APOD Retrospective: June 20

A nostalgic look back at Astronomy Picture of the Day
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APOD Retrospective: June 20

Post by bystander » Tue May 08, 2012 3:42 am

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2015 "Beautiful Nebula discovered between the Balance [Libra] & the Serpent [Serpens] ..." begins the description of the 5th entry in 18th century astronomer Charles Messier's famous catalog of nebulae and star clusters. Though it appeared to Messier to be fuzzy and round and without stars, Messier 5 (M5) is now known to be a globular star cluster, 100,000 stars or more, bound by gravity and packed into a region around 165 light-years in diameter. It lies some 25,000 light-years away. Roaming the halo of our galaxy, globular star clusters are ancient members of the Milky Way. M5 is one of the oldest globulars, its stars estimated to be nearly 13 billion years old. The beautiful star cluster is a popular target for Earthbound telescopes. Of course, deployed in low Earth orbit on April 25, 1990, the Hubble Space Telescope has also captured its own stunning close-up view that spans about 20 light-years near the central region of M5. Even close to its dense core at the left, the cluster's aging red and blue giant stars and rejuvenated blue stragglers stand out in yellow and blue hues in the sharp color image.

2014 In this night skyscape setting stars trail above the western horizon over Rio de Janeiro, Brazil, a venue for the 2014 World Cup. Gentle arcs from the bright, colorful stars of Orion are near the center of the frame, while the starfield itself straddles planet Earth's celestial equator during the long exposure. Of course, trails from more local lights seem to create the strident paths through the scene. Air traffic smears an intense glow over an airport at the far right, while helicopters fly above the city and boats cruise near the coast. Striping the waterfront are tantalizing reflections of bright lights along Rio's central beaches, Botafogo and Flamengo. Near the horizon, the brightest fixed light is the famous Cristo statue overlooking Rio at night.

2013 Sharp telescopic views of magnificent edge-on spiral galaxy NGC 3628 show a puffy galactic disk divided by dark dust lanes. Of course, this deep galactic portrait puts some astronomers in mind of its popular moniker, The Hamburger Galaxy. The tantalizing island universe is about 100,000 light-years across and 35 million light-years away in the northern springtime constellation Leo. NGC 3628 shares its neighborhood in the local Universe with two other large spirals M65 and M66 in 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.

2012 Today's solstice, the astronomical beginning of summer in the north, is at 23:09 UT when the Sun reaches the northernmost declination in its yearly trek through planet Earth's sky. While most in the northern hemisphere will experience the longest day of the year, for some the Sun won't set at all, still standing just above the horizon at midnight as far south as about 66.6 degrees northern latitude. Of course, as summer comes to the north the midnight Sun comes earlier to higher latitudes. Recorded near midnight, this time series from June 6 follows the Sun gliding above a mountainous horizon from a latitude of 69 degrees north. The remarkable scene looks north over the Norwegian Sea from Sortland, Norway. The 2012 transit of Venus is already in progress, with Earth's sister planet in silhouette at the upper left against the bright disk of the midnight Sun.

2011 In the final move of its kind, NASA's space shuttle Atlantis was photographed earlier this month slowly advancing toward Launch Pad 39A, where it is currently scheduled for a July launch to the International Space Station. The mission, designated STS-135, is the 135th and last mission for a NASA space shuttle. Atlantis and its four-person crew will be carrying, among other things, the Multi-Purpose Logistics Module Raffaello to bring key components and supplies to the ISS. Pictured above, the large Shuttle Crawler Transporter rolls the powerful orbiter along the five-kilometer long road at less than two kilometers per hour. Over 15,000 spectators, some visible on the right, were on hand for the historic roll out.

2010 What are those strange filaments? Background galaxies. Gravity can bend light, allowing huge clusters of galaxies to act as telescopes, and distorting images of background galaxies into elongated strands. Almost all of the bright objects in this Hubble Space Telescope image are galaxies in the cluster known as Abell 2218. The cluster is so massive and so compact that its gravity bends and focuses the light from galaxies that lie behind it. As a result, multiple images of these background galaxies are distorted into long faint arcs -- a simple lensing effect analogous to viewing distant street lamps through a glass of wine. The cluster of galaxies Abell 2218 is itself about three billion light-years away in the northern constellation of the Dragon (Draco). The power of this massive cluster telescope has allowed astronomers to detect a galaxy at the distant redshift of 5.58.

2009 This surprising view of the Full Moon rising on June 7 was captured with a telephoto lens from a seaside balcony near Nice, France. The orange Moon's dark markings and odd shape put the photographer in mind of an alien creature's face staring down at the passing ship. Of course, the Moon's distorted appearance is due to the unusual bending (refraction) of light rays creating multiple images or mirages, similar to sunset and sunrise mirages. The effects are most pronounced when temperature layers in the atmosphere produce sharp changes in air density and refractive index. Acting over long sight-lines to the rising and setting Sun or Moon, the refraction significantly alters the path of light rays creating merged, distorted images. Such mirages are also associated with the Green Flash.

2008 Today's solstice marks the northernmost point of the Sun's annual motion through planet Earth's sky and the astronomical beginning of the northern hemisphere's summer. But only two days ago, the Full Moon nearest the solstice rose close to the ecliptic plane opposite the Sun, near its southernmost point for the year. Astronomer Anthony Ayiomamitis recorded this dramatic picture of the solstice Full Moon rising above Cape Sounion, Greece. The twenty-four hundred year old Temple of Poseidon lies in the foreground, also visible to sailors on the Aegean Sea. In this well-planned single exposure, a telescopic lens makes the Moon loom large, but even without optical aid casual skygazers often find the Full Moon looking astonishingly large when seen near the horizon. That powerful visual effect is known as the Moon Illusion.

2007 Something was about to happen. Just two days ago, two of the three celestial objects easily visible during the day appeared to collide. But actually, Earth's Moon passed well in front of the distant planet Venus. The occultation was caught from Switzerland in the hours before sunset. Moments after this image was taken, the Moon, visible as the crescent on the right of the above image, eclipsed Venus, appearing near half phase on the lower left. Clouds that once threatened to obscure the whole event, were visible on the far left. About 90 minutes later, Venus re-appeared just to the right of the bright crescent.

2006 Is this a picture of a sunset from Earth's North Pole? Regardless of urban legends circulating the Internet, the answer is no. The above scene was drawn to be an imaginary celestial place that would be calm and peaceful, and therefore titled Hideaway. The scene could not exist anywhere on the Earth because from the Earth, the Moon and the Sun always have nearly the same angular size. This is particularly apparent, for example, during solar eclipses. Still, the scene drawn is quite striking, and the crescent part of the "moon" shown is approximately accurate given the location of the parent star. In reality, the North Pole of Earth looks different. Starting earlier this month, the North Pole even has a web camera returning near-live pictures.

2005 What would it be like to see a sunset on Mars? To help find out, the robotic rover Spirit was deployed last month to park and serenely watch the Sun dip below the distant lip of Gusev crater. It was a tough job, but some robot had to do it. Now on Earth a red sunset is caused by two effects -- by blue light being preferentially scattered out of sunlight by oxygen and nitrogen molecules in the atmosphere, and by scattering off a small amount of impurities like volcanic dust. (The magnitude of the first effect was computed in one of Albert Einstein's most cited papers.) Although Mars lacks oxygen and nitrogen, it is covered in red dust frequently hoisted into the atmosphere by fast but thin winds. Analyses of images like the above photograph show that at least some Martian days are capped by a sunset significantly longer and redder than typical on Earth. For up to two hours after twilight, sunlight continued to reflect off Martian dust high in the atmosphere, casting a diffuse glow. The result helps atmospheric scientists understand not only the atmosphere of Mars, but atmospheres across the Solar System, including our home Earth.

2004 Season's greetings! Today or tomorrow, depending on your time zone, the Sun reaches its northernmost point in planet Earth's sky marking a season change and the first solstice of the year 2004. In celebration, consider this delightfully detailed, brightly colored image of the active Sun. From the EIT instrument onboard the space-based SOHO observatory, the tantalizing picture is a false-color composite of three images all made in extreme ultraviolet light. Each individual image highlights a different temperature regime in the upper solar atmosphere and was assigned a specific color; red at 2 million, green at 1.5 million, and blue at 1 million degrees C. The combined image shows bright active regions strewn across the solar disk, which would otherwise appear as dark groups of sunspots in visible light images, along with some magnificent plasma loops and an immense prominence at the right hand solar limb.

2003 Dark nebulae snake across a gorgeous expanse of stars in this wide-field view toward the pronounceable constellation Ophiucus and the center of our Milky Way Galaxy. In fact, the central S-shape seen here is well known as the Snake Nebula. It is also listed as Barnard 72 (B72), one of 182 dark markings of the sky cataloged in the early 20th century by astronomer E. E. Barnard. Unlike bright emission nebulae and star clusters, Barnard's nebulae are interstellar dark clouds of obscuring gas and dust. Their shapes are visible in cosmic silhouette only because they lie in the foreground along the line of sight to rich star fields and glowing stellar nurseries near the plane of our Galaxy. Many of Barnard's dark nebulae are themselves likely sites of future star formation. Barnard 72 is a few light years across and about 650 light years away.

2002 Big and beautiful spiral galaxy M81, in the northern constellation Ursa Major, is one of the brightest galaxies visible in the skies of planet Earth. This superbly detailed view reveals its bright nucleus, grand spiral arms and sweeping cosmic dust lanes with a scale comparable to the Milky Way. Hinting at a disorderly past, a remarkable dust lane runs straight through the disk, below and right of the galactic center, contrary to M81's other prominent spiral features. The errant dust lane may be the lingering result of a close encounter between M81 and its smaller companion galaxy, M82. Scrutiny of variable stars in M81 (aka NGC 3031) has yielded one of the best determined distances for an external galaxy -- 11.8 million light-years.

2001 A total eclipse of the Sun is that special geocentric celestial event where the Moon passes exactly in front of the solar disk. During a fleeting few minutes of totality, fortunate earthdwellers located within the path of the Moon's dark shadow can witness the wondrous shimmering solar corona sharing the sky with stars and bright planets. The next total solar eclipse will occur tomorrow, June 21. Since the Sun is still near the maximum of its 11 year activity cycle, careful eclipse-watchers will also likely see the spectacle of bright solar prominences lofted above active regions around the Sun's edge. In fact, a telescopic view could be similar to this stunningly detailed image -- a picture of the solar eclipse of August 1999 taken at the beginning of totality from Kecel, Hungary. The upcoming 2001 June 21 event will be visible as a partial eclipse from some of South America and much of Africa, but will only be total along a 125 mile wide path that tracks across land through Southern Africa and Madagascar. Of course, if you can't travel to Africa tomorrow (and you're not already there), web sites plan to offer live views from the Moon's shadow!

2000 If Ganymede orbited the Sun, it would be considered a planet. The reason is that Jupiter's moon Ganymede is not only the largest moon in the Solar System, it is larger than planets Mercury and Pluto. The robot spacecraft Galileo currently orbiting Jupiter has been able to zoom by Ganymede several times and snap many close-up pictures. Ganymede, shown above in its natural colors, sports a large oval dark region known as Galileo Regio. In general, the dark regions on Ganymede are heavily cratered, implying they are very old, while the light regions are younger and dominated by unusual grooves. The origin of the grooves is still under investigation.

1999 Pictured above is one of the world's premiere radio astronomical observatories: The Very Large Array (VLA). Each antenna dish is as big as a house (25 meters across) and mounted on railroad tracks. The VLA consists of 27 dishes - together capable of spanning the size of a city (35 kilometers). The VLA is the most sensitive radio telescope ever, and, through interferometry, can resolve a golf ball-sized radio source 150 kilometers away (0.04 arcsec). The VLA is continually making new discoveries, including determining the composition of galaxies, passing comets, quasars, HII regions, and clusters of galaxies. The VLA is also used to receive the weak radio signals broadcast from interplanetary spacecraft. The VLA is located in New Mexico, USA. A significant upgrade of VLA's capabilities is planned.

1998 Q: What was made by humans and is 6.5 billion miles away? A: Pioneer 10 - and last year was the 25th anniversary of its launch. More than 9.5 light-hours distant, Pioneer 10 is presently about twice as far from the Sun as Pluto, bound for interstellar space at 28,000 miles per hour. The distinction of being the first human artifact to venture beyond the Solar System is just one in a long list of firsts for this spacefaring ambassador, including; the first spacecraft to travel through the asteroid belt and explore the outer Solar System, the first spacecraft to visit Jupiter, the first to use a planet's gravity to change its course and to reach solar-system-escape velocity, and the first spacecraft to pass beyond the known planets. Pioneer 10's mission is nearing an end - now exploring the distant reaches of the heliosphere it will soon run out of sufficient electrical power to operate science instruments. However, the 570 lb. spacecraft will continue to coast and in 30,000 years or so it will pass within about 3 light years of a nearby star known as Ross 248. Ross 248 is a faint red dwarf just over 10 light years distant in the constellation Taurus. (Note: This year Voyager 1, launched 21 years ago but traveling faster than Pioneer 10, became humanity's most distant spacecraft.)

1997 NGC1850 is a large cluster of stars located a mere 166,000 light-years from Earth in our neighboring galaxy the Large Magellanic Cloud (LMC). The colors in this beautiful Hubble Space Telescope composite image of the cluster reveal different populations of stars. Yellowish stars are the main cluster stars, sun-like main sequence hydrogen burners about 50 million years old. The white stars are massive, hotter, and younger, about 4 million years old. Radiating strongly in ultraviolet light, they represent a loose cluster themselves, perhaps within 200 light-years of the main cluster. Massive stars which formed in the older main cluster have long since disappeared, ending their lives in spectacular supernova explosions. Did expanding debris from these supernovae trigger the formation of the nearby younger cluster? Probably so. In any event, a few million years from now a similar fate awaits the massive stars of the younger cluster - burning brightly but briefly before they explode sending new clouds of stellar debris into space.

1996 In November of 1969, homeward bound aboard the "Yankee Clipper" command module, the Apollo 12 astronauts took this dramatic photograph of the Sun emerging from behind the Earth. From this distant perspective, part of the solar disk peers over the Earth's limb, its direct light producing the jewel like glint while sunlight scattered by the atmosphere creates the thin bright crescent. Today at 10:24 pm Eastern Daylight Time is the Summer Solstice. From an earthbound perspective, the solar disk will climb to its greatest northern declination marking the Northern Hemisphere's first day of Summer and creating the longest day -- with over 15 hours of daylight near latitude +40 degrees.

1995 The Pleiades star cluster, M45, is one of the brightest star clusters visible in the northern hemisphere. It consists of many bright, hot stars that were all formed at the same time within a large cloud of interstellar dust and gas. The blue haze that accompanies them is due to very fine dust which still remains and preferentially reflects the blue light from the stars.

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