A look at celestial happenings in the Northland

MUCH colder by the lake

Did you notice how orange the moon was last night even when it was well above the horizon? Hazy, dusty air was hard at work scattering and absorbing the cooler colors in its light, leaving us a partially-eaten peach of a moon. I don't think the haze will clear out until thunderstorms arrive tonight and tomorrow to clean things up.


Venus and Mercury will be hanging out together in the western sky over the next week. Look to the left of the sun around 8:30 p.m. to find Venus. It's very low -- only three fingers - held out in front of you - above the horizon. Mercury will slowly swing to the left of Venus over the next four to five nights.

If it's clear tonight, and anytime over the next few nights, you can see Venus and Mercury together very low in the western sky about two fists to the left of the sunset point. The best time to see them is at 8:30 p.m. Once the haze departs, Venus will be fairly easy to spot but Mercury might require binoculars.


This illustration depicts a lake of liquid hydrocarbons under the smoggy skies of Saturn's moon Titan. Credit: NASA

Saturn used to be part of the trio including Mercury and Venus but is now too close to sun to be visible. It will hide in the solar glare for the next few weeks and then re-emerge in the morning sky. All the while, the Cassini Mission has been keeping its electronic eyes on the planet and its moons. One of the mission's most important discoveries was the detection of liquid hydrocarbons in one of the largest lakes on the moon Titan.


The footprint-shaped Ontario Lacus in Titan's south polar region (upper left). It's filled with frigid liquid ethane. Back on Earth, ethane is found mixed with natural gas and is used by the chemical industry. Cassini used a radar mapping instrument to penetrate Titan's haze and see the surface. Photo: NASA/Cassini

Cassini has photographed hundreds of smooth, dark, lake-like patches on Titan's surface but finally confirmed the presence of liquid ethane in Ontario Lacus near the moon's south pole this summer. That makes Titan the only other world in the solar system besides Earth with liquid on its surface.  Before you jump into Ontario Lacus, which is a little larger than Lake Ontario, be sure you're wearing your special insulated bathing suit. The temperature of the ethane is around 300 degrees Fahrenheit below zero. We have a saying here in Duluth that it's always colder by the lake, referring to Lake Superior's chill waters and winds. Future astronauts to Titan will understand the real meaning of "colder" when they explore the beaches of Ontario Lacus on some far-off day. 


Saturn's moon Titan, which is about 3000 miles in diameter, is covered in a dense haze created by evaporation of ethane and other materials from its surface lakes and rivers. Credit: NASA

Ethane evaporates from Ontario and the other lakes just like water does on Earth. It mingles with other hydrocarbons and forms a dense, pink haze in Titan's nitrogen atmosphere. Titan's a billion miles from the sun and bitter cold all over. With all those organic or carbon-based compounds like ethane and methane around, scientists believe Titan resembles the early Earth before life developed.  In liquid form, they could serve to gather and mix other chemicals in the environment to create more complex organic molecules. 

If we warmed this moon up a little, is it possible that Titan might become fertile ground for a second beginning of life in the solar system? I'd love to think that life is that ready to get up and go, given half a chance.

Posted by: rking@duluthnews.com on 8/21/2008 at 10:26 AM | Comments (0) | Permalink

A plunge into the deeps


The dark and dusty nebula, Barnard 68, in the constellation Ophiuchus the Serpent Holder. It almost looks you're standing along the edge of a deep pond looking down into its dark depths. Photo credit: VLT, ANTU + FORSI

Yesterday we talked about how dust, water vapor and pollutants redden the sun and moon as they approach the horizon. This same color shift occurs in outer space between the stars where it's called interstellar reddening. Fine dust spread across space absorbs the cooler colors in a star's light, causing it to appear warmer-hued than it really is.

The photo of the dark nebula Barnard 68 demonstrates reddening in dramatic fashion. Dark nebulas are clouds of dust and gas that appear dark because no stars shine from within to illuminate them.  Notice that stars are obvious around its perimeter but fade rapidly and redden as you "step off" the edge. This globule of cold dust is seven light-months (about 1000 times the size of our solar system) across and 500 light years away. Over time, it will eventually contract, and the dust will coalesce to form a cluster of brand new stars. When that happens, the dark nebula will be transformed into a bright nebula like the famous one in Orion.

To see through the dust, astronomers have developed infrared (heat sensing) and radio telescopes that penetrate the dark clouds and reveal stars in the making.


Dead tree branches reach toward the moon Monday night. Fast moving clouds became a blur during the 5-second time exposure. The small colored disk around the moon is called a corona. Photo: Bob King / News Tribune

On Monday night, we had some great clouds flying across the waning gibbous moon. As they came and went, a beautiful colored disk appeared, melted away and reformed as more clouds took their place. These small disks, called coronae, are blue in the center and red on the outside. They're fairly common, and caused by the diffraction of light by millions of tiny, similar-sized droplets in the clouds.


A riot of color is created by diffraction in oily water droplets in this photo taken in the alley behind the Duluth News Tribune. Coronas, oil films and bubbles all reveal white light's hidden color in a process called diffraction. Photo: Bob King

This same phenomenon can be seen on oily water droplets in streets and alleys after a rain. Light reflects off both the top of the thin oil film and from its underside. The two rays that leave the surface interfere with one another and create patterns of vivid colors. The exact same thing happens with soap bubbles. To learn more about diffraction, click here for a great website.

No matter where you look, light's always playing games.

Posted by: rking@duluthnews.com on 8/20/2008 at 12:30 PM | Comments (0) | Permalink

Red-faced at sunset


The full moon is surrounded by the first rays of morning sunlight reflecting off the mountains, as it disappears into a notch created by peaks of the Sierra Nevadas in California. This image was taken this past Sunday, August 17. Photo: Andrew Kirk

Andrew Kirk from California wrote yesterday asking for more information about the origin of the sun and moon's red coloration as they approach the horizon. He sent a wonderful photo of the full moon just before it was swallowed by the peaks of the Sierra Nevadas.


Light from the sunset or sunrise sun (left) and the noonday sun are a study in color contrasts. You'll recall that the sun's white light is really a combination of all the colors of the rainbow spectrum. Thicker air near the horizon removes most of the cool colors from the sun's light, leaving red and orange.  Photos: Bob King / News Tribune

Most of the air in our atmosphere is concentrated in the bottom 10 miles. Compared to our planet's diameter, this airy shell is about as thin as the plastic wrap you drape over a slice of pie to reheat in the microwave. But it's enough to keep most of the life we're familiar with chugging along. The lower you go in the atmosphere, the thicker the air is and the greater the concentration of suspended dust particles, pollutants and water vapor.


The sunset sun takes the long path through our atmosphere and is heavily filtered, while the noon sun meets little resistance. Illustration: Bob King / News Tribune

When the sun or moon is up high, the path their light takes is much more direct. It cuts through the extremely thin outer atmosphere, zips through the lower 10 miles  and lights up our shining faces. The air molecules on this short path only remove a portion of the sun's blue light and scatter it to create a blue sky. To the eye the sun appears nearly white.

Now take a look at the sunset sun (which also applies to sunrise). Its light comes in at such a low angle that it passes through hundreds of miles of the dense and dusty lower atmosphere. All the colors are scattered or absorbed by the air except for the warm reds and oranges. These have the ability to penetrate where the others can't. The same is true for the moon of course.

If you look back at Andrew's photo, you'll see the mountains glowing pink in the sun's early light. It hadn't risen yet for him but the mountaintops, being much higher, caught the light sooner. This sunrise-sunset mountaintop lighting is often referred to as the Alpenglow, after the mountain range in Europe. Officially, Alpenglow is the deep-red, reflected glow of twilight immediately after sunset or before sunrise. Either way, the warm tones painted by the air's magic make a memorable sight. In Andrew's words: "A very short-lived event, modest ... but also miraculous." 


A vividly orange moon rises over Lake Superior Sunday night. Photo: Bob King / News Tribune

Posted by: rking@duluthnews.com on 8/19/2008 at 9:35 AM | Comments (0) | Permalink

A twinkling harp sails overhead


The moon rises over an incoming ore boat as seen from Skyline Parkway in Duluth last night (August 17). Details: 280mm lens at f4, 1/8" exposure at ISO 400. Photo: Bob King / News Tribune

It's a joy to walk the moonlit night wearing a T-shirt. Baked by the sun earlier in the day, and slightly toasted from cooking chicken on the grill, I was in no hurry. The moon, which I'd photographed earlier, struggled now to free itself from insistent clouds. When it finally succeeded, the forest came to life in an inviting mix of bright branches and dark hollows. Nothin' but a T-shirt. I think the deep bone chill of February was finally exorcised last night.

The moon will be up again at 8:45 p.m. tonight if you want to catch it rising. Stay out a bit later and you can learn a new constellation -- Lyra the Harp. We met Lyra casually before in this blog, but have never taken the time for a proper introduction.


Lyra the Harp is a small and rather faint constellation but with Vega as your guide, it's worth trying to see. To the ancient Greeks, Lyra represented a lyre or small harp given to Orpheus by Apollo. -- created with Stellarium

Start with Vega, the fifth brightest star in the sky and the western apex of the Summer Triangle. Look due south and almost overhead around 9:30 p.m. Vega stands out brightly, even in twilight. From there, see if you can find the four stars in a parallelogram that comprise the harp. The brightest of them is the lower left star called Gamma Lyrae.

The star above and left of Vega is Epsilon Lyrae, the famous double-double star we met in a blog last June. It's a tough split with the naked eye but binoculars will easily slice it into a physically orbiting pair of suns. You don't have to look far from Vega to see the harp. The entire constellation is small enough to cover with three outstretched fingers held at a horizontal.


The Ring Nebula in Lyra, located 2300 light years away, is a one of the best known deep sky objects of the summer sky. The faint star at center is a white dwarf and only about as big as the Earth. Light radiating from the star illuminates the nebula. -- Photo: Jim Misti

Hidden between the two bottom harp stars is one of the coolest nebulas of the summer sky, the famous Ring Nebula. You'll need a small telescope to see this smoky, celestial doughnut. The Ring is a cloud of gas and dust expelled by a star much like the sun. As the star evolved, its outer atmosphere wafted into space as an enormous expanding bubble. Radiation pouring from the searingly hot core of the star, now called a white dwarf, excites the gas to glow, creating this beautiful celestial trinket.

Posted by: rking@duluthnews.com on 8/18/2008 at 11:29 AM | Comments (0) | Permalink

The blue hour


A 1820 painting titled Der Abend (The Evening) by Caspar David Friedrich depicting two people on a twilight walk. Twilight is sometimes referred to as "the blue hour" when the natural illumination we receive comes from the sky.

Yesterday's sunset was welcome relief after a hot, humid day. The cool evening air made everything right. Darkness is coming on quickly now, not only because the sun is setting earlier, but also because twilight is shorter than it was in early summer. Twilight is caused by sunlight still illuminating the upper atmosphere even as the sun has set for us ground dwellers. That wonderful diffuse glow bathes the landscape in a soft light that's easy on the eyes. With the sun gone, we put away the day's activities and allow ourselves a little time to relax.


The brightness of the twilit sky depends on how far the sun is below the horizon. One degree is equal to 2-times the apparent size of the sun or moon. Illustration credit: Canadian Space Agency

There are actually three twilights as defined by common use:

For the northern states, twilight lasts up to 2 1/2 hours during early summer. Add that to 9 o'clock sunsets and you don't see true night until after 11:30 p.m. The sun sets tonight at 8:14 p.m. and twilight is currently one hour, fifty minutes. Astronomical night begins just after 10 o'clock. No wonder it gets dark so early! Ah, but there is the moon. It's only a day past full tonight, and rises around 8:30 p.m. Don't expect to see black skies at 10 for a few more evenings because of moonlight.


This diagram shows how the angle of the sun's path affects the length of twilight. Its steeper path in August places the sun further below the horizon  compared to June. The change in angle has to do with the changing position of the sun in the sky as we orbit it over the course of a year.  Illustration: Bob King

The shorter twilights are caused the steepening path the sun takes as we move towards fall. If the sun hits the horizon at a shallow angle, as it does in late spring-early summer, it takes much longer for it to sink 18 degrees below that line. By mid-August its path cuts at a sharper angle and night begins sooner.


Lunar twilight? Sorry, no atmosphere, no mellow glow. The bonus is getting to see the sun and the stars in the same black sky. And without the atmospheric glare, all you'd have to do to spot the current trio of evening planets is block the sun with your hand. -- created with Stellarium

In the southern U.S., the sun's path is generally steeper than in the North. Twilights in Tucson are typically last 1 1/2 hours all year long. What about other planets? Any planet with an atmosphere and a surface will have twilight hours ideal for strolling or walking your space dog. Twilight on Mars lasts longer than on Earth because of so much light-reflecting dust suspended high in its atmosphere. Because the moon has virtually no air of any kind, there's no twilight there. Being able to observe the sun and the stars at the same time? Sounds like an astronomical paradise.

Posted by: rking@duluthnews.com on 8/17/2008 at 10:06 AM | Comments (0) | Permalink