Sky's Up July-September 2017 - Page 20

2 3 4 5 Why do we need to wear special glasses during a solar eclipse? I have looked at the Sun before and my eyes are fine. A quick glance at the Sun is dazzlingly bright and leaves an afterimage that can take minutes to disappear. A longer look at the Sun risks damaging the jelly-like liquid in your eye and burning your retina, the very delicate tissue that gives you sight. The tiny image generated by your cornea + lens focuses on your retina, just where your sharpest eyesight is (called the fovea). Your retina can be damaged, just the way a magnifying glass focusing sunlight on paper or wood will burn the material. If you burn your retina anywhere, the damaged cells will not be replaced and you will have a blind spot. If you burn your fovea, reading and similar detail-oriented sight will be impossible. It can even take a day before the blind spot becomes noticeable. The “problem” with eclipses is that people want to see what’s going on and may not know the damage they can do to their eyes if they stare at the Sun. Professionally made eclipse glasses cost $2.00 or less each. They are safe to use because they only deliver about 1 part in a few hundred thousand of the Sun’s light. DANGER!! Stacking sunglasses, crossing polarizers (sunglasses or photographic polarizers), or even using photographic neutral density filters will NOT protect your eyes. It is not just the visual wavelengths (from violet to red) that can cause damage. Solar ultraviolet and infrared rays also damage eyes and can get through some filters that appear to be safe and dark in visual wavelengths. Get safe solar filters to enjoy the partial eclipse and protect your eyesight. 6 If the Sun is blocked, how will looking at it hurt your eyes? Yes! On the ground, the sky is so bright near the Sun that even coronagraphs, instruments designed to study the Sun’s outer atmosphere, can only give us limited measurements. In space, coronagraphs flying on spacecraft like the Solar Heliospheric Observatory (SOHO) are built to show the outer corona. They use an occulting disk (like an artificial Moon) that is significantly oversized so they can study the outer (upper) corona and events in it in great detail. But the part of the corona that is most easily studied during an eclipse is also completely blocked by the occulting disks. The picture below is a composite made from three different telescopes on SOHO. At the center is an image of the Sun in extreme ultraviolet. It is surrounded by the black occulting disk blocking the inner corona while revealing the middle corona, colored red. The limit of the red image is the size of an occulting disk in a different coronagraph that records the outer corona, visible in the blue image. (Note that all the colors presented are artificial. Our eyes don’t see in the ultraviolet. The color of the corona we see during an eclipse is white.) NASA is sponsoring Citizen CATE, placing 60 similar telescopes across the U.S. to record the inner corona over the 90 minutes it will be visible from somewhere in the U.S. The images will be combined into a movie so scientists, for the first time, can look at detailed motions and study any waves in the corona over a long time span. There are also weather observations that can be made to see how the cooling along the path of totality and outside the path affect weather. A free smart phone app called “GLOBE Observer” found at guides the user through making observations of clouds (or not) and relaying the data to a central archive for use by scientists. Join the fun! This composite image from the Solar Heliospheric Observatory (SOHO) shows a direct, extreme ultraviolet image of the Sun in the center, the occulting disk and narrow field coronagraph’s field of view (black and red, respectively), and the field of view outside the larger occulting disk (center to red edge) of the wider-field coronagraph. The inner corona is not observed. This is one frame of a video showing a coronal mass ejection, easily identified on the right side. When the Moon completely covers the Sun, called totality, the Sun will not hurt your eyes. That is the time to put your safe solar filter glasses in your pocket and admire the spectacular corona and the many other effects associated with a total eclipse. When the Moon is not completely covering the Sun, during the partial phases before and after totality, SAFE FILTERS MUST BE USED. If you don’t, you can burn little crescents into your retina, creating blind spots or worse. Will the eclipse happen at the same time everywhere? No. The eclipse starts at dawn in the Pacific Ocean and ends at sunset in the Atlantic Ocean. When the center of the Moon’s shadow reaches the coast of Oregon on eclipse-morning it will spend the next 90 minutes or so crossing the United States before exiting the Atlantic coast of South Carolina. Because of the t