Roy G. Biv pays a visit

A primary and secondary rainbow drape the eastern sky yesterday evening (August 6). The colors are arranged in the order of the spectrum: red-orange-yellow-green-blue-indigo-violet, a.k.a. "roygbiv".Also visible is Alexander's Dark Band between the two bows. Photo: Bob King / News Tribune

Perhaps you saw the beautiful double rainbow yesterday evening that a brief thundershower left in its wake. Rainbows are not too uncommon during the summer months. Quickly moving storm clouds typically travel from west to east. As the western sky clears, sunlight breaks through to illuminate raindrops still falling in the other half of the sky.

The primary bow is formed when sunlight is bent and spread into its component colors as it enters, reflects and then exits a raindrop. Diagram: UW-Stout physics and used by permission

As long as the sun is lower than about four outstretched fists above the horizon, its light will intercept raindrops at the right angle for a rainbow to form. Raindrops act like tiny lenses, and bend or refract the sun's white light, literally spreading it into a rainbow of colors. Let's follow a light beam. Light enters the drop, gets bent and begins to spread into its component colors. Next it reflects off the back of the drop, spreading out even before it finally exits. Millions of raindrops are doing this all at the same time, mingling their light to create the familiar bow.

Rainbows can appear very substantial but they're made of nothing but sunlight as translated by raindrops. There's an astonishing beauty in that.

The secondary bow is formed in a similar way as the first, but instead of reflecting once inside the drop, light reflects twice and exits at a different angle. This extra reflection also causes the colors to be reversed, with red on the bottom instead of the top. When light leaves the raindrops some of it's also refracted below the main bow and above the secondary bow, but none between the bows. This is the origin of Alexander's Dark Band, first described by Alexander of Aphrodisias in 200 A.D.

Supernumerary arcs below yesterday's primary rainbow. These colorful bows are formed when light is diffracted as it exits the raindrops. Photo: Bob King / News Tribune

If you look closely at a well-developed rainbow, you'll often spot a series of vividly colored purple and green fringes below the main bow called supernumerary bows. They were unbelievably bright in yesterday's rainbow. Supernumeraries form through another process called diffraction.

Computer simulation of an interference pattern created by two expanding circular light waves. If a wave trough and crest coincide, the pattern is darker than either waves average brightness (destructive interference). If wave crests or troughs coincide, the pattern is brighter (constructive interference). Fig. 8-6 from The Rainbow Bridge (Penn State Press, 2001) by Raymond L. Lee, Jr. and Alistair B. Fraser; also available at  www.usna.edu/Users/oceano/raylee/RainbowBridge/Chapter_8.html

Drop a pebble into water and you'll create a nice pattern of waves. Each wave crest is separated by a wave trough. If you drop two pebbles at the same time some distance apart, some of the crests from one set of spreading waves will overlap the troughs of the other, canceling each other out. Other crests will reinforce each other to create even higher crests. You'll recall that light is made of waves, too. As individual light waves enter and then exit a raindrop, they behave like our water waves. Some light waves are canceled out and some are strengthened, creating a pattern of bright and dark colored fringes. These are the supernumeraries. To learn more about light diffraction and interference, here's a good website. 

There are even more rainbows possible beyond the secondary. They're called higher order or tertiary bows and are very rarely seen. The first of these -- called the third order bow -- forms a broad, faint arc around the sun itself. It's only 24% as bright as the primary. I've searched but have yet to see a photograph of one. Click here to see a simulation of what to expect.

Next time you're enjoying a spectacular rainbow, turn around and look right above the sun. If you see the tertiary, we'll know at the very least that there's a pot of good luck at the end of your rainbow.

Posted by: rking@duluthnews.com on 8/7/2008 at 10:19 AM | Comments (1) | Permalink

Something to hang your coat on

The Milky Way courses right through the Summer Triangle constellations. You can use Sagitta the Arrow to guide you to Brocchi's Cluster, a fine binocular object, or take an alternate route. The cluster is on a straight line from Altair to Vega so if you point your binoculars at Altair and make a beeline for Vega, you'll run right into it. Details: 20mm lens at f/2.8, ISO 1600, 30-second time exposure. Bob King/News Tribune

Did you see the Sagitta arrow last night? Comparing Sagitta to Delphinus, I thought Delphinus was a little easier to see because it lies outside the distracting band of the Milky Way. Sagitta is a little more challenging because of competition from neighboring star clouds. You can see this in the photo above. 

A closeup of the Coathanger as it would appear in binoculars. Details: 41mm lens at f/4, ISO 800 and 4-second time exposure. Photo by Tony Mitchell

We can now use Sagitta to help us find a striking asterism called Brocchi's Cluster but better known the Coathanger. From a dark sky site, Brocchi's is just visible to the naked eye as small, glittery patch in a dark area of the Milky Way just above the right end of the Sagitta arrow. Binoculars really breath life into the little group, revealing a spangled coathanger floating upside down in the vast lane of interstellar dust. Four stars comprise the hook, while six or more make up the supporting bar. The effect is quite striking, and once you see it, you'll wonder where this gem has been hiding all your life.

The Coathanger was first written up as long ago as 964 A.D. by Persian astronomer Al Sufi in his Book of Fixed Stars. Much later, in the 1920s, amateur astronomer D.F. Brocchi created a map of the group to calibrate light-measuring devices called photometers. Brocchi's has been the cluster's name since, but the more informal "Coathanger" has been gaining ground for obvious reasons.

Despite its name, Brocchi's Cluster is not a true, physically connected group but a chance alignment called an asterism -- an unrelated but picturesque arrangement of stars. Whatever it is, every time I'm observing with amateur astronomers during the summer months, everyone loves to feast their eyes on the Coathanger. While you're out tonight (August 6), looking for a place to hang your hat, watch for the last good evening pass of the International Space Station. It will appear in the west at 8:54 p.m., and arc about three fists high in the southwestern before disappearing in the southeast.

Brocchi's Cluster as it would look in a small telescope. Photo: Gil-Estel

My grandma was famous for remarking "Oh my stars!" when she was surprised or amazed by the stories and accomplishments of her grandkids. She always gave us candy when we visited, and she or grandpa would quietly slip each of us a dollar before we left. Great people.

Grandma Bauer had a variety of different broaches in her jewelry box. I can't remember any one in particular but they were colorful and glittered with rhinestones and gold. If there were ever a perfect nominee for a celestial broach, it would be Brocchi's Cluster. I can imagine her pinning the bejeweled coathanger on her blouse, patting it flat and remarking once again: "Oh my stars!" 

Posted by: rking@duluthnews.com on 8/6/2008 at 9:42 AM | Comments (0) | Permalink

An arrow in the Milky Way's heart

The view looking southwest around 9:15 p.m. tonight August 5. The four-day-old moon is joined by the star Spica in Virgo. -- created with Stellarium

A toenail clipping of a moon will dangle in the sky this evening after sunset. If you look toward the southwest around 9:15 p.m., you should see it without any trouble. Nice to get our moon back after its eclipse mischief last Friday.

A fisheye view of the entire sky around 10:30 p.m. The zenith is the point directly overhead. Deneb, Vega and Altair form the Summer Triangle. Notice that Vega, the brightest star in the triangle, is almost at the zenith. Brilliant Jupiter in the southeast joins the triangle to form an enormous backwards "Y" figure that covers a huge swath of sky. --created with Stellarium

As we wend our way into August, the sky gets darker earlier. By 10 o'clock, there are lots of stars visible, including the Summer Triangle high in the southeast. Last Friday we used Altair, the bottom star in the triangle, to find Delphinus the Dolphin. Altair will also guide us to another pocket-sized constellation, Sagitta the Arrow. To use the circular, all-sky map, rotate it in your computer so that the direction at the bottom of the chart is the one you face. For example, when you want to see what's up in the eastern sky, turn the map 90 degrees (1/4 turn) counterclockwise while facing east. The map is set up here with the view facing south.

Looking southeast at Altair, the bottom star in the Summer Triangle. Delphinus is an outstretched fist to the left or east of Altair, while Sagitta is one fist above. -- created with Stellarium

Like Delphinus, Sagitta is a dim collection of stars but its compact form makes it relatively easy to find. Look one fist above Altair to spot the little arrow. If you live in the country or a suburban area, both constellations shouldn't pose a problem. City dwellers would best keep a pair of low power binoculars on hand as a visual aid.

I hope our good forecast holds for tonight and that you're successful finding the Arrow. Tomorrow we'll use the arrow to guide our gaze to a most curious object you're more likely to find in your closet than in the Milky Way.

The Grinch bear looking for some tasty snacks meets its starry alter ego. Photo: Katherine King. Constellation from Stellarium.

A couple weeks ago my daughter Katherine spotted an Ursa Major in Duluth's Woodland neighborhood. She whipped out her camera and bagged a photo of the constellation namesake on the prowl. She was tickled by how it walked -- the feet reminded her of the Grinch in the book How the Grinch Stole Christmas by Dr. Seuss. May the bear give you inspiration as you pad your way across the sky tonight.

Posted by: rking@duluthnews.com on 8/5/2008 at 10:50 AM | Comments (5) | Permalink

Fueled by starlight and good people

William Wiethoff waits in anticipation as amateur astronomer Jeff Setzer from West Bend, Wis. aims his 22" reflecting telescope at a distant globular cluster. Hobbs Observatory dome in the background. Photo: Bob King/News Tribune

I'm back after a good night's sleep. I had a blast at the Northwoods Starfest at Hobbs Observatory this weekend. Friday night was up until dawn. Saturday night was clear until an ominous blanket of cloud blocked the sky like that UFO in the movie Independence Day. What to do? We hurried our telescopes back into cars and vans and then made a beeline to the mess hall. An acre of cheetos, spicy chips, candy, fruit juice, coffee and grapes beckoned. After food and converstation, most of us were ready for bed until someone called out that the sky was clearing again. I looked up at the Milky Way, felt the baggage under my eyes, and declared sleep the victor. Others unpacked a few telescopes or just kicked back in chairs watching meteors till 3.

Refracting telescopes for sale at one of the tables at Saturday's swap meet. Photo: Bob King/News Tribune

We looked at all kinds of stuff through every kind of telescope -- comets, bright familiar clusters, a faint supernova, a nebula shaped like Saturn and one shaped like North America, Jupiter and many more. I snagged a bargain at Saturday's swap meet, paying two bucks for a wonderful model of the solar system with moveable planets orbiting an illuminated sun.

Dr. Jennifer L.B. Anderson takes questions after her talk from (left to right) Starfest participants Jim Schaff, Deane Clark and Dale Eason. Photo: Bob King / News Tribune

Friday night's talk by Dr. Jennifer L.B. Anderson, an experimental impact cratering scientist, who uses the Vertical Gun range Laboratory at NASA's Ames Research Center to make and study impact craters in the lab, was outstanding. She combined her fascinating research into crater-making with an earthy sense of humor. "I'm interested in anything you can throw a rock at," said Anderson. Referring to the extinction of the dinosaurs and subsequent rise of the mammals due to an asteroid impact, Anderson quipped: "If it wasn't for that impact, a dinosaur would probably be here talking to you right now."

We learned how she used a special gun to fire a small steel ball at a target of sand at extremely high speeds. The resulting hits are studied using lasers and cameras to better understand what happens when meteorite meets solid ground.

There were additional talks on comets, satellites and space vehicles, radio astronomy and the upcoming International Year of Astronomy 2009. And if that wasn't enough, you could wander around the reserve's butterfly house or hike nearby trails.  

French toast stuffed with apples, scrambled eggs and the best sausages this side of Mars at Saturday's breakfast. All the participants were well fed and well taken care of by members of the Chippewa Valley Astronomical Society. Photo: Bob King / News Tribune

Everyone who attended Starfest enjoys the night sky just like the readers of this blog. I asked a few of the participants what they thought was the best thing about astronomy. Here's what they said:

"The ooh and the ahh." - Dave Evans, Saginaw
"We adults become like kids in the backyard chasing butterflys with a butterfly net again." - Eric Norland, Normanna Township
"People spend all their money on trips, and it's all in your backyard." - Jim Harstad, Aurora
"I do it because it's fun. I like learning new stuff and it keeps you humble. Every point of light out there hasn't heard of us." - Ben Huset, Twin Cities
"Lately for me, it's imaging (taking pictures). They're pretty and you can see a lot more than with your eye." - Mike Cain, Jim Fall, Wis.
"The absolute beauty of the skies!" - Shelly Dziak, Reedsburg, Wis.
"Space is vast and limitless. Theories are always changing. The natural beauty of the universe." - Russell Mannigel, Spooner, Wis.
"I like learning from other people. Unplugging -- I work computers all day. Afterword, I can take this out (telescope) and just observe. I met my wife through astronomy." - Jeff Setzer, West Bend, Wis.
"I like the moon the best. All the craters and how it got beat up in the past." - Sarah Beede, Duluth
"The quality of the people." - Greg Furtman, Webster, Wis.
"I think it's a joy to the eye." - Kevin Kienholz, Two Rivers, Wis.
"It can fulfill any interest -- from computing to stargazing." - Jim Schaff, Hermantown
"Sharing the discovery of excitement and understanding." - William Wiethoff, Port Wing
"I bought a telescope for my son for a reward for his good grades in school. In two weeks, that scope was mine. I like the awesome scope of the universe we're part of. It puts our small, human problems in perspective." - Mike Dziak, Reedsburg, Wis.

And the road goes ever on.

Posted by: rking@duluthnews.com on 8/4/2008 at 3:47 PM | Comments (2) | Permalink

Till dawn do us part

I'm writing from Hobbs Observatory near Eau Claire, Wisconsin, the site of the annual Northwoods Starfest put on by the Chippewa Valley Astronomical Society. There's fantastic camaraderie here among people who love the sky. Outside the observatory, a sandy field looks like a primitive stage for a symphony orchestra of telescopic instruments. Refractors, reflectors, Cassegrains and more are poised for the night, their owners eager to play them toward galaxies, star clusters and planets.
Last night I did what I've wanted to do for years -- stay up all night. I didn't think it would really happen. After the "midnight snack", a table stacked with an excellent assortment of the finest junk foods, I felt weary and ready to collapse into my sleeping bag. But I followed my friends back under the inviting sky and ten minutes later, began anew.
The hours spun by as we searched for a challenging supernova in a distant galaxy, returned again to study the Jupiter's cloud belts and oohed and aahed at one meteor after another.
By 4 a.m., dawn touched the eastern sky. Now I didn't want the night to end.
I was a vagabond riding a railcar to freedom. It didn't matter how late or how tired. We were gonna ride this baby until the horizon glowed pink .

... more tomorrow.  

Posted by: rking@duluthnews.com on 8/2/2008 at 8:40 PM | Comments (4) | Permalink