Fata Morgana and Mirages July 22, 2012Posted by pswanso233 in Astronomy, physics.
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I was scanning through the archives of the Astronomy Picture of the Day, and I saw this one which I thought looked really cool:
Interested, I looked up Fata Morgana and learned that it was a type of superior mirage. Not knowing what a superior mirage was, I had to understand what causes mirages and what the difference between an inferior and a superior mirage was.
A mirage is a real optical phenomenon, rather than a hallucination. Mirages can actually be photographed, whereas hallucinations cannot be. Mirages are caused by temperature differences in the Earth’s atmosphere. It’s here that I should probably introduce Snell’s Law and refraction, which is the bending of light through different materials. Every transparent subtance has what’s called an index of refraction, which is the ratio of the speed of light in vacuum to the speed of light in that substance. A high index of refraction indicates that light travels very slowly through the substance, whereas a low index means it doesn’t slow down much. For example, the index of refraction of water is 1.33. This means that light travels 1.33 times slower through water than it does air or vacuum. This is why a pencil looks bent if you put it in a water filled beaker while still allowing part of it to be in the air.
A high index of refraction also means light will bend more if it travels through that substance. So how exactly does this apply to mirages? Cold air is denser than warm air, so light has a harder time going through it; therefore its index of refraction is higher. If light rays from a distant source travel from cold air to hot air, they will bend away from the direction of the temperature gradient. As these light rays reach your eye, your brain traces it as though they came from a line straight ahead, similar to your eye interpreting a virtual image through a convex lens.
An inferior image is a type of mirage where an image appears to be below a real object. A common example would be a desert mirage, where the viewer thinks that there’s an oasis on the horizon. This is caused because sand tends to heat up quickly, so the air around the sand is hot and the air above it is cooler. The image you’re actually seeing is actually the sky, which is why it looks like water.
A superior image is the opposite case, where the image appears above the horizon. This is caused by what’s called a temperature inversion, where hot air exists above cold air. This tends to be more common at sea.
Anyway, as I said before, a Fata Morgana is a special case of a superior mirage. They can be seen from anywhere on Earth, but tend to be most common in Polar Regions and higher altitudes.
The special case of the superior mirage of a Fata Morgana occurs when the temperature inversion is high enough such that the light bends through it in such a way that the curvature of the light is higher than the curvature of the Earth. The viewer should be present in an atmospheric duct, which is where light rays and other electromagnetic waves bend with the curvature of the Earth. This is why these images tend to be rarer than other types of mirages.
A Fata Morgana usually looks very bizarre, and can produce stacked images on top of each other. They can also change rapidly, as if the temperature gradients change in such a way that the light no longer bends with the curvature of the Earth, they become regular superior mirages and don’t necessarily appear on the horizon anymore.
Fata Morgana is named for “The Fairy Morgana”, Morgan le Fay, who opposed King Arthur and Queen Guinevere in Arthurian legend. She was a sorceress who had affairs with some of Arthur’s knights and was also Arthur’s half-sister.
One other cool and (sort of) funny I learned is that, back in the early 1900’s, some explorers found what they called the “Crocker Land”, which was a supposedly large island that existed between Canada and Greenland. A very expensive team was sent to survey the island, but the mission cost over $100,000 (a huge sum at the time), because the island they saw was, in fact, a Fata Morgana. They were even warned by some of the natives of Greenland that it was an illusion, but they pressed on anyways and were unable to explore the Crocker Land.
There is also some interest that perhaps a Fata Morgana contributed to the sinking of the Titanic:
Gathering the Wrong Light July 21, 2012Posted by pjhsscience in Astronomy, Observatory, telescopes.
Tags: Astronomy, cosmology, Light Pollution, Observatory, science, space, stars, telescope
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Imagine for a moment, driving at night through the vast and unpopulated expanses of the western deserts of North America. Frequently, some of the most amazing photos of our night sky are taken from locations such as these and for very good reason. The only light visible is that which is being projected from the stars above. Back to yourself in the car now, you are approaching a town, a rather large town. As you get closer the lights from above start to fade as your eyes are drawn toward the glowing city. It’s not that street lamps and stoplights are more of an amazing site than our celestial blanket; it’s just that those lights are quickly becoming the only thing visible. You are experiencing the plague of metropolitan exorbitance, a form of pollution, light pollution.
Light pollution is one of the newest forms of pollution plaguing modern society. Before electric grids the night sky, even in large cities, was still an intriguing sight. As technology evolved and electricity flowed we were able to combat our limited night vision by lighting the night. As the world at night become brighter we covered the sky by uncovering what lies beneath us at night.
Lighting too has evolved throughout time. We are becoming more familiar with the glow of HID, or high intensity discharge lights, while becoming less familiar with the arrangement of the heavens. To get a view of just how encroaching light pollution can be we need only look at the animal kingdom. Lighting areas where light is not naturally present at night is having a major effect on nocturnal animals. Sea turtle hatchlings are often confused by brightly lit beaches and wander away from safe havens. Migration patterns of many species of waterfowl have been altered due to excess lighting. Feeding is a naturally performed at night for nocturnal creatures and feeding patterns have brought unwanted guests to our doorsteps due to light pollution. Lights attract bugs and bugs attract bats.
Astronomers from amateur to professional can all agree that light pollution is a great disturbance. Before even viewing a star astronomers without an enclosure cannot expect to have full dark adaption at night. The tools of astronomy are also plagued by light pollution. For instance, the Mt. Wilson Observatory just outside of Los Angeles is now operating at 11% of its original capacity due to the glowing L.A. night sky. While some stars may be visible in areas of high light pollution galaxies and nebula are greatly dimmed and very difficult to see even with advanced telescopes. New observatories are increasingly being constructed in remote areas in order combat light pollution but remote construction brings higher costs.
Limiting magnitude can be described as the faintest apparent magnitude of a celestial body capable of being detected and dependent upon equipment. Light pollution has a direct and sustained impact on the limiting magnitude in a given area. The limiting magnitude of the human eye under a completely dark sky is somewhere in the range of 7.6-8.0. At the other side of this scale, imagine yourself staring up at the night sky in a brightly lit inner-city setting. The limiting magnitude of your eye has been reduced by fifty percent to 4.0 or less. That comparison is simply applied to eyeball astronomy though, what about astronomers looking to make an observation. Under a dark sky with a 32 centimeter reflecting telescope you might just make some observations at the 18th magnitude. Again, we travel to the city where you set up your scope and find that you will only be making observations at the 13th magnitude.
For those in areas affected by light pollution there are some methods of circumventing it. Astronomers often employ narrow or high-band filters that do not allow light of certain spectral lines to pass through a telescope. The spectral lines targeted are those emitted by common vapor lamps including mercury and sodium. Though a good tool, these filters do limit the use of higher magnification.
If you wish to calculate how much light pollution will affect your astronomy work there is a simple equation to employ. The equation, I=0.01Pd-2.5 where I is the increase in sky glow, P is the population of the targeted city and d is the distance to the center of the city, works very well. This law is commonly referred to as Walker’s Law. Merle Walker proposed this relation after taking measurements of sky glow in several California cities. If you used this calculation and yielded a value of .03 that would mean that at the midway point between the horizon and zenith angle in the direction of the city the current sky would be 3% brighter than the natural background.
It is easy to see that combating light pollution would be of great benefit to society in general, the cost savings alone are staggering. Every year we waste one billion dollars lighting the night sky. Remediation of this problem is not as difficult as one might think; in fact, light pollution is the easiest of all forms of pollution to fix. Replacing old style lamps that radiate light in all directions with lamps that focus light downward is one remediation tactic. Also, we have to realize that lighting is not always necessary and we should take steps to remove lighting where it is not needed. Changing output is another effective method. Extremely bright bulbs are used in a number of lighting applications where they are not needed, limiting energy output not only reduces light pollution but also saves money.
We often light outdoor areas without a thought as to what we are losing. We may gain a little extra ease of night time navigation but we lose light at the same time. The light we lose is the light from nebula, galaxies and stars. This light has traveled a great distance, often many light years. This light has traveled those great distances through the vast reaches of outer space. This light ends its journey within our atmosphere at the hands of our lighting. Light pollution is a problem we have created but a problem that we can fix. Take a moment to look at the heavens through a dark sky and ask yourself if it is worth saving. My answer is yes.
Volcanism on Icy Io July 20, 2012Posted by epscienceblog in Astronomy, moon, Solar and Space weather.
Tags: Io, Solar System, Tidal force, volcano
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As we have studied the Universe, one of the main ways that we have learned in the past is by using the Earth as a comparison, using all that we know about our planet as a reference for the other galactic bodies that we explore. What is amazing now is the shift that is taking place, where we are beginning to use what we learn about other celestial bodies and apply that information to our own planet. We learn even more about Earth as advances are being made in the exploration of our universe. One example of how we are applying our knowledge of other bodies is Io. The new information that is being gained from Io gives clues to the processes that occurred on Earth when it was young.
Io is an icy satellite of Jupiter 628,866,000 km from Earth, far enough from the sun that its surface temperature is 175 K (-143°C or -230°F) and is covered in sulfur dioxide frost. Io’s yellow tinged crust is not fractured, therefore, it is not thought to have tectonic activity. Despite these two factors, Io has the most volcanic activity in our solar system, spewing out over 100 times as much lava as all Earth’s volcanoes combined and may have as many as 300 active volcanoes.
The surface of Io is much different than previous expectations had dictated, and contains potential clues to the history of Earth. When the Voyager spacecraft missions took images of Io in1979, NASA was surprised to see that Io was not full of craters, as had previously been thought. It was assumed that Io would be cratered much like our moon. Yet Io hardly had any craters at all, instead it had irregular pits and blotches of color. When the images were carefully examined, volcanic plumes and lava flows were discovered. Infrared spectrometry also detected abundant sulfur and sulfur dioxide in the volcanic plumes.
The sulfur on Io’s cold crust is solid, though when heated inside the crust, it explodes much like steam in a geyser on Earth. The sulfur cools as it is ejected and may fall back down as “snow” on Io’s surface. The lava flows on Io can range in color from orange to red to black and are found around the active vents. The Galileo spacecraft monitored volcanic areas in the late 90s and found that the active lava flows of Io were between 1700 to 2000 K (around 1450 to 1750°C, or 2600 to 3150°F). Earth’s lava temperatures are around 1300 to 1450 K. The lava on Io is probably ultramafic, containing magnesium and iron that have higher melting points. Ultramafic lava is found on Earth, but was formed when the Earth was young and the interior was much hotter than today.
The volcanoes on Io are mostly caldera-like, containing large pools of lava, though some are fissures or cracks where the molten material can flow over the surface. Loki Patera is a caldera with a diameter of 200 km, which makes it the largest in the solar system. Some of the volcanoes form fountains, umbrella-shaped flows that spread up and out over large distances. The Prometheus plume is a volcanic region that has been seen in almost every image taken of Io from 1979 to 1997, suggesting that it has been continually erupting for years.
Aside from the similarities, we can also learn from the stark differences between Earth and Io. While the heat in Earth’s core is mainly due to the radioactive decay of uranium, thorium and potassium, Io’s extreme internal temperature is caused by gravity. Io is the closest natural satellite to Jupiter and is one of Jupiter’s four largest moons. Due to the close proximity to Jupiter and the slightly elliptical orbit of Io, the gravitational pull on Io ebbs and flows, creating contraction and expansion on Io’s crust. The next two moons closest to Io, Europa and Ganymede, also interact gravitationally with Io and increase the forces on Io as they routinely pass by. The speed of the moon’s orbits are not the same; during the same period of time Ganymede orbits once, Europa orbits twice and Io orbits four times around Jupiter. The differences in the orbits cause the moons to line up often. This increases the gravitational pull on Io from both Europa and Ganymede as well as from Jupiter. This continual pull creates tidal heating causing temperatures that melt the rock within Io and fuels the intense volcanic activity. The process of squeezing and flexing is similar to how a ball of clay will soften and warm as a person kneads it. However, the heating of Io is unlike what we have ever experienced on Earth. The tidal heating on Io adds as much energy as 24 tons of TNT exploding every second. Io’s surface receives 2.5 watts of power to each square meter, compared to 0.06 watts per square meter on the Earth’s crust from global heating. The only areas on Earth that are comparable to Io’s average are in Earth’s volcanic areas.
GUEST POSTS THIS WEEK July 20, 2012Posted by jcconwell in Astronomy.
Tags: Eastern Illinois University, EIU
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Over the next few days we will have a special treat. Some guest bloggers from my summer astronomy class for science teachers will be commenting on some cosmic and terrestrial topics that caught their interests this summer.
OBSERVATORY OPEN HOUSE 9:00PM TONIGHT June 29, 2012Posted by jcconwell in Observatory.
Tags: Eastern Illinois University, EIU, Observatory
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Come to the Saturn viewing tonight. Last month was a little cloudy, but tonight looks clear. Viewing this close to the summer solstice, not only is it HOT (103 today), but the sun sets the latest of the year. So we will begins at 9:00PM TONIGHT. Parking is at the campus lot near the Methodist church. Because of construction on 4th street you may have to approach from the South.
TRANSIT OF VENUS TODAY June 5, 2012Posted by jcconwell in Astronomy, planets, Solar and Space weather, Sun.
Tags: Eastern Illinois University, Solar System, Transit of Venus
Come to the Charleston public library to view the transit of Venus. From 4:30pm to 6:00PM TODAY!!
OPEN HOUSE TONIGHT! May 25, 2012Posted by jcconwell in Observatory.
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Beginning at dusk, about 8:45PM local time, rain or shine. If it’s clear, we will be viewing Saturn through the 16″ telescope. We will continue until about 10:30 or people stop showing up.
Your guide to the Lyrid meteor shower April 21, 2012Posted by jcconwell in Astronomy, meteor.
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Tonight and tomorrow night, look up at the sky for a spectacular light show.
The Lyrid Meteor Showers happen annually, but this year’s “moonless” night and lack of cloud cover for the western two-thirds of the United States will make for better views.
The moon is in its new phase – meaning the side facing Earth isn’t lit up by the sun, NASA’s meteor shower expert Bill Cooke told Space.com. Last year, the moonlight made it harder to see the Lyrid show.
“The Lyrids are really unpredictable,” Cooke told Space.com. “I’m expecting 15 to 20 Lyrid meteors an hour. Back in 1982, they outburst to nearly 100 per hour. You really can’t predict with this.”
Space.com reports that the Lyrid shower – which takes place as the Earth passes through dust from comet Thatcher – has been watched by humans for more than 2,600 years.
The meteor shower’s name comes from the constellation Lyra.
The best times to watch are after midnight and just before dawn. Look to the northeast and pick a viewpoint well away from city lights. The darker the sky, the brighter the meteors will appear.
NASA recommends watching with the naked eye instead of through a telescope or binoculars.
Bob Holmes and ARI on tonight on “Heartland Highways” April 20, 2012Posted by jcconwell in Astronomy.
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If you live near us in Charleston Illinois, then might want to catch WEIU-TV tonight. At 7:00PM, an episode on the program “Heartland Highway”, will feature Bob Holmes and his work at the Astronomical Research Institute. Tucked away in the corner of Coles county Illinois, about 15 miles due east of the EIU campus, ARI is building the largest privately owned telescope in the world.
FUNDRAISER AT YERKES OBSERVATORY February 19, 2012Posted by jcconwell in Astronomy, Observatory.
Tags: ARI, Astronomy, Eastern Illinois University, EIU, Observatory, Yerkes Observatory
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You are invited…
Yerkes Observatory event Friday Feb. 24th – 7:00 PM
In 2012, Yerkes will be engaged in a series of fund-raising events to support the restoration and upgrades of Yerkes telescopes and support funding for Yerkes Education Outreach programs. On Friday evening February 24th, Yerkes will host the first of these events.
Supporting SKYNET and Yerkes telescopes
Funds from this first event will be used specifically to upgrade the mirror coating and operation of the Yerkes 41″ reflector, and to support the redesign of the optics of the reclaimed Hands-On Universe 30” telescope by Robert Holmes of the Astronomical Research Institute. Both of these telescopes are operable through SKYNET (http://skynet.unc.edu/), a world-wide network of telescopes, used by scientists, and teachers and students associated with our Yerkes Education Programs and our Collaborators, including Hands-On Universe (HOU) and International Asteroid Search Campaign (IASC).
Limited participation, register now!
Participation will be limited to 100 guests; cost $50 per person. There will be several scientists, engineers, educators and students attending to mingle with the guests to discuss SKYNET, our participation in SKYNET and the plans we have to restore Yerkes telescopes. If weather permits, guests will also be invited to do some stargazing through the Yerkes great refractor. Wear warm clothes (domes are not heated) and shoes appropriate for climbing narrow stairs; flashlights are suggested as well.
It is our hope to find benefactors among the guests who will be interested in a contribution beyond the initial $50.
Name___________________________________________ Address___________________________________________ City______________________________ State _____________ Zip__________ YES, _____________ Person(s) will attend @ $50 per person
Check enclosed for $_________________
Checks payable to: University of Chicago, Yerkes Observatory
Send checks to Yerkes Observatory, 373 W. Geneva Street, Williams Bay, WI 53191 Additional information, phone: 262-245-5555, fax: 262-245-9805
You may also register online at http://astro.uchicago.edu/yerkes/yo_feb24/index.html