APOD Retrospective: March 20

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

Postby bystander » Tue Mar 20, 2012 4:11 am

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Today's date marks an Equinox and a New Moon. Remarkably, while the exact timing of both geocentric events occur within a span of only 13 hours, the moon also reaches its new phase only 14 hours after perigee, the closest point in its orbit. That makes the Equinox New Moon the largest New Moon of 2015, though hard to see since that lunar phase presents the Moon's dark, night side to planet Earth. Still, in this well composed image of a young lunar phase from late January you can glimpse both night and day on the lunar surface, the night side faintly illuminated by Earthshine next to the day side's brightly sunlit crescent. But some will see today's Equinox New Moon in silhouette! The Equinox Solar Eclipse will be total across stretches of the Arctic Ocean, visible in partial phases from Europe, northern Africa and western Asia.

Today is the equinox. The Sun crosses the celestial equator heading north at 16:57 UT, marking the northern hemisphere's first day of spring. To celebrate, consider this remarkable image following the Sun's yearly trek through planet Earth's sky, the first analemmas exposed every day through the technique of solargraphy. In fact, three analemma curves were captured using a cylindrical pinhole camera by daily making three, separate, one minute long exposures for a year, from March 1, 2013 to March 1, 2014, on a single piece of black and white photographic paper. The well-planned daily exposures began at 10:30, 12:00, and 13:30, CET from a balcony looking south from the Kozanów district in Wrocław, Poland. That year's two equinoxes on March 20 and September 22 correspond to the mid-points, not the cross-over points, along the figure-8 shaped curves. Apparent gaps in the curves are due to cloudy days. Two solid lines at the lower left were both caused by a timer switch failure that left the pinhole shutter open.

The Great Nebula in Orion, an immense, nearby starbirth region, is probably the most famous of all astronomical nebulas. Here, glowing gas surrounds hot young stars at the edge of an immense interstellar molecular cloud only 1500 light-years away. In the above deep image in assigned colors highlighted by emission in oxygen and hydrogen, wisps and sheets of dust and gas are particularly evident. The Great Nebula in Orion can be found with the unaided eye near the easily identifiable belt of three stars in the popular constellation Orion. In addition to housing a bright open cluster of stars known as the Trapezium, the Orion Nebula contains many stellar nurseries. These nurseries contain much hydrogen gas, hot young stars, proplyds, and stellar jets spewing material at high speeds. Also known as M42, the Orion Nebula spans about 40 light years and is located in the same spiral arm of our Galaxy as the Sun.

Click to play embedded YouTube video.
Video Credit: LRO, SVC, NASA

What is the history of the Moon? The Moon was likely created from debris expelled when a Mars-sized object violently impacted the Earth about 4.5 billion years ago. Just after gravitationally condensing, as imagined above, the glowing-hot surface of the Moon cooled and cracked. Rocks large and small continued to impact the surface, including a particularly large impact that created Aitken Basin about 4.3 billion years ago. A Heavy Bombardment period then continued for hundreds of millions of years, creating large basins all over the lunar surface. Over the next few billion years lava flowed into Earth-side basins, eventually cooling into the dark maria we see today. As always, relentless impacts continued, forming the craters we see today, slowly diminishing over the past billion years. Today the cooled Moon we know and love is as dark as coal and always keeps the same face toward Earth. Exactly how the Moon formed initially, and why lunar maria are only on the Earth side, remain active topics of research.

Did you see the Full Moon last night? Near the horizon, the lunar orb may have seemed to loom large, swollen in appearance by the famous Moon illusion. But the Full Moon really was a large Full Moon last night, reaching its exact full phase within an hour of lunar perigee, the point in the Moon's elliptical orbit closest to planet Earth. A similar near perigee Full Moon last occurred on December 12, 2008. The difference in the Moon's apparent size as it moves from perigee to apogee, its farthest point from Earth, is about 14 percent. Of course, a nearly Full Moon will rise again tonight, lighting the skies on the date of the Equinox or equal night. The Full Moon also looms large in this well-planned, telescopic lunar portrait. Captured earlier this year, the rising lunar orb is dramatically matched to the 2,500 year old Parthenon in Athens, Greece.

Ghostly Zodiacal light, featured near the center of this remarkable panorama, is produced as sunlight is scattered by dust in the Solar System's ecliptic plane. In the weeks surrounding the March equinox (today at 1732 UT) Zodiacal light is more prominent after sunset in the northern hemisphere, and before sunrise in the south, when the ecliptic makes a steep angle with the horizon. In the picture, the narrow triangle of Zodiacal light extends above the western horizon and seems to end at the lovely Pleiades star cluster. Arcing above the Pleiades are stars and nebulae along the plane of our Milky Way Galaxy. Recorded on March 10 from Teide National Park on the island of Tenerife, the vista is composed of 4 separate pictures spanning over 180 degrees.

Today, the Sun crosses the celestial equator heading north at 11:44 UT. Known as an equinox, this astronomical event marks the first day of spring in the northern hemisphere and autumn in the south. It also marks the beginning of Norouz, the Persian (Iranian) new year. Equinox means equal night. With the Sun on the celestial equator, Earth dwellers will experience nearly 12 hours of daylight and 12 hours of darkness. Of course, in the north the days will grow longer, the Sun marching higher in the sky as summer approaches. To celebrate the equinox, consider this scenic view of the setting Sun from the island of Naxos in the Aegean Sea. Recorded last June, the well-planned image captures the Portara (big door) in a dramatic silhouette. Measuring about 6 by 3.5 meters, the Portara is the large entrance to the Greek island's ancient, unfinished Temple of Apollo.

Today, the Sun crosses the celestial equator heading north at 0548 UT. Known as the equinox, the geocentric astronomical event marks the first day of spring in the northern hemisphere and autumn in the south. Equinox means equal night and with the Sun on the celestial equator, Earth dwellers will experience nearly 12 hours of daylight and 12 hours of darkness. Of course, for those in the north, the days will grow longer with the Sun marching higher in the sky as summer approaches. To celebrate the equinox, consider this colorful view of the setting Sun. Recorded last June from the International Space Station, the Sun's limb still peeks above the distant horizon as seen from Earth orbit. Clouds appear in silhouette as the sunlight is reddened by dust in the dense lower atmosphere. Molecules in the more tenuous upper atmosphere are preferentially scattering blue light.

What's happening to the Moon? Drifting around the Earth in 2006 July, astronauts from the International Space Station (ISS) captured a crescent Moon floating far beyond the horizon. The captured above image is interesting because part of the Moon appears blue, and because part of the moon appears missing. Both effects are created by the Earth's atmosphere. Air molecules more efficiently scatter increasingly blue light, making the clear day sky blue for ground observers, and the horizon blue for astronauts. Besides reflecting sunlight, these atmospheric molecules also deflect moonlight, making the lower part of the moon appear to fade away. As one looks higher in the photograph, the increasingly thin atmosphere appears to fade to black.

Are "super-Earths" common around other star systems? Quite possibly. Unexpected evidence for this came to light recently when a planet orbiting a distant star gravitationally magnified the light of an even more distant star. Assuming the planet's parent star is normal red dwarf, the brightening is best explained if the planet is about 13 times the mass of the Earth and orbiting at the distance of the asteroid belt in our own Solar System. Given the small number of objects observed and similar determinations already obtained for other star systems, these super-Earths might be relatively common. Astronomers speculate that the planet might have grown into a Jupiter-sized planet if its star system had more gas. Since the planet was not observed directly, significant uncertainty remains in its defining attributes, and future research will be aimed at better understanding this intriguing system. The above drawing gives an artist's depiction of what a super-Earth orbiting a distant red dwarf star might look like, complete with a hypothetical moon.

Today, the Sun crosses the celestial equator heading north, marking the Vernal Equinox -- the first day of spring in the Northern Hemisphere and autumn in the south. Equinox means equal night and with the Sun on the celestial equator, Earthlings will experience 12 hours of daylight and 12 hours of darkness. For those in the Northern Hemisphere, the days will continue to grow longer with the Sun marching higher in the sky as summer approaches. A few weeks after the northern Autumnal Equinox of 1994, the Crew of the Shuttle Endeavor recorded this image of the Sun poised above the Earth's limb. Glare illuminates Endeavor's vertical tail (pointing toward the Earth) along with radar equipment in the payload bay. The space shuttle is expected to return to flight later this year with the launch of STS-114.

Twice a year, at the Spring and Fall equinox, the Sun rises due east. In an emphatic demonstration of this celestial alignment, photographer Joe Orman recorded this inspiring image of the Sun rising exactly along the east-west oriented Western Canal, in Tempe,Arizona, USA. But he waited until March 21st, one day after the northern Spring equinox in 2001, to photograph the striking view. Why was the rising Sun due east one day after the equinox? At Tempe's latitude the Sun rises at an angle, arcing southward as it climbs above the horizon. Because the distant mountains hide the true horizon, the Sun shifts slightly southward by the time it clears the mountain tops. Waiting 24 hours allowed the Sun to rise just north of east and arc back to an exactly eastern alignment for the photo. Today's equinox finds the Sun on the celestial equator at 0649 Universal Time.

Astronomically speaking, at the Equinox on March 21, 0100 UT (March 20, 8:00 PM ET) the season changes. For this Equinox the Sun rises due east as it crosses the celestial equator heading north. In celebration, consider this spectacular sunrise analemma! An analemma is the figure-8 loop you get when you mark the position of the Sun at the same time each day throughout the year. In this remarkable case, 38 separate exposures (and 1 foreground exposure) were recorded on a single piece of film between January 12 and December 21, 2002 at 0600 UT. The tilt of planet Earth's axis and the variation in speed as it moves around its elliptical orbit combine to produce the predictable analemma curve. The top and bottom of the figure-8 correspond to the Solstices -- the Northern and Southern limits of the Sun's sky motion. The two Equinoxes find the Sun at points along the anelemma curve exactly half way between the Solstices. Here, the analemma's Southern portion is partly hidden by mountains. In the foreground lie the stone ruins of the Tholos at the ancient site of Delphi, Greece.

Looking out from the bottom of the world, strange and spectacular sights are sometimes observed. Such was the case during the long Antarctic night of 1998, as awesome aurora sub-storms were photographed above scientific outposts. Visible in the left foreground of the above photograph is the Martin A. Pomerantz Observatory while the now defunct SPIREX telescope canvas dome is visible to its right. The outside temperature at the time this photograph was taken was about -73 Celsius (-100 Fahrenheit), although a slightly heated box sheltered the camera.

Welcome to the equinox! Moving northward in Earth's sky, today the Sun crosses the celestial equator at 13:31 Universal Time bringing Spring to the north and Fall to the south. The change of season is known as an equinox as the Sun rises due east on the horizon and sets due west -- providing an equal night, 12 night and 12 daylight hours, for both northern and southern hemispheres. In this picture from March 8, the Sun peers over the eastern horizon at the space shuttle Discovery's dramatic morning launch on mission STS-102. Having delivered supplies and taxied crew to the International Space Station, Discovery will remain in orbit for this first day of northern hemisphere Spring. Discovery is scheduled to land at Kennedy Space Center in Florida early tomorrow.

Have you ever seen the planet Mercury? Because Mercury orbits so close to the Sun, it is never seen far from the Sun, and so is only visible near sunrise or sunset. If trailing the Sun, Mercury will be visible for several minutes before it follows the Sun behind the Earth. If leading the Sun, Mercury will be visible for only several minutes before the Sun rises and hides it with increasing glare. An informed skygazer can usually pick Mercury out of a dark horizon glow with little more than determination. Above, a lot of determination has been combined with a little digital trickery to show Mercury's successive positions during the middle of last month. Each picture was taken from the same location in Spain when the Sun was 10 degrees below the horizon and superposed on the single most photogenic sunset.

Looking toward the south from low Earth orbit, the crew of the Space Shuttle Endeavor made this stunning time exposure of the Aurora Australis or southern lights in April of 1994. Aurora are visible at high northern latitudes as well, with the northern lights known as Aurora Borealis. They are caused by high energy electrons from the Solar Wind which are funneled into the atmosphere near the poles by the Earth's magnetic field. The reddish colors occur at the highest altitudes (about 200 miles) where the air is least dense. At lower altitudes and greater densities green tends to dominate ranging to a pinkish glow at the lowest. The familiar constellation of Orion the Hunter is clearly visible above the dark horizon in the background. Because of the shuttle's orbital motion, the bright stars in Orion appear slightly elongated.

No, it's not breakfast ... but looking down from an orbiting spacecraft, the odd intersecting ridges covering this area of Mars do present a waffle-like appearance. The cause of the ridge pattern is unknown but it suggests that more complex layered deposits lie below. The south polar region in this Mars Global Surveyor image measures about 8.5 by 12 miles and is spread with a layer of bright, seasonal carbon dioxide frost. Mysterious dark spots which pepper some of the interridge areas are 60 to 300 feet across. Their exact nature is also unknown, but these spots have apparently defrosted early and lack the bright layer of frozen carbon dioxide.

Today marks the Vernal Equinox, the first day of Spring for planet Earth's northern hemisphere. Despite recent attempts by other spectacular and dramatic celestial events to take center stage, Comet Hale-Bopp remains the most popular object in the sky (according to APOD access logs!) and is likely to make this spring memorable for many. Gorgeous pictures of the comet with its delightful tails - this one taken March 16 - make this outbreak of "comet fever" understandable. Will the Earth pass through the lovely tails of Hale-Bopp? No, but the Earth has made similar journeys in the past. In fact, tales are often told of our planet's 1910 passage through comet Halley's tail. Anticipation of this event caused hysteria as it followed close on the heels of the spectroscopic detection of CN, poisonous cyanide, as a gaseous constituent of cometary tails. However, stretching for millions of miles, awe-inspiring comet tails are actually an extremely tenuous, nearly perfect vacuum and don't pose a danger to life on Earth.

The Orion Nebula is visible to the unaided eye as a fuzzy patch near the famous belt of three stars in the constellation Orion. The above picture captures a part of the Orion Nebula that primarily reflects light from bright Orion stars. This reflection nebula appears blue because the blue light from the neighboring stars scatters more efficiently from nebula gas than does red light. The dark lanes are composed of mostly interstellar dust - fine needle-shaped carbon grains.

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