Shine on Harvest Moon for me and my gal

The Harvest Moon from a few years back rises over Lake Superior at Brighton Beach in Duluth, Minn. Photo: Bob King

Autumn blew in last weekend with a chill and flourish of color. This weekend we get fall v2.0 with an appearance by the Harvest Moon, the full moon closest to the autumnal equinox.

The Harvest Moon harks back to our agrarian past when farmers could continue harvesting by its light for several evenings in a row. Because the angle of the full moon’s path to the horizon is very shallow in September and October, the time difference between successive moonrises is only about 20-30 minutes instead of the usual 50-60. With moonrise happening on the heels of sunset, farmers could harvest their crops into the night in the days before electric lighting.

The angle of the moon’s path to the horizon makes all the difference in moonrise times. At full phase in spring, the path tilts steeply southward, delaying successive moonrises by over an hour. In September, the moon’s path is nearly parallel to the horizon with successive moonrises just 20+ minutes apart. Times are shown for the Duluth, Minn. region. Illustration: Bob King

The moon, planets and sun all travel along the ecliptic, an invisible circle in the sky that defines the plane of the solar system. During early fall, the ecliptic runs nearly parallel to the eastern horizon for the northern hemisphere. As the moon scoots eastward along this path at the rate of one fist held at arm’s length each day, its rising times vary by a half hour or less. For several nights in a row a big moon seems to rise at nearly the same time – a constant companion of sorts.

Exactly the opposite happens 6 months later in spring, when the moon’s path is tipped at a steep angle to the horizon. While it moves the same amount each night – one outstretched fist – the moon is much farther below the horizon on successive nights, delaying moonrise by an hour or more. You’ll see the full moon one night and wonder why it’s taking so long to rise the next.

We all love to see a big orange moon on the horizon. If you want to know when it’s coming up in your neighborhood this weekend, go to the U.S. Naval Observatory website , select the link (data for the sun and moon for one day or the whole year) and specify your location. When the table pops up, be sure to add an hour for Daylight Saving Time to the times shown. For Duluth, Minn. the full moon rises at 6:21 p.m. tomorrow night (Sept. 29) or 30 minutes before sunset, 6:46 p.m. on Sunday and 7:14 p.m. Monday. Happy gazing!


The title of this blog refers to Shine On, Harvest Moon, a Tin Pan Alley song from the early 1900s. Click the video to see and hear it sung by two of my favorite comedians – Laurel and Hardy.

Curiosity finds ancient streambed but no fish … yet

A rock outcrop called Link pops out from beneath the ubiquitous red dust blanketing Mars’ surface. A fracture has exposed fresh gravels that scientists say could only have been created by flowing water. Credit: NASA/JPL-Caltech/MSSS

This looks so much like Earth. I see gravels like these in Duluth’s abundant creeks and rivers. Scientists are calling it the first evidence of streambed gravels on Mars. A fracture in an outcrop called “Link” shows rounded pieces of gravel eroding from a layer of white rock. Called a sedimentary conglomerate, the outcrop was formed by the deposition of water and is composed of many smaller rounded rocks cemented together. Water transport is the only process capable of making rounded gravels like these.

The name “Link” is derived from a rock formation in Canada’s Northwest Territories, where there’s also a lake with the same name. “Hottah”, named after another lake in the Northwest Territories, also bares similar sedimentary rocks.

Another exposed bedrock site named “Hottah” showing water-born gravels cemented into a rock layer. Where the rock erodes, pebbles fall to the ground. Click to see larger version. Credit: NASA/JPL-Caltech/MSSS

“The shapes tell you they were transported and the sizes tell you they couldn’t be transported by wind. They were transported by water flow,” said Curiosity science co-investigator Rebecca Williams of the Planetary Science Institute in Tucson, Ariz.

OK, we all know by now that Mars once had flowing water, but there’s nothing like seeing the evidence come in piece by piece. We’re on a collective hunt that one day may take us to some steamy hot spring or even a river coursing through a deep underground cave.

“From the size of gravels it carried, we can interpret the water was moving about 3 feet per second, with a depth somewhere between ankle and hip deep,” said Curiosity science co-investigator William Dietrich of the University of California, Berkeley.

This image shows the topography around the area where rover landed back in August. The cross marks the actual landing spot. Higher elevations are colored in red; lower elevations in cooler colors. An alluvial fan, or fan-shaped deposit where debris spreads out downslope, is highlighted in lighter colors. On Earth, alluvial fans are often formed by water flowing downslope. Credit: NASA/JPL-Caltech/UofA

That sounds even more like a typical small stream or creek on Earth, making visualization of this ancient Martian waterway that much easier. The slabs of busted rock are tilted layers of ancient streambed that remind me a lot of a more recent concrete parking lot I watched an excavator dig up recently.

Left picture is a closeup of Mars gravel with a particularly round pebble highlighted. It’s just under 1/2 inch across. At right, rocks are rounded into pebbles by the action of water in Amity Creek in Duluth, Minn. Photo: Bob King

The rounded shapes indicate a lengthy transport from further up Gale Crater’s rim, most likely from where the larger channel called Peace Vallis (above) feeds into the alluvial fan. Curiosity is poised along the edge of that fan as it ambles toward Glenelg, an area where three terrains of scientific interest converge: light-colored bedrock, a region rich in small craters and pebbly ground similar to where Curiosity touched down. Read more details of the discovery HERE.

The house, the window and the moon

The moon reflects from a window in a building near my home earlier this week. Details: 25 second time exposure at f/3.5, ISO 800 and 16mm lens.  Photo: Bob King

A couple nights ago I was out walking and happened onto a beam of moonlight. A chance step in the right spot lined up my gaze with a reflection of the quarter moon from a window pane in a building not far from home. The sudden flash made me stop and study the reflection of the moon in the glass. Depending on how I moved my head, any unevenness in the glass stretched and squeezed the moon’s image much like a carnival mirror.

I was also struck with how bright the moon appeared even though just past half. Shadows were distinct and the reflection was glaring. Much of that has to do with contrast – the moon is brilliant against a dark sky –  and the ability of our eyes to adapt to darkness. We’ve all felt our eyeballs tingle in mild pain when walking from the dark into a brightly lit room.

The reality is that the moon reflects almost the same amount of light as worn asphalt — 12%. It’s a dark world. Desert sand is far more reflective (40%) and Earth sends back 33% of the light it receives. Very charitable, I think.

Star atlases, like view of Orion from Cambridge Atlas 2000.0, show star brightness as different sized dots. The smaller the dot, the fainter the star. Photo: Bob King

How does the moon compare with the sun? Though the same apparent size, the sun is overwhelmingly brighter than the moon. How much is easy to figure out by comparing their magnitudes.

Astronomers use the magnitude scale to measure star and planet brightness. Each magnitude is 2.5 times brighter than the one below it. Saturn, which shines at 1st magnitude, is 2.5 times brighter than the 2nd magnitude North Star, which in turn is 2.5 times brighter than a 3rd magnitude star and so on.

The larger the magnitude, the fainter the star. If something is very bright, its magnitude is a negative number. Sirius, the brightest star sparkles at magnitude -1.4 and Venus brighter yet at -4.4.

A first magnitude star is 2.5 x 2.5 x 2.5 x 2.5 x 2.5 (about 100) times brighter than a 6th magnitude star. The sun shines at magnitude -26.7 and the full moon at -12.7, a difference of 14 magnitudes. Do the math and you’ll see that the sun is 372,529 times brighter than the moon. Looking at it another way, you’d need 372,529 full moons to equal the sun’s radiance.

The full moon shines brightly at magnitude -12.7 much brighter than the brightest planet Venus. Photo: Bob King

The entire sky is 360 degrees all around, both the half we see and the other half under our feet. You can measure the area of the sky the same as you’d measure carpet for your living room. It comes out to 41,253 square degrees for both hemispheres. Since the moon spans only 0.2 square degrees, you can obviously fit lots of them in the sky. But can you fit enough to match the sun’s brilliance? Hmmm. 41,253 divided by 0.2 = 206,265 full moons. That’s 166,264 shy of the total number of moons (372,529) needed to equal the sun’s dazzle.

So even if you packed the entire visible sky with full moons, you’d still need an additional northern hemisphere and half a southern hemisphere’s worth of sky filled with the remaining 166,264 moons to finally match the power of the sun. Amazing, isn’t it, how bright our star truly is. Or is it that the moon’s just doggone faint?

New Hubble image takes us to the brink of the Big Bang

The Hubble eXtreme Deep Field picture was made over 10 years from time exposures totaling 2 million seconds or 23 days. It shows about 5,500 galaxies in a patch of sky less than one-tenth the size of the full moon. Click for larger version. Credit: NASA, ESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

Hubble sweeps us off our feet again. Using the orbiting 94-inch telescope astronomers have taken the deepest photo of the universe yet. They’re calling it the Hubble Xtreme Deep Field or XDF. It’s based on the older Ultra Deep Field picture made in 2003-2004 of a tiny patch of sky southwest of Orion in the constellation Fornax the Furnace measuring 3 arc minutes or one-tenth the size of the full moon. That picture took 11 1/2 days to expose (1 million seconds) and recorded light from galaxies only a stone’s throw from the Big Bang.

This new image drills into the original ultra deep field, adding another million seconds of exposure time and squeezing even more galaxies from a smaller 2.3 x 2 arc minute peephole. And yes, it takes our cosmic gaze back another 200 million years.

Illustration showing the size of the piece of sky compared to the moon photographed over 10 years by the Hubble Space Telescope. The patch measures 2.3 x 2 arc minutes. Credit: NASA, ESA

The faintest galaxies in the photo are ten billion times fainter than the dimmest star the human eye can see. For those familiar with star magnitudes, a measure of celestial brightness, the picture reaches down to magnitude 31. The picture resembles a sandwich piled high with slice upon slice of time, sampling relatively nearby space as well as the most distant recess of the universe we’ve seen yet.

Some of the galaxies are so far away we see them when the universe was less than 5% of its current age.

See all those tiny blue “stars” peppering the image? They’re young, small galaxies – seeds as it were – that later merged with similar small galaxies to build the stately spiral and monster elliptical galaxies we see in the current era. The blue color comes from fresh generations of blazingly hot blue stars formed in the violence of collision and subsequent star birth.

This image separates out the Hubble eXtreme Deep Field by the distances of objects within it. The galaxies are at vastly different distances from each even though they all appear close together in the XDF photo because they lie along the same line of sight. This illustration breaks down the view into three zones. Credit: NASA/ESA

The large, fuzzy red galaxies are what’s left of collisions between earlier galaxies that failed to ignite new star formation. Their color tells us that their stars are aging. There’s even a smattering of beautiful spiral galaxies similar to our own Milky Way. The farthest objects are 13.2 billion light years away and formed 450 million years after the origin of the univers in the Big Bang.

Closeup of a small section of the XDF with the galaxy UDFj-39546284, candidate for the most distant galaxy yet discovered. We see it as it was 13.2 billion years ago. It’s a small object packed with blue stars. Credit: NASA/ESA

“The XDF is the deepest image of the sky ever obtained and reveals the faintest and most distant galaxies ever seen. XDF allows us to explore further back in time than ever before,” said Garth Illingworth of the University of California at Santa Cruz, principal investigator of the Hubble Ultra Deep Field 2009 program.

A few days ago we looked backward in time through the lens of a huge boulder dropped by a glacier 10,000 years ago. Hubble transports us to a time when Earth was still an unrealized possibility.

New comet C/2012 S1 (ISON) could be spectacular sight in fall 2013

Comet C/2012 S1 ISON was discovered on Sept. 21 in pictures taken with 15.7-inch reflecting telescope of the International Scientific Optical Network (ISON) in Russia. This photo was taken on Sept. 22. Credit: Ernesto Guido, Giovanni Sostero and Nick Howes

A new comet was discovered inching across Cancer the Crab in the morning sky may knock your socks off next fall. Based on a preliminary orbit, it could become a very bright object beginning in November 2013 for both northern and southern hemisphere sky watchers. C/2012 S1 (ISON), its formal name, was found by Russian amateurs Vitali Nevski and Artyom Novichonok of the International Scientific Optical Network (ISON), a network of observers who track man-made space debris.

When first photographed on Sept. 21 the comet was an incredibly faint 18.8 magnitude and appeared much like a star with only a “whiff” of a coma around its icy nucleus. It will slowly cruise through the constellation Gemini for many months while growing gradually brighter. And I do mean gradually. Even as late as next June, ISON will only shine at 14th magnitude; that’s scraping the bottom of the bucket for most amateur telescopes.

The situation improves next September when 8-10 inch scopes should pick it up as a small blob in Cancer around 11.5 magnitude. From late October through late-November, things get much cheerier with C/2012 S1 brightening sharply from 7th to 1st magnitude in the eastern sky at dawn. I’m sure I’ll be setting my alarm for a look Thanksgiving morning.

The sharply curving path of Comet C/2012 S1 (ISON) shown from mid-November through mid-December 2013. The comet will be only 1.1 million miles from the sun on Nov. 29. That’s only a little more one sun-diameter. Maps created with Chris Marriott’s SkyMap software

When closest to the sun at perihelion on Nov. 29 it’s predicted to shine a spectacular -7 magnitude or almost 10 times brighter than Venus. Before you say WOW, you need to know the comet will lie only 4.4 degrees north of the sun (very close!) on that date and probably be hidden in the solar glare. Then again, we might spy it in daylight through binoculars by taking proper precautions to keep the sun out of the field of view. Some of us saw the last daylight comet C/2006 P1 McNaught in January 2007 this way.

The comet will be visible low in the southwest just after sunset in bright twilight with an upward pointing tail. The numbers are its brightness or magnitude. For reference, Venus is -4.4, Vega is 0 and the Big Dipper stars are +2 or second magnitude.

A second round of excellent visibility commences immediately after perihelion as S1/ISON performs a  hairpin turn around the sun and banks north into Ophiuchus and Hercules in early December.

While the comet fades during this time, it’s likely to have a spectacular tail and be as bright as magnitude -4. Both hemispheres will get great views, with the northern favored as Christmas approaches. Indeed, northerners will see it at both dusk and dawn. The comet will pass nearest Earth at a distance of about 37 million miles in January 2014. On the 8th it will appear only 2 degrees from the North Star.

That makes two potentially bright comets in 2013 – the other is C/2011 L4 PANSTARRS. It’s forecast to shine as brightly as Vega next March. While I like what I see, it’s important to remember that comets don’t always perform as expected. Any prediction of a comet’s brightness is subject to change, sometimes radically. I’ve seen a few wax much brighter than expected, while others have gone nowhere but downhill. More needs to be known about C/2012 S1′s orbit before an accurate forecast can be made. Let’s just say things look very promising for now.

The Great Comet of 1680 over Rotterdam painted by Lieve Verschuier. Notice the lack of city lights. Some of the people are using cross-staffs to measure the comet’s altitude and tail length.

One other interesting tidbit about C/2012 S1 (ISON) is that its orbit appears very similar to the Great Comet of 1680 also known as Kirch’s or Newton’s Comet. The two may even be related. Kirch’s comet was discovered on November 14, 1680 by German astronomer Gottfried Kirch.  After passing extremely close to the sun, it brightened so much it was plainly visible to the naked eye in mid-afternoon in early December.

One eyewitness report described it as having a “very fiery tail” that stretched 70 degrees long or more than 2/3 the way from horizon to zenith. Newton was working on his great treatise “Principia” at the time and used the comet’s motions to test the predictions of his theory of gravity.

Will C/2012 S1 (ISON) become a Great Comet, too? I’ll look into my crystal ball when more data becomes available.

Morning mystery glow announces arrival of zodiacal light

The zodiacal light (left) points upward from the eastern horizon and “touches” the Milky Way (right) Sunday morning. The bright object is the planet Venus. Details: 16mm lens at f/2.8, ISO 800 and 2-minute exposure on a tracking mount. Photo: Bob King

When fall comes around I know it’s time to get up at dawn to see the glory of the zodiacal light. It may not be easy to set the alarm for 4:30 a.m. and wander out into 32-degree cold, but you take it a step at a time. Pull on your pants, zip up jacket, pack camera in car and drive to a dark location with an open view to the east.

I’ve never been able to shake the feeling of sneaking around or breaking the rules when I get up in the early morning hours. As a teenager I remember tiptoeing from the bedroom to the front door trying hard to avoid the “creaky spots” in the floor. The last thing you wanted to do was wake up dad.

Illustration showing the tapering wedge of zodiacal light extending upward from the eastern horizon to the Milky Way around 5 a.m. in late September. The Belt stars of Orion are at right. The light is centered on the ecliptic, the path the sun follows through the zodiac during the year. Created with Stellarium

Seen from a dark sky the zodiacal light is large wedge of ghostly light tilting up from the eastern horizon, reaching past the brilliant planet Venus all the way to the band of the winter Milky Way where it touches Orion’s shoulder. The bottom end is brighter and wider than the tapering tip. Many people are surprised when they see the light cone for the first time – it’s much bigger than they think. Brightness-wise the wedge looks like dawn itself at its base, but the top glows only as brightly as the fainter parts of the Milky Way.

The zodiacal light is so called because it’s centered on the zodiac, that ring of 12 constellations defined by the sun’s apparent path through the sky during the year. Right now the sun shines in Virgo; in December it will have moved to Sagittarius and next summer will ascend into Taurus.

Twice a year the zodiacal light makes a good show from mid-northern latitudes when it’s tilted up high at dusk in spring and at dawn in fall. This week it angles up through Leo, Cancer and Gemini.  Just like the sun, the angle of the zodiacal wedge to the horizon varies during the year. When tipped at a low angle, it’s obscured by thick air and haze. When tipped up high, you can’t miss it from a dark sky.

Comets like 168P/Hergenrother, currently visible in the fall evening sky, are the source of most of the material that creates the zodiacal light dust cloud. Credit: Michael Jaeger

The wonder of the zodiacal light is that it’s made of billions of dust particles shed by countless comets orbiting approximately in the plane of the solar system between Jupiter and the sun.

Heat from the sun vaporizes comet ices which are gummed up with dust and small rocks. Some of that liberated dust strikes Earth’s atmosphere, burning up as occasional random meteors we see on any night of the year. Much of it settles into a vast, rarefied cloud in the plane of the solar system where it’s illuminated by the sun like pollen shaken from a pine tree. Dust closer to the sun reflects its light more brightly; dust farther away less so. That’s why the zodiacal light is brighter at its base – which is closer to the rising sun – than at its tip.

Much comet dust slowly spirals into the sun over time. To keep a steady supply available, comets ancient and new have contributed the dribs and drabs that make the zodiacal light an arresting sight. To see it best, find a dark sky location with a great view to the east and start looking about 2 hours before sunrise. For my town, that’s between 5 and 5:30 a.m. 90 minutes before sunrise, you’ll still make out the glowing light but also notice dawn gaining ground.

Sunday morning was special. The cone of comet debris stretched all the way to the blizzard of stars comprising the band of the Milky Way. Comet dust, like all dust, is a gift of the stars. Seeing the eerie light literally reach for the stars hit me like a cosmic version of a baby touching her mother’s face.

Approximate view (not to scale) showing the dust cloud from comets that extends across the inner solar system. Lit by the sun, we see it as the zodiacal light. Illustration: Bob King

Tomorrow through Thursday morning are ideal times to see it for yourself. By Friday the moon will be up at dawn and spoil the view. The next moonless period begins on Oct. 13 and continues through the end of the month. Mark your calendar for an early morning adventure.

Lifestyles of the hydrogen-rich and erratically famous

A huge glacial erratic versus a small human on the Superior Hiking Trail this week. Photo: Bob King

Wait a minute. What is this thing? Asteroid, alien spacecraft, rock of doom? No, it’s only a 20-foot-high glacial erratic astride the Superior Hiking Trail 5 miles east of Finland, Minn. The guidebook indicated its location but I wasn’t prepared for the sight of this behemoth. Erratics are boulders plucked by glaciers and deposited miles from their source. This one , composed of the mineral anorthosite, appeared to have been dropped in the middle of nowhere, hence it’s otherworldly appearance.

A very tight closeup of lichens (center) and crystal structure (right) on a small patch of the erratic. Photo: Bob King

When seen up close up, the bland, gray exterior of the rock proved to be composed of coarse crystals. Tiny patches of tough lichens hid parts of the weathered surface.

The boulder probably parted company with the glacier 10,000 years ago. Anything around that long in one place becomes a time machine into the past. Through its dark crystal panes, we glimpse a long-gone world of mile-thick ice and numbing cold. Given enough time, life’s tiny fungal tentacles, working in tandem with nature’s freeze-thaw cycle, will reduce this titan to shards and finally soil.

The Dawn spacecraft used its gamma ray and neutron detectors to discover hydrogen hot spots on the asteroid Vesta this past year. Red indicates the greatest amount of hydrogen; gray the lowest. Credit: NASA/JPL-Caltech

Another big rock – this one in the asteroid belt – shares an even more ancient past than my erratic. Rather than rocks dropped by ice, Dawn was hit with hunks of water-rich asteroids from the asteroid belt called carbonaceous chondrites. The probe found Vesta’s equatorial zone laced with hydrogen from water chemically bound to the rocks as -OH, also called hydroxyl. Free water’s formula is OHH, described more simply as H2O.

“The source of the hydrogen within Vesta’s surface appears to be hydrated minerals delivered by carbon-rich space rocks that collided with Vesta at speeds slow enough to preserve their volatile content,” said Thomas Prettyman, the lead scientist for Dawn’s gamma ray and neutron detector (GRaND) at the Planetary Science Institute in Tucson, Ariz.

Hundreds of small pits inside Vesta’s crater Marcia may have formed when late bombardment heated earlier materials deposited by water-rich asteroids. Heated by impact, water bound in rocks escaped to create the pits. Credit: NASA/JPL-Caltech/University of Arizona/MPS/DLR/IDA/JHUAPL

If any ice itself were to survive, you’d think Vesta’s polar regions would be the best places for preservation just as on the moon. The moon’s rugged terrain and an axis tipped just 1.5 degrees to the plane of Earth’s orbit create permanently shadowed havens for ice in craters at its north and south poles. Unlike the moon, Vesta’s axis has a considerable 29-degree tilt. As it rotates and orbits the sun, both north and south polar regions are repeatedly exposed to sunlight just as they are on Earth. If ice once languished there, it’s long gone.

The Sutter’s Mill carbonaceous chondrite, which fell on April 22, 2012 in California, is dark colored like most of its class. Photo: Bob King

In fact, most of the hydrogen was found in darker-colored rocks encircling the equator. Since carbonaceous chondrites are themselves dark and water-rich compared to other meteorites, they’re a good match for what Dawn found on Vesta.

More evidence for ancient water comes from strange clusters of pits measuring about 100-800 feet across discovered in the 40-mile diameter crater Marcia. They resemble similar features on Mars that likely formed when water within the rocks vaporized explosively during an impact leaving behind pothole-shaped depressions.

Similar pits on Mars from water boiling from the surface by the heat of impact. Credit: NASA/JPL-Caltech

It’s thought that a second round of high-speed impacts accomplished the same on Vesta. Marcia’s center is pocked with pits and has very low levels of hydrogen, consistent with water boiling off into space when the crater was formed.

Water. It’s always at the center of the story when it comes to space exploration. Earth’s water is believed to have arrived the same way as Vesta’s through comet and “wet” asteroid bombardment. Much later, water would build the glacier that plucked the boulder that now reposes alongside a woodland trail. You never know what adventures may lie ahead when you go with the flow.

Uranus makes a rare pass by a star in Pisces

To get oriented, face east around 10 o’clock and find the Great Square of Pegasus. Uranus is located one “square width” below and in line with the Square’s left side and forms an isosceles triangle with Delta and Omega Piscium. Charts created with Chris Marriott’s SkyMap

Last week at a star party I aimed my telescope at Uranus for the first time this season and was pleasantly surprised to find it next door to 44 Piscium (PYE-see-um), a star of identical brightness. One by one people lined up for a look. We had fun comparing the two colors – Uranus an obvious blue against the yellow-orange of 44 – and trying to figure which was the planet and which the star. That was apparently too easy, since everyone had no trouble telling them apart.

As we zoom in a little, Uranus and 44 Piscium stand out better. Delta and Omega Piscium will guide binocular users to the pair. Uranus is magnitude 6 and just visible with the naked eye from dark rural skies. Stars are shown to magnitude 7 with the view facing around 10 this evening (22nd).

Since then, Uranus has been edging closer to the star night after night. Tonight (Sept. 22) and tomorrow they’ll be at their closest and form a striking “double star” through binoculars and telescopes. How close? Only 1.4 arc minutes  or about 1/20 the diameter of the full moon. Those with excellent skies will see the pair as a single, faint, unresolved star, while binoculars will show them as a pair of close-set “eyes” staring straight back at you. In the coming nights, the planet will slowly pull away to the west but remain near 44 through the end of the month. Finding the 7th planet and seeing it groove through the sky is a very worthwhile observing project that requires only the simplest of equipment.

Our final chart is close in, so you can use it to track Uranus’ movement to the right (west) over the coming nights. Positions are shown for 10 p.m. CDT. The blue dots show the planet on the nights of closest approach.

Uranus always appears identical to a star with the naked eye and binoculars, but a small telescope magnifying 60x or higher will not only show the color difference between star and planet but also reveal Uranus as a tiny disk. 44 Piscium will remain a flickering point of light even at high power – a fine side-by-side example of the difference in appearance between a star-like planet and a star. I wonder if the colors will be visible in binoculars? Only one way to find out. Good luck!

Happy equinox! Half-moon dabbles in the occult tonight

Yellow sugar maple leaves contrast with the bark of a paper birch yesterday. Photo: Bob King

Happy first day of fall! The sun crossed the celestial equator on its way south at 9:49 a.m. (CDT), sending a chill up the spines of those who love summer. Many of us look forward to cooler days and nights and that first frost. Here in Duluth, Minn. leaves began changing as early as the end of August. We’re now well into the color season with every hue from lemon to scarlet to brown falling to the ground.

Tonight all those colorful trees will cast shadows under a first quarter moon. First quarter refers to the moon having completed a quarter of its orbit around the Earth. This takes about 7 days. In another 7 days the half moon will fill out into a full moon. We get a special edition of the full moon this month – the Harvest Moon. I’ll have more on exactly what makes it special early next week.

Don’t pass up the chance to look at the moon through a small telescope over the next few nights.  The number of craters visible is incredible.

Illustration of the moon tonight (September 22) showing the star Mu Sagittarii hovering above the earthlit half of the moon minutes before occultation as viewed from Duluth, Minn. North is up. Created with Christ Marriott’s SkyMap software

Let’s return to tonight. The moon will occult the star Mu Sagittarii (Sa-ji-TARE-ee-eye) this evening around 8:20 p.m. (CDT). An occultation is the astronomical term to describe one celestial object covering up another.

Mu is a moderately bright star – magnitude 3.8 – and should be visible next to the moon this evening in binoculars. A telescope will show it plainly. If your sky’s free of haze you may even be able to make out the dim, earthlit half of the moon. Try putting the bright half out of the field of view to see it best.

The fun begins when the moon draws very close to the star. Minutes before the cover-up, you can see the moon move in real time as it moves in for the kill. With just seconds remaining,  Mu may seem to hover forever at the precipice, and then – PFFFT! – it’s gone. Whether you’re looking through telescope or binoculars, the star will blink out with surprising suddenness because the moon lacks an atmosphere.  If there was air up there, Mu would gradually dim and disappear. Even without special instruments, early astronomers could be certain there was little air on the moon by observing occultations.

An 8th magnitude star appears to sit on the moon’s edge moments before it’s occulted on April 21, 2007. Credit: Herbert Raab

Times for the occultation vary depending on where you live. In Duluth, Minn. it happens very close to 8:20 p.m., in Chicago 8:21 p.m. Central time, Atlanta at 9:21 p.m. Eastern time, Denver at 6:53 p.m. Mountain time and LA at 5:15 p.m. Pacific time. Depending on your latitude, Mu will disappear at a slightly different spot along the moon’s left or eastern edge.

In the northwestern U.S., the star will just miss the moon, appearing to graze its northern limb. If you’re keen to observe the event, write to me in the Comments section with the name of the nearest medium-sized or large city to your home, and I’ll send you back a time.

Is NASA hiding something? No, but the Earth is

Pictures of the sun snapped every 15 minutes by the orbiting Solar Dynamics Observatory. Data appears to be missing from the middle five frames. Credit: NASA

So what’s up with those blank squares? You’re looking at a screen capture of a page of pictures of the sun in photographed in ultraviolet light by NASA’s Solar Dynamics Observatory (SDO). The photos were shot 15 minutes apart starting Wednesday evening into Thursday morning this week.

Since SDO circles Earth in a geosynchronous orbit about 22,000 miles high, it “sees” the sun continuously both day and night from a vantage point high above Mexico and the Pacific Ocean. About 1.5 terabytes of solar data or the equivalent of half a million songs from iTunes are downloaded to antennas in White Sands, New Mexico every day. The space station, which orbits much closer to Earth, would make a poor solar observatory since Earth blocks the sun for half of every 90 minute orbit.

SDO’s eclipse season started around 1 a.m. September 6 when the observatory shot a photo of the Earth (top middle) cutting across the sun. Credit: NASA

Did I say SDO watches the sun continuously? Well, not quite. Twice a year for a period of about three weeks around the equinoxes, the Earth gets in the way of the sun from the space craft’s point of view, causing a total solar eclipse. The latest round of eclipses began on September 6 and will conclude on the 26th.

Now you know the reason for the blank frames – it’s a conspiracy by the Earth to block out the sun. The blackness is none other than the planet itself.

Normally the Earth is out of the way of the sun from SDO’s perspective but twice a year its orbit and Earth’s orientation to the sun cause Earth eclipses. Credit: NASA

Total eclipse happens every day between 1 and 2 a.m. local time (Mountain Daylight Time) when the Earth blocks the sun from SDO’s view. In similar fashion, we experience a solar eclipse on the ground when the moon covers up the sun. You can watch for pictures of the partial eclipse as Earth gets out of the way sometime next Tuesday the 25th by going to the SDO website. Follow these simple steps to find and view the images:

* Click on the Data tab and select AIA/HMI Browse Data
* Click on the Enter Start Date window, select a start date and click Done
* Click on Enter End Date and click Done
* Under Telescopes, pick the color (wavelength) sun you want
* Select Images in the display box
* Click Submit at the bottom and then browse the pictures

Not only does the Earth cross the sun from the observatory’s perspective, so does the moon (left) on occasion. The moon’s”bite” smaller and sharper. Earth’s atmosphere gives our planet a soft, diffuse edge compared to the airless moon’s. Photo at right was taken on September 6, 2012 at eclipse season start. Credit: NASA

While watching an eclipse of the sun by the Earth is one of the joys of living in the space age, there are other cool things to see from SDO’s perspective. Look at the drastic difference between the moon’s sharp outline and Earth’s fuzzy edge. Our planet “bites softly” into the sun because its substantial atmosphere grades from thick to thin, filtering the sunlight that passes through it. The moon’s a big baldy. With no air to grade and soften the light, the sun shines crisply right up to its edge.


Video of a partial eclipse of the sun by Earth. Refraction of light by Earth’s atmosphere causes the sun to bend at its edges. Credit: NASA SDO / Stanford University for HMI

We’ve seen how air can also bend or refract sunlight in strange ways, going so far as to “lift” the sun  into view when it’s still below the horizon.  You can see the same effect in a brand new way in this short video of an SDO partial eclipse. Watch the sun’s edge bend as the Earth rolls by. Compare it to a similar eclipse by the moon below.


Moon eclipsing sun via SDO 

SDO orbits about 22,000 miles above Earth, tracing out a figure-8 (called an analemma) above the Pacific and Mexico every 24 hours. Credit: NASA

SDO amazes with its spectacular pictures of the sun taken in 10 different wavelengths of light every 10 seconds; additional instruments study vibrations on the sun’s surface, magnetic fields and how much UV radiation the sun pours into space.

It’s the latest, greatest “Swiss Army knife” used by scientists to pry open the inner workings of the sun. The eclipses, while a gap in the data stream,  are a sweet bonus all their own.