth's magnetic field, and create colors by exciting atoms at different heights. Conversely, rainbows are created by sunlight backscattering off falling raindrops, and different colors are refracted by slightly different angles. Unfortunately, auroras can’t create waterfalls, but if you plan well and are lucky enough, you can photograph them together. The featured picture is composed of several images taken on the same night last November near the Skógafoss waterfall in Iceland. The planning centered on capturing the central band of our Milky Way galaxy over the picturesque cascade. By luck, a spectacular aurora soon appeared just below the curving arch of the Milky Way. Far in the background, the Pleiades star cluster and the Andromeda galaxy can be found. Your Sky Surprise: What picture did APOD feature on your birthday? (post 1995)
Copyright: Stefano Pellegrini
Η Αστρονομική Εικόνα της Ημέρας από τη NASA (NASA Astronomy Picture of the Day) είναι μια δωρεάν υπηρεσία που παρέχει καθημερινά μια εντυπωσιακή εικόνα από το σύμπαν, την λήψη της οποίας έχει πραγματοποιήσει κάποιος από τους αστρονόμους της NASA ή από κάποιον από τους δορυφόρους ή τα τηλεσκόπια που η NASA λειτουργεί. Οι εικόνες που εμφανίζονται καλύπτουν μια ευρεία γκάμα από θέματα, συμπεριλαμβανομένων των αστερισμών, των γαλαξιών, των πλανητικών συστημάτων, των κομητών, των αστρικών σωμάτων και των παρατηρητηρίων. Κάθε εικόνα συνοδεύεται από μια σύντομη εξήγηση και πληροφορίες σχετικά με το τι παρατηρείται στην εικόνα.