Morning of the blue moon

What - a blue moon? Thanks to scattering, you can see a colored moon during the morning hours this week. Photo: Bob King

A whirlwind of work pins me to the wall today, so I’ll have to postpone writing about my favorite molten rocks – the tektites – until tomorrow. Did you see the moon this morning? The last quarter hung high in the southwestern sky around 8 a.m. Very easy to see even with the sun well up in the east. Looking more closely, I couldn’t help but notice that the lunar seas, those big gray spots that are actually vast volcanic plains, were tinted blue like the waters of nearby Lake Superior.

Shorter wavelength blue light is scattered efficiently by air molecules, while the other colors can better penetrate the air and travel directly to your eye. Credit: Pingry School at

We all know that white light is composed of all the colors of the rainbow from violet to red. When sunlight strikes the atmosphere, tiny molecules of nitrogen and oxygen scatter the blue part of sunlight much more than the other colors, painting the sky its familiar blue.

The moon, which is not bright enough during the day to overwhelm the scattering effect, is likewise tinted as if viewed through a blue filter. Take a look for yourself at the moon’s appropriately sky-blue waters the next clear morning.

The moon has returned to easy visibility in the daylight hours because it’s headed in the sun’s direction as it thins from last quarter to morning crescent on its way to the next new moon. And since the sun is approaching its highest point in the sky (summer solstice), it follows that the moon also gains altitude as it tracks sunward.

Visitors to Mars will someday see with their own eyes its butterscotch skies caused by suspended dust kicked up by Martian winds. Credit: NASA

Mars’ sky is very different from Earth’s, colored instead by iron oxide dust kicked up by the planet’s incessant winds. The dust absorbs the blue end of the rainbow and reflects back the red and orange light, giving the sky a pink or butterscotch hue. Mars atmosphere is much thinner than Earth’s. If you could somehow vacuum up all the dust, it would appear very dark blue, similar to what you see out the window of a high-altitude aircraft.

Shocking news about Earth’s 182 craters

Meteor Crater, also known as Barringer Crater, was excavated by an iron meteorite 54 yards wide 50,000 years ago. It's 3/4 mile in diameter and 570 feet deep. Credit: NASA

Yesterday we touched on Mars and moon craters. Earth also had the pleasure of meteorite and asteroid pummeling over its 4.6 billion-year history. While it’s easy to spot craters on the moon with little more than 10x binoculars, finding them on our planet takes scientific sleuthing. Most have long since been erased by water and wind erosion, glaciers, volcanoes and the ever-changing jigsaw puzzle of our planet’s crustal wanderings known as plate tectonics.

Can you spot the crater? The Sudbury Basin in Ontario, Canada is the large, tilted oval across the center of the picture. It's 39 miles long x 19 miles wide. Sudbury, the second largest crater on Earth, formed during an impact 1.85 billion years ago . Originally round, its shape has been distorted by geological processes. A second crater called Wanapitei (upper right) formed 37 million years and is now a lake. Credit: NASA

To date 182 confirmed craters or impact structures have been discovered on terra firma. Most blend into the landscape and wouldn’t be known if it weren’t for telltale shock features found in local rocks. Others, like Meteor Crater near Flagstaff, Arizona, stick out like a sore thumb.  In only a small minority have fragments of the impactor been found around the craters – Meteor Crater, Kamil (Egypt), Henbury (Australia), Sikhote-Alin (Russia), Wabar (Saudi Arabia) and Campo del Cielo (Argentina) to name a few.

Iron-nickel fragment from Meteor Crater. The metal crystal pattern is peculiar to iron meteorites and not found in earthly iron. Photo: Bob King

The pieces are almost always iron-nickel meteorites. Though much rarer than stony meteorites, irons are more resistant to weathering than rocky meteorites and more likely to survive passage through the atmosphere intact.

Without meteorites as clues, how do scientists know whether a suspected Earth crater has an extraterrestrial origin? This or that hole or depression might just as well be an ancient lake bottom or sculpted by a volcanic explosion.

No meteorites were found at the Tenoumer crater in Mauritania in Africa but shocked rocks and frothy impact melt glass like the 4-inch wide slice (inset) clinched its extraterrestrial origin. Click photo to learn more. Credit: NASA and Bob King (inset)

The key discovery was made by geologist Eugene Shoemaker in 1960 when he uncovered strongly shocked forms of quartz called coesite (KOH-site) and stishovite in sand and rocks in Meteor Crater in Arizona.

The 1.1 mile diameter Lonar Crater in India is filled with a saltwater lake. Shocked quartz as well as impact breccia mixed with glass (inset) were found within the crater's rocks. Lonar formed between 35,000 and 50,000 years ago. Click photo to learn more. Credit: Wiki and Bob King (inset)

Under the extreme pressures and temperatures experienced during a large meteorite impact, quartz is transformed into these new minerals. No volcano nor any other force on the surface of the Earth, except the detonation of a nuclear bomb, has the power to alter quartz in this way.

Sand-sized quartz grain from the Chesapeake Bay impact structure showing two sets of shock lines at angles to one another. Credit: U.S. Geological Survey

Scientists could now use shocked quartz as a litmus test to identify impact craters. And since the mineral abounds on Earth, all they need to do is gather pieces from a suspected impact crater and bring them back to the lab. There they examine the specimen under a powerful microscope for tiny crystals of coesite and stishovite.

Meteoric impact also leaves behind “tracks” as multiple sets of shock veins within quartz crystals (above).

Space station astronauts and surveillance satellites find occasional new craters from orbit that are later confirmed by ground expeditions. Even regular folks have spotted potential impacts from the comfort of their homes and offices using Google Earth.  The recent Kamil crater in Egypt was discovered in satellite imagery by a former museum curator in 2008. An expedition to the remote hole in 2009 turned up 1.6 metric tons of iron meteorite fragments!

Beautiful shattered rock called breccia in specimens from Lake Wanapitei (left) and Gardnos Crater in Norway. Click to learn more. Photo: Bob King

Along with shocked quartz, meteorite impacts blast, tumble and mix local rocks into a tutti-frutti of fragments that are compacted over time into breccias (breh-chuhs) . The heat and pressure of a strike also melts rocks, forging curious varieties of glass called impact melts.

Breccias and impact-made glasses are known collectively as impactites. Some impactites even contain bits of the original impactor. While not as exotic as meteorites, these altered rocks represent Earth’s inaction with meteorites and are fascinating materials in their own right.

Year 2000 map of Earth impact craters. Several of the featured craters are shown. Click to visit the Earth Impact Database.

Tomorrow we’ll look at another type of impact debris on Earth – the enigmatic tektites.

Far-out flora: Daisies bloom on Mars

The humble daisy, flower of summertime comforts.

You know it’s summer in my neighborhood when the daisies return. They appear when the threat of below freezing temperatures at night disappears. One of the most cheerful-looking flowers, the daisy’s charm continues into the night, when its pure white petals seem to glow in the dark.

Pattern-finding is one of the hallmarks of the human species. When we find a pattern in one object that resembles or matches the pattern in a completely different object, we often assume they’re connected in some way. Sometimes they are, sometimes not.

Crater in Arabia Terra with "petals" of fluidized lava. Click photo to enlarge. Credit: NASA/JPL/ASU

Yesterday I received a link to fresh photos of Mars taken by the combined visual and infrared camera on the Mars Odyssey craft in orbit about the Red Planet. One of the images immediately reminded me of my beloved daisies. It depicts an unnamed crater in the densely cratered Arabia Terra region of northern Mars.

Another unnamed Martian crater surrounded a series by a beautiful halo of fluid, hilly ejecta. Click to enlarge. Credit: NASA/JPL/ASU

Despite the appearance of having sprouted petals,  we’re looking at lobes of ejecta blasted out by the impact the gouged the crater. Martian craters are very different from those on the moon and Mercury. Many are surrounded by petals like this one that were probably created by the heat of impact melting ice trapped in the planet’s crust. Now a liquid, the water mixed with the impact materials and spread outward into overlapping mud-like flows that quickly refroze into rocky petals.

A trio of lunar craters with their rays of secondary craters created when material shot up by impact fell back to the surface peppering the area with secondary craters. Left to right: Aristarchus, Kepler and Copernicus. Credit: Jim Misti

Astronomers call them rampart craters because the outer lobes of the ejecta blanket are rimmed by ridges or ramparts.

If you look at most lunar craters, it’s obvious that material was blasted high above the surface and then rained back down in a big splat around the main crater hole. We see these ejecta blankets as bright rays around craters like Tycho and Copernicus.

In petaled craters, some of the material from the impact appears to have flowed across the surface instead of going airborne. Rampart craters have been experimentally duplicated right here on Earth by shooting projectiles into mud.

Mars' magnificent lobed Tooting crater, 17 miles in diameter. Credit: NASA

If nothing else, lobed craters indicate that Mars has water – and potentially life – beneath its surface. Daisies have simply taken the next step and made water serve life, creating an ejecta blanket of white and yellow blooms along our roadsides every June.

Mercury spices up the evening as the Heavenly Palace glides by

Mercury will be low in the northwestern sky over the next couple weeks. The picture shows the view tonight about 30 minutes after sunset. The stars shown are for reference and may or may not be visible because of a bright sky. Maps created with Stellarium

Sure it’s not Venus but at least the western sky won’t be planetless this month. Mercury has returned to the evening twilight stage. It’s bright but low. Go out about a half hour to 40 minutes after sunset and look northwest to the left of the sunset point to find it. The speedy planet is about 6 degrees above the horizon 30 minutes after sunset tonight. In the coming week, it will get easier to spot  – especially for northern observers with our long twilights – as it departs the sun’s glare and gains altitude.

Meanwhile, Venus has moved to the other side of the sun and is now in the morning sky though still too close to see with the naked eye. By late June it will pop up in the east in morning twilight, and in mid-July will gather with Jupiter, the star Aldebaran and the blade-thin moon for a spectacular dawn conjunction.

A pass of the International Space Station below the Big Dipper back in April this year. Photo: Bob King

I know some of you saw the space station pass by a couple times last night. I was out between 10:30 and 10:40 p.m. and saw both the station and the Chinese Tiangong 1 (Heavenly Palace 1) satellite at the same time buzzing along different paths in the sky.

The Chinese space laboratory module, a precursor to a full-blown Chinese space station, was sent into orbit for rendezvous and docking exercises with the manned Shenzhou spacecraft. Although Heavens Above indicated it would appear as a bright 1st magnitude star, the ship was much dimmer at 4th magnitude. If you’d like to see it, click on the link, log in and select your city, then click on the Tiangong 1 link. Don’t be surprised if it’s fainter than predicted.

A ring of 1st and second magnitude stars in the constellations (from left) Gemini, Auriga, Perseus and Cassiopeia ornament the northwestern horizon at nightfall.

We’ve had a few partly clear nights lately, so I’ve been enjoying watching the ring of bright fall-winter stars arrayed along the west and north horizons.  From left to right as you face to the northwest they are: Pollux and Castor in Gemini, Mirfak in Perseus, Menkalinan and Capella in Auriga and the W of Cassiopeia with Schedar and Caph nearly equal in brightness. For some of you living in the southern U.S. one of more of these luminaries may have already been gobbled up by your local horizon.  Taken together, the bunch look like guards stationed at distant outposts around the western terminus of the sky.

Get your bib number on, the space station marathon starts now

Four passes of the International Space Station (ISS) over the Duluth, Minn. region are shown tonight June 7-8. The ISS is making similar multiple passes over the northern hemisphere for the next few nights. Each panel shows the entire 360-degree view of sky overhead. Created with Stellarium

For some of us, planning for the Venus transit was a marathon. Figuring out what telescope and camera to use, where to watch and then standing in the hot sun for a couple hours took planning and effort. But the sweat of any endeavor makes the next that much easier.

Ready for another marathon? Good. Tie your shoelaces tight and let’s get outside. Every year for about a week in early June, the International Space Station’s flight path combined with the position of Earth’s shadow place the ship in nearly continuous sunlight for the northern hemisphere. That means we get to see passes every 90 minutes from dusk till dawn. Tonight there will be four and tomorrow we’ll max out at five.

The space station orbits the Earth as seen from the Atlantis space shuttle in 1995. Credit: NASA

I’ve stayed up late enough to observe three sky crosses but never had the energy for the complete marathon. By the third it felt like the crew on the ship was keeping an eye on me. It would really be fun to photograph an entire night’s worth with a fisheye lens and superimpose the images into one picture. Any takers?

Photos of the Venus transit taken by astronaut Don Pettit from aboard the space station. Click photo to see closeup photos numbered 7-14 in the Expedition 31 gallery. Credit: NASA/Don Pettit

Pass times for the Duluth, Minn. region. For times for you town, please click over to Spaceweather’s satellite flyby page or log in to Heavens Above, select your location and click the ISS link.

The space station travels from west to east across the sky and may occasionally show bright flares.

* Four passes tonight and Friday morning June 7-8 starting at 10:30 p.m., 12:06 a.m., 1:43 a.m. and 3:19 a.m.
* Five passes Friday-Saturday June 8-9 at 9:36 p.m., 11:12 p.m., 12:48 a.m., 2:25 a.m. and 4:02 a.m. An observing marathon lasting nearly 6 1/2 hours!
* Four overnight passes Saturday-Sunday June 9-10 at  10:17 p.m., 11:54 p.m., 1:51 a.m. and 3:07 a.m.
* Four passes Sunday-Monday June 10-11 at 11 p.m., 12:36 a.m. and 2:13 a.m.

Transit of Venus a huge hit across the country

Amanda Haglund of Moose Lake holds a white board with a projected image of the sun and Venus (dot at upper left) during the transit. Photo: Bob King

It was awe-inspiring to watch a planet step in front of the sun Tuesday. I hope all of you had success in your observations of the Venus transit. I drove down to Moose Lake, Minn. (about 45 miles south of Duluth) to join my friend Glenn Langhorst for two bowls of homemade chile and one very tasty transit.  Glenn had his 4-inch refractor ready to go when I pulled up. 10 minutes later my own 4-inch scope was on its legs and ready to run.

We waited, now exactly sure where Venus would first bite into the sun’s disk, and a little after 5:04 p.m. were thrilled to see a dark indent in the sun’s northeast limb. As Venus slowly swelled into a black disk, we easily saw the thin, white rim of glowing atmosphere on the part of the planet still poking into the blackness beyond the sun’s disk. The deeper the planet moved onto the sun, the easier to see the arc became.

This picture, taken through an 8-inch Dobsonian reflector with a solar filter, shows the thin white outline of Venus' atmosphere beyond the sun's edge. "At first I thought I was imagining a thin white line with the naked eye through the telescope, but after I unloaded the cameras, I noticed it did vaguely show up on the images," said Larry Regynski. Thanks and credit to: Larry Regynski

How about this perspective? Venus floats against the corona beyond the sun's edge in this photo taken by the Solar Dynamics Observatory (SDO) at 12:25 a.m. today after the transit finished. Click image to see a whole page of SDO stills and movies of the transit. Credit: NASA/HDI

Having never witnessed this Venusian “ring of fire” before and not quite knowing what to anticipate, Glenn and I were blown away by the sight. We noticed another interesting thing – the white rim was much easier to see through a glass filter yielding an orange image than through optical mylar with a blue image.

Two views of the Venus transit through a 4-inch telescope. At left, Venus looks like a notch made with a paper punch. The other dark flecks are sunspots. Photos: Bob King

Next was the black drop effect. No long after the atmospheric arc disappeared, Venus stuck to the inner limb of the sun for what seemed an entire minute before finally separating. The bridge or ligament connecting the planet to the sun’s inner edge was black at first but turned gray just before it disappeared. Now free of the sun’s limb, Venus began its leisurely traverse of the solar disk for the next few hours. Was there a more perfect black circle – sphere ever in the world? The sight was mesmerizing.

Kevin Nudell watches Venus and the sun through an 8" Meade telescope. He and his father, Bob Nudell, shared both Tuesday's and the 2004 transit together. Credit: Bob Nudell. Photo at right by John Sponauer

Venus wasn’t obvious with the naked eye through a filter at the beginning of the transit, but a half hour later, after it had fully entered the sun, you couldn’t miss it. The planet reminded me of a large naked eye sunspot.

After a few more photos, it was time to take a break and eat that chile.

Glenn Langhorst holds his canine observing companion Lucy while watching Venus' progress through the telescope. Photo: Bob King

We returned to do more viewing and picture taking until the sun dropped into the treetops. If you like really big images, projection is for you. We used my scope to project an image of the sun an entire foot in diameter. Boy, was Venus ever obvious!

Venus was an easily visible black dot through a Nikon 55-300mm lens at 300mm from Anamosa, Iowa. Thanks and credit to: Steve Wendl

I’ve included a few photos of the transit and would love to expand our little gallery with more. Send a pic or two my way at and I’ll try to include it later today.

Dan Staley took this photo at Observatory Park in Denver. His daughter Payne observes the transit through a hydrogen-alpha telescope.

Nice photo of the black drop effect at the end of second contact. Credit: William Wiethoff

Matthew Connell captured both the transit and the scene from southern Oregon. "There was thick cloud cover most of the afternoon and quite a bit of rain. We got a good look twice, for a total of maybe 20 minutes," said Connell. "We were VERY glad to get a short window of opportunity!"

Members of the Arrowhead Astromical Society in Duluth, Minn. including William Wiethoff (center right) share views of the transit with the public Tuesday afternoon. Credit: Eric Norland

This composite photo required patience and careful timing. Rick Klawitter of Port Angeles, Wash. observed the transit from 2:30 to 9:10, taking about 20 pictures per hour. "It took 15 to 20 photos and several attempts to create this composite picture. Credit: Rick Klawitter

Today’s transit of Venus: The sight of a lifetime

The full moon rises over a hotel on Duluth's Park Point last night at 9:45 p.m. Photo: Bob King

I’ve been preoccupied with the Venus transit and sometimes forget there are other things happening in the sky. Last night the rising moon brought this to my attention. Like an unexpected gift, it stood beautifully orange and oval when I left work for home. Of course I had to change course to stop and enjoy the view.

Edmond Halley

In yesterday’s blog we talked about things to watch for during today’s passage of Venus across the sun. You might wonder if a transit is still of any scientific interest. It was Edmund Halley of Halley’s Comet fame who first suggested that Venus transits could help us determine Earth’s distance from the sun.

Back in the early 18th century, astronomers using Kepler’s laws of planetary motion could only determine the relative distances of the planets from the sun. Earth was understood to be one astronomical unit distant (1.0 A.U.), Mercury 0.4 A.U.,  Jupiter 5.2 A.U.s and so forth. No one knew the real or absolute distances in miles.

In 1677 when Halley was watching a transit of Mercury, it occurred to him that widely spaced observers across the globe could use a transit of Venus to determine its distance from the sun. Armed with that number, simple multiplication would then give the distances to the other planets. Halley suggested that observers record small variations in the position and timing of Venus’ entry and exit from the sun’s disk from various locations around the Earth during the next transit on June 6, 1761.

My daughter Katherine demonstrates how to see parallax using her finger and eyes. Photo: Bob King

The apparent shift of Venus against the more distant sun as seen from widely spaced locations is called parallax. You can see parallax at close range by holding up your index finger at arm’s length from your face. Close one eye and then the other and watch your finger shift against the distant backdrop. If you measure the amount of shift and know the distance between your eyes, you can determine the distance to your index finger with simple trigonometry.

18th century French cartoon depicting the transit of Venus. The first known observation of a Venus transit was on December 4, 1639 by Englishman Jeremiah Horrocks. He used his telescope to project an image of the sun on white paper.

Halley figured the same idea would work on Venus provided observers were spaced far apart (in effect, making for a wide set of multiple “eyes”) , knew their locations exactly and could time Venus entry and exit from the sun precisely. Although Halley didn’t live to see the 1761 transit, his idea was embraced by the scientific community. Astronomers were dispatched around the world in the first international scientific enterprise of its kind.

The black drop effect frustrated the efforts of astronomers to derive accurate distances to the planets. Credit: NASA/TRACE/LMSAL

Despite careful timings, the infamous black drop effect made them anything but precise. If Venus had cleanly separated from the sun when it entered the disk, accuracy would have been excellent. Instead, the planet appeared to “stick” to the inner limb (an atmospheric affect) anywhere from a few seconds up to a full minute for some observers, confounding the timings. When the results were analyzed, astronomers could do no better than place Earth’s distance from the sun at between 77.8 and 96.2 million miles.

Observers met with similar frustrations during the 1769, 1874 and 1882 transits. An accurate Earth-sun distance was finally determined by Scottish astronomer David Gill who photographically measured Mars’ parallax against the distant background stars in 1877. For an excellent, in-depth explanation of how to measure parallax using Venus, click HERE.

The Venusian "ring of fire" is visible just beyond the sun's edge in this satellite photo taken during the 2004 transit. Credit: NASA

During the current transit, another worldwide effort is underway to study Venus miniature “ring of fire”. A narrow ring of light caused by sunlight scattered by the planet’s thick carbon dioxide atmosphere briefly makes an appearance when the planet enters and the exits the sun’s disk. Astronomers will use nine different coronagraphs spaced around the world to probe the temperature and density structure of Venus’s middle atmosphere, or “mesosphere,” where the sunlight is refracted. A coronagraph is a special telescope designed to block the sun and its glare, so astronomers can study the inner corona or in this case, Venus and its atmosphere.

By analyzing how the ring brightens and fades with time, researchers can figure out the temperature and density of this critical layer from pole to pole. To read more about the effort, please check out the full NASA press release.

Extrasolar planets are discovered by measuring the change in brightness of the host star when a planet orbits in front of it.

To date 775 extrasolar planets beyond the solar system have been discovered. Scientists find many of them when they transit their host stars, robbing them of a tiny fraction of their light in a repeated and predictable way.

Observing extrasolar transits, astronomers can determine sizes, masses and orbits of planets we can’t see directly because they’re so far away. Starlight filtering through an exoplanet’s atmosphere can even leave its imprint on the star’s light, revealing the composition of the air.

In another transit study, astronomers will point the Hubble Space Telescope  at the moon to observe the transit of Venus. What? The sun’s too bright and would damage Hubble’s sensitive instruments. Instead, the scope will measure the tiny drop in the amount of sunlight bouncing off the moon as Venus passes in front of the sun.

A spectrum of the sun shows dark lines where elements like calcium and hydrogen reveal themselves by absorbing a bit of sunlight. Scientists will look for similar absorptions in light reflected off the moon to study the composition of Venus' atmosphere.

During the transit, Venus’ atmosphere will absorb specific wavelengths of sunlight that will go missing from the light reflected off the moon. A spectrograph, an instrument which spreads light into its component rainbow colors, will spot these missing colors and determine what chemical components in Venus’ atmosphere cause them. The Hubble effort will help inform efforts to study the atmospheres of exoplanets – an idea nothing short of brilliant.

Venus almost ready to go on stage. Sunlight scattered from the planet's atmosphere reveals the full outline. This picture was taken yesterday June 4 at Observatory Rimavska Sobota in Slovakia. Thanks and credit to: Pavol Rapavy

I hope you have clear skies today. If you’re threatened with clouds and mobile, check out the Clear Sky Chart to find a better location. If you’re socked in, you can always watch the transit via live webcast at Planethunters or NASA EDGE. Remember, the transit starts just a few minutes past 6 p.m. Eastern time, 5 p.m. Central, 4 p.m. Mountain and 3 p.m. Pacific. You can watch it at the Alworth Planetarium on the UMD campus here in Duluth, Minn. from 5 p.m. till sunset. If you take a photo, please share it with us by e-mailing me at:

Cool stuff to see during Tuesday’s transit of Venus

A gorgeous image of this morning's partial lunar eclipse from southern Alberta photographed by author and astrophotographer Alan Dyer. This was the kind of eclipse chasing I like," said Dyer. "Just to the end of my driveway." Details: Canon 60Da and 18-200mm Sigma lens at 115mm and at f/5.6 for 0.4 sec at ISO 160. Copyright: Alan Dyer

Soon the trifecta will be complete. Originally a horse racing term, but now often used to refer to three important things coming together at the same time, tomorrow’s transit of Venus will be the third in a series of celestial alignments that included the May 20 annular eclipse of the sun and this morning’s partial lunar eclipse.

Although I got up at 4:45 a.m. with hopes of seeing the moon touched by Earth’s shadow, clouds eclipsed the entire sky. I hope some of you were more fortunate. The weather looks good in my neighborhood for tomorrow’s (June 5) Venus transit, which across North America begins within a couple minutes of 22:05 Universal time or 6:05 Eastern Daylight, 5:05 p.m. Central, 4:05 p.m. Mountain and 3:05 Pacific time.

In Duluth, Minn. we should see the first sign of Venus’ silhouetted self touch the northeastern limb of the sun at 5:04 p.m. This moment is called first contact. 18 minutes later the opposite side of the planet will touch the inner limb of the sun at second contact. A little more than 6 hours later Venus will again touch the inner limb on the western side of the sun during third contact before slowly exiting the sun’s disk (fourth contact).  While you may applaud at transit end, don’t expect an encore until the year 2117.

Black polymer eclipse glasses are great for observing the transit. Venus will be visible as a very small black dot without optical aid. Photo: Bob King

By now, you’ve probably planned out how you’re going to view the event. Methods include special mylar or glass filters coated to reduce the sun’s light UV and infrared light to a safe level. You can also use a #14 welders glass, pinhole projection using a cardboard box or project the sun’s image onto white poster board with binoculars. If you plan on projecting the sun’s image and still need help on how, click HERE for an excellent guide.

It’s important to set up at least a half hour before the transit begins, so you don’t miss the first 20 minutes and /or the last 20 minutes, arguably the most exciting times for viewing. Here are things to watch for:

Venus' atmosphere shows as a bright rim just beyond the sun in the picture taken by the Transition Region and Coronal Explorer (TRACE) spacecraft during the 2004 transit. Credit: NASA

* Aureole of Venus’ atmosphere – This is a faint, extremely narrow annulus or ring of light visible around the planet at first contact through a telescope. Use as high a magnification as possible without blurring the image.  As Venus moves farther into the sun, the ring will shrink to a arc and then disappear.

What you’re seeing is Venus’ thick atmosphere backlit by the brilliant sun. The air on Venus is mostly carbon dioxide and about 100 times as dense as Earth’s. Lower down, clouds of sulfuric acid sprinkle the red-hot, 800-degree surface with a particularly nasty version of acid rain. The sequence of events are reversed during third and fourth contacts.

Venus’ ring is very similar to the ring of light around the Earth that an astronaut on the moon would see when we on Earth experience a total lunar eclipse. Picture the astronaut watching the big silhouetted Earth cover the sun, its circumference aglow with the oranges and reds of low-angled sunlight. That very same light, bent by the atmosphere into Earth’s shadow, colors the moon during eclipse.

You can create a similar black drop effect with your fingers. Photo: Bob King

* Black drop effect — I described this in an earlier blog, but let’s have one more go at it. As the last bit of Venus moves into the sun’s disk at second contact, you might see the planet momentarily shaped like a teardrop. The side of Venus’ disk closest to the sun’s inner limb will appear to stretch toward it momentarily as a black ligament before “snapping” free.

You can see a similar effect by holding out a hand against a smooth, light backdrop and barely touching thumb and index finger. It’s caused by a combination of blurring from turbulence in Earth’s atmosphere and the natural darkening along the sun’s inner edge.

* Blackness of planet vs. sunspots — Venus will easily win out for being blacker than the darkest sunspots. As of this morning there are seven sunspot groups out, so it’ll be easy to compare.

* Happy alignment — Expand your mind and picture the whole scene as you’d see it far above the Earth in outer space with the sun, Venus and Earth, Venus all sitting pretty in a row.

If it’s cloudy at your place tomorrow,  you can always watch the live NASA webcast. Locally here in Duluth, the Marshall W. Alworth Planetarium on the UMD campus will have people and telescopes on hand for anyone wanting to come by for a look starting at 5 p.m.

Tomorrow we’ll look at what scientists hope to learn from the transit.

Lunar eclipse only hours away

The full moon rises atop Earth's rising shadow in early spring 2010. The full moon tomorrow morning June 4 will undergo a partial eclipse. Photo: Bob King

All ready for tomorrow morning’s partial lunar eclipse? Earth’s shadow will cover a maximum of 37% of the moon shortly before and after sunrise for the Midwest, South and and Western U.S. You might be scratching your head wondering if it’s worth getting up at 4  or 5 a.m. to see such a wimpy eclipse. I understand your reluctance. We all have our thresholds.

While I might drive a couple hundred miles for a total lunar eclipse, I wouldn’t for a minor partial. Here in Duluth, Minn. only a few percent of the moon will be covered before it sets shortly after sunrise. If skies are clear, I’ll still be out there. Just about any eclipse is rare enough to expend some effort. After all, the next one – a total – won’t happen until April 2014. And honestly, I like the picture in my head of sun, Earth, moon and me all lined up.

The shadow of the planet rises into the sky for an observer facing east as the sun sets in the west. The situation's reversed for the rising sun, with the shadow in the west as the sun rises in the east. Illustration: Bob King

Eclipses are about shadows. We can see the cause of this eclipse anytime it’s clear around sunset. Face opposite the setting sun and you’ll see a long, purple-gray band rising in the eastern sky. That’s the Earth’s shadow. Standing here on the planet, the shadow rises and eventually fills most of the sky by local midnight. We don’t see it at night of course because it’s dark out and the shadow blends into the sky.

The Earth's shadow has two parts, the dark inner umbra and the outer penumbra.The penumbra is not fully dark because a portion of the sun shines into it. Credit: image courtesy of Courtney Seligman

We can see the space station and other satellites disappear into or re-appear out of the shadow on many nights depending on the satellite’s altitude and season of the year.

If we could see the Earth's shadow at the moon's distance it would look like this - a double bullseye covering a surprisingly small patch of sky. Shadow size is approximate. Created with Stellarium

Earth’s shadow stretches out behind the planet to the moon and beyond. At the moon’s average distance of 239,000 miles, Earth’s two-part shadow, consisting of the dark umbra and sun-touched penumbra, covers only about 2.3 degrees of sky or a bit less than five full moons lined up side by side.  The distance between the two stars at the end of the bowl of the Big Dipper is twice as wide. That’s a small target.

Times and the moon's path through the outer part of Earth's umbra are shown for tomorrow morning. Add an hour for Eastern time, subtract an hour for Mountain and two hours for Pacific. Credit: Tom Ruen with my own additions

Since the moon’s orbit is inclined to the Earth’s, it easily misses the shadow most full moons. Only occasionally does it hit the mark.

If you’ve been following the transit of Venus updates, you’ll recall that Venus’ inclined orbit is the reason transits are so rare. The same is true with the moon.

I hope you do take a look at tomorrow’s  eclipse. Don’t let the routine early risers have all the fun. For more details on the event, check out my earlier blog and this NASA pdf file.

Tycho (button-like crater below and left of center) and its rays dipped in the colors of another partial eclipse on August 16, 2008. Click photo to see more eclipse photos. Thanks and credit to: Pedro Ré

During the eclipse, you’ll see the shadow cover the bottom or southern half of the moon. This is Tycho territory. Tycho, a 53-mile- diameter crater, is crowned with the most spectacular and largest halo of “rays” of any lunar crater. The rays formed when material from the impact that created Tycho rained back down onto the moon’s surface to excavate strand upon strand of fresh secondary craters. Tycho and its rays, normally a blaze of white streaks centered on the brilliant crater at full moon, will be tempered a mellow yellow and orange during eclipse.

Venus takes another step toward the sun plus out-of-this-world star trail photos

Looking like a faraway moon, Venus shines in Earth's sky yesterday at dusk. Photo: Bob King

Venus made an otherworldly appearance between cloud layers last night 15 minutes after sunset, but hazy skies and low altitude made it impossible to see its little companion Mercury. Tonight might be the last night you’ll see Venus in the evening before Tuesday’s transit.

Look for it 2-3 degrees above the northwestern horizon right after sunset. Once found in binoculars, try to spot it with your naked eye.

Venus at 10 a.m. today photographed through a 4-inch scope. The cusps of the crescent extend a little around the shadowed side of the planet. Because of turbulence, Venus looks much thicker than a live view. Photo: Bob King

Today Venus is only five degrees east of the sun, with its nightside turned earthward. Only a super-slender crescent remains. When Venus is in its “full moon” phase, 100% of the planet is illuminated by the sun; today it’s down to 0.4%.

You’d think with so little surface area in sunlight, the planet would be much fainter than its -3.8 magnitude, but as the crescent narrows, its size increases as Venus approaches the Earth. That helps to compensate in good part for the shrinkage in area.

The planet has gradually faded from its maximum brightness of -4.6 magnitude reached on April 30. Before it passes in front of the sun and becomes a magnitude-less black dot, Venus will fade a tiny bit more to -3.7. To measure the brightness of an object in space, astronomers assign it a magnitude, a counterintuitive scale where the larger the number, the fainter the object.  The brightest celestial objects have negative number magnitudes. Topping the list is the sun at -27!

Don Pettit created this amazing image by combining 18 long-exposure images taken with a camera mounted inside the International Space Station (ISS) on March 16, 2012. Cities show as streaks of orange below the station and the bright blotches are lightning. Since the ISS circles Earth once every 90 minutes, long exposures record stars as circular arcs centered on the station's north and south "poles". Credit: NASA/Don Pettit

NASA astronaut and flight engineer Don Pettit,  famous for his many stunning photos taken from the International Space Station (ISS), plans to be the first person to observe and photograph a Venus transit from outer space. He’ll be using a Nikon D2Xs camera and an 800mm telephoto lens equipped with a white light solar filter. Pettit will got the approval from NASA to remove the non-optical quality, internal window panes called “scratch panes” from the space station’s cupola so he can get sharp images.  If I could only do that in an airplane.

Another set of time exposures taken through one of the space station's cupola windows. Click photo for more a larger version and more details on how he made the pictures. Credit: NASA / Don Pettit

Petitt plans to post the photos during the transit. Since he’s a big Twitter guy, you check him out at astro_pettit. It should be interesting to see if Don will see the black drop effect without any atmosphere in the way.

On a final note, I inadvertently used the wrong set of NASA transit times in this earlier blog (now corrected). Though only several minutes off, they were incorrect.

Click HERE for a table listing transit start, maximum and end times for a list of U.S. cities, HERE for Canadian cities and HERE for the rest of the world. Be aware that the times for Memphis and Minneapolis in the U.S. list are incorrectly shown as Eastern. Subtract an hour for the correct one. Here’s another interactive time link.