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Tips and Quips on the Total I trust you know enough not to stare directly at the mid-day sun. The sun can be blindingly bright. Just accidently catching an eyeful is uncomfortable - you’ll see afterimages for several minutes and may get an instant headache. Yet you’ve certainly noticed there are times during the day when it’s safe to look at the sun. You stare directly at it with the naked eye when watching a beautiful sunset. Next time you do, remind yourself that you are focused on an object 93 million miles away. At that distance it appears deceptively small. You can blot it out with your thumb held out at arm’s length. Heck, you can do it with your pinky; the apparent size of the sun is about half the width of that pinky nail out at arm’s length. Normally you raise your whole hand to shield yourself from the sun’s harsh glare, by placing your eyes directly in your hand’s shadow. That doesn’t plunge you into utter darkness. There’s still plenty of scattered light from the blue sky above and the objects around you (including your own body) that still allow you to see detail on the back of your hand. Standing right in the shadow of a wall or building there’s still plenty of scattered light to see by. Duck into an alleyway. A small amount of scattered sunlight from a limited area of sky can still be enough to light your way. We notice when clouds pass overhead. The temperature suddenly drops a bit and daylight dims slighty. That’s hardly like nightfall though. What if, however, the sun were blocked by an object over 2,000 miles wide? Your neighborhood, the entire city, and a good part of the county would be plunged into darkness! That’s what will happen in the middle of the day in Lincoln, Nebrsaska, on Monday August 21st, when our 2,159-mile wide get’s in the sun’s way. Just two minutes past 1pm that day, we will pass beneath the moon’s shadow for a full minute and 24 seconds. When we do, night will suddenly appear, and the stars will come out! In fact you will be able to see Jupiter, Mercury, Mars, and Venus ( out in our daytime skies this summer where they ordinarily can't be seen) lying along a line across our sky that passes right through the position of the eclipsed sun! This is no ordinary, everyday occurance. It’s a once-in-a-lifetime event! Where will YOU be on August 21? Lincoln, NE will be one of the best spots on earth to view one of nature’s grandest spectacles!

Dan Claes, Professor and Chair Department of Physics and University of Nebraska

Tips and Quips on the Total Eclipse We tend to think of the air as completely transparent. That’s not exactly true. Were the atmosphere truly transparent, what would you expect to see when you stepped outdoors on a cloudless afternoon? Of course you’d see the sun overhead (but it would be even more dangerous to look directly at). It would not appear against a clear blue sky, but engulfed by a night-dark sky full of stars. We don’t see stars in the daytime not because they aren’t there, but because the atmosphere scatters sunlight (preferentially the bluer shades) out of it’s straightline path to earth. That scattered light scatters again and again bringing dazzling sky-blue color from all directions we look overhead. That’s why being in the shadow of a large object (a wall, a building, a plane passing overhead) doesn’t exactly place you in the dark. Scattered light still comes from all directions of the sky. An obstacle the size of the moon (2,159 miles across), however, passing overhead would leave little room for sunlight to sneak past to illuminate our skies. Sunlight would be blocked from nearby objects, unable to be scattered toward us. This is what will happen moments past 1pm on Monday August 21st. What will we see then? The moon is far enough away that even it’s 2,159 mile diameter does not blot out the entire sky. Just a spot of it. Enough to blot out the sun. The moon will cast a shadow on the ground nearly 70 miles across. Like a passing cloud’s shadow, the moon’s shadow will move (across our state at about 1575 miles/hour). But unlike a passing cloud, this shadow is huge enough to plunge us into night-like skies. The stars WILL come out! The planets Jupiter, Mercury, Mars, and Venus happen to be in our daytime skies this summer and you will be able to see them lined up across the sky. Don’t expect midnight darkness. The entire hemisphere isn’t shadowed by the moon (remember the shadow is about 70 miles across). Tens of miles off toward the horizon in any direction, sunlight will be reaching the ground and illuminating the sky. Along our horizons (for the full 360 degrees) we will see a dusklike glow. Hope for good weather and cloudless skies! Should the weather be uncooperative, you will still be able to tell when totality occurs. It will be the difference between an overcast aftrernoon, and (for a minute and a half) nightfall.

Dan Claes, Professor and Chair Department of Physics and Astronomy University of Nebraska

Tips and Quips on the Total Eclipse Totality (when the moon blocks the face of the sun completely and we experience nighttime darkness in the middle of the afternoon) will last 1 minute and 24 seconds here in Lincoln, Nebraska. But the moon takes nearly 3 hours to pass the sun in their race across our sky. At 11:37 a.m. the moon’s leading edge will first touch the edge of the sun. Over the next hour and 25 minutes the dark disk of the will crawl across the face of the sun, blocking more and more of it’s light, creating a gradually thinning crescent sun.

Do NOT look directly at the sun unless you have a special pair of “eclipse viewing” glasses. Regular sunglasses will not protect your eyes. The cardboard “eclipse” glasses have lenses of optical Mylar that filter out most of the sun’s light (particularly it’s dangerously damaging infrared and ultraviolet radiation) and allow you to look at the sun at any time (just make sure the lenses have not been scratched or damaged first). You can pick these up at several Lincoln locations and special events over the next couepl weeks or order them online. Using them you will be able to watch the steady progess of the partial leading up to totality. You can also safely look down at the image of the sun projected by a simple pin-hole camera. Better yet, you’ll be able to view live-streamed images the University of Nebraska will be sharing from video cameras mounted on two of the Department of Physics and Astronomy’s telescopes: the Student Observatory atop the Memorial stadium garage (a 16-inch Cassegrain Reflector, ordinarily used for night-time viewing, that will be covered with an H-alpha filter to permit viewing of the sun) and the Minnich Telescope (a dedicated solar telescope used by students to study sunspots and solar prominences, mounted on the Jorgensen Hall south entrance stairwell tower). We’ll post the url for that feed soon, and you should be able to pick it up on your computer or smart electronic devices on the 21st. 2:29 p.m. the show is over. Enjoy it while it lasts! Dan Claes, Professor and Chair Department of Physics and Astronomy University of Nebraska

Tips and Quips on the Total Eclipse What makes total solar so rare and this August 21st so special? Imagine drawing a huge circle on the tabletop (or floor) before you to represent the earth’s orbit about the sun. Pick a spot on this orbit (through which the earth passes once a ) and draw a smaller circle about it. That’s the moon’s orbit about the earth. As the earth slides along its path about the sun, the moon keeps encircling it. In this simple model the moon swings around to block the sun once every . That would suggest solar eclipses happen routinely. Notice halfway between each solar eclipse, the moon would move into earth’s shadow and we’d see a just as often. Why do we not get solar eclipses every month? The orbits in our model were drawn on the same flat surface. This moon’s orbit lies in the same plane as the orbit of the sun. That’s not true of the real earth-moon-sun system. Like adjusting the rear-view mirror on a car, imagine tipping the moon’s orbit a bit (about 5.15o) out of the plane of the sun’s orbit (the “”). That’s enough that most of the time the moon is above or below the ecliptic. The new moon that comes around once a month usually does not block the sun, and instead throws it’s shadow into empty space, missing the earth entirely. There are now only a couple points in its orbit (where the moon’s orbit crosses the earth’s orbital plane) where the moon’s shadow can strike the earth. For a solar eclipse to occur, the moon has to be at a “lunar node”, one of the two places where the moon’s orbit crosses the Earth’s orbital plane. And it has to be at the “new moon” phase. Another complication: the earth’s equator is tipped with respect to the ecliptic (and tipped differently than the moon’s orbit - 23.45 degrees). When the moon’s shadow does reach the earth, it will be at a different location each time. Solar eclipses usually occur somewhere on earth a couple times a year, but given the earth’s surface is ¾ oceans they usually happen in hard to reach regions (including the poles!). For most of these eclipse the moon is only near one of the nodes, as we get the far more common “partial” eclipse. TOTAL SOLAR ECLIPSES are rare, and, when they occur, they are not viewable to everyone on earth. To be able to enjoy a total solar eclipse without needing to travel any further than your own yard? Priceless!

Dan Claes, Professor and Chair Department of Physics and Astronomy University of Nebraska

Tips and Quips on the Total Eclipse Whenever you walk beneath shade trees you can take pleasure in the mottled pattern of light at your feet. On your next midday stroll, take a closer look. You will not see perfectly outlined shapes of leaves, but more of a blurry pattern of patchy spots of light. Those patches remind me of spattered drops of white paint - faintest at points of individual sunlight drops, and brighter wherever the spots overlap or bunch together. You are looking at a vast collection of softly focused circles - a pattern of multiple projected images of the face of the sun itself. The layers of overlapping leaves overhead effectively produce hundreds, sometimes thousands, of pinhole cameras! Take a squint at https://petapixel.com/2012/05/21/crescent-shaped-projections-through-tree- leaves-during-the-solar-eclipse/ Find it hard to believe? Rather than waiting impatiently for the solar eclipse to reach totality, walk a tree-lined boulevard through the late morning of August 21st. You will be amazed to see thousands of perfect crescent shapes cast before you, projected images of the waning sun! You can have fun with this. A sheet of poster board with a small pinhole, held to your side parallel to the ground, will not only produce a shadow of you and the rectangular card you hold, but a bright vivid crescent where the hole should be. Cross the extended fingers of your two hands to make a lattice and see what you project. See: https://eclipse.aas.org/sites/eclipse.aas.org/files/Waffle-Fingers_RTF_512x323.gif

Or bring some of your kitchen tools with you: http://www.eclipsewise.com/solar/SEhelp/images/PSE2000-Projection.jpg

Dan Claes, Professor and Chair Department of Physics and Astronomy University of Nebraska

Tips and Quips on the Total Eclipse Although not visible (it's ordinarily completely washed out by the sun's intensity) the sun has an atmosphere of it's own, called the corona. You cannot see it with the naked eye (so don't even think about trying!). This highly active atmosphere is most often studied by space telescopes (like the Hubble) with special filters selectively imaging the sun in specific wavelengths of light.

Credit: ESA/NASA Hubble Telescope

With the sun conveniently blocked during a total solar eclipse and backlighting the moon, its glowing atmosphere can be observed around the edges of the dark side of the moon. This "ring of fire" will be visible to us in Lincoln on August 21st moments past 1:02 p.m.

Photo courtesy of NASA

Studying the spectrum of light in the corona, scientists can measure its temperature, which will help us understand how the plasma in that atmosphere is heated (it's a bit of a mystery why the atmosphere reaches millions of degrees while the sun's surface beneath it is a measly 5,500° Celsius (about 10,000° Fahrenheit). They may also be able to measure velocities of ions in the gas, and learn more about the sun's magnetic field. Most of us will just oooh and aaahhh! This is the part I am really looking forward to! Note: You may have heard that during this short period of totality its safe to observe the ring of fire without protective eyewear. From NASA's https://eclipse.gsfc.nasa.gov/SEhelp/safety2.html "The only time that the Sun can be viewed safely with the naked eye is during a total eclipse, when the Moon completely covers the disk of the Sun. It is never safe to look at a partial or annular eclipse, or the partial phases of a total solar eclipse, without the proper equipment and techniques." So, yes, when the eclipse is TOTAL you actually can look at the ring of fire that will be visible for that minute plus with the naked eye. Then you are only seeing the glow of the sun's atmosphere...not its' glowing surface. But I wouldn't tell kids that! The danger is staring too long (or too soon or too late).

Dan Claes, Professor and Chair Department of Physics and Astronomy University of Nebraska