Thursday, December 14, 2017

What to expect when you’re expecting a total solar eclipse

From the University of Illinois

The solar eclipse on August 21, 2017, will truly be an American Eclipse because everyone in North America will see at least a partial eclipse. In the continental U.S. the lowest partial eclipse will be 48% in the northern tip of Maine. Although the entire U.S. will have at least a partial eclipse that day, the real action is going to be the little swath where totality will occur. Whereas a partial eclipse is interesting to see, totality is another thing entirely.

At first a solar eclipse appears slow, as just a small part of the sun is blocked by the moon. You won’t be able to tell anything is happening unless you look at the sun. You won’t need a telescope, but you will need protective eyewear. Solar viewing glasses are available at The Vienna Times office and other locations throughout the region. If you look at the sun with the viewing glasses, you begin to see a small portion of the sun being obscured. In Johnson County, the process will start at 11:53 a.m. on Monday, August 21.

As you watch the sun, it becomes more and more obscured by the moon, and you will begin to notice differences in your surroundings. When the sun is a little more than half covered, the color of the sky will slowly become a duller shade of blue. Shadows on the ground will appear sharper as the apparent size of the sun shrinks. If you look in the shadows of trees, the gaps in the shadows will begin to look weird. It may take a while for you to realize it, but there will be many crescent shapes in the shadows. The tiny gaps in the leaves will act like multiple pinhole cameras, projecting the sun’s image to the ground. When an eclipse is not happening, they are there too, as circles, but these don’t catch our attention since they blend together.

Very slowly, almost imperceptibly, as we move toward totality, the western sky will become a little darker than the eastern sky (the shadow is approaching!), and slowly the colors of the landscape become more muted and gray-ish. The sun becomes a smaller and smaller crescent, but is still very bright. (You will need eye protection up until totality.) When there is about 5 minutes until totality, the western sky becomes darker and darker, beginning to shroud the sun in darkness.

With only a few minutes until totality (around 1:20 p.m. in Johnson County), shadow bands may be seen right before (and after) totality. These are rapidly shimmering bands of shadow on the ground or any flat surface (lighter colored surfaces show this best). The bands are formed when the final bits of sunlight from the last wedge of the remaining crescent of the sun are distorted by the earth’s atmosphere. You can see similar effects by shining a flashlight over a lit candle; the eddies of motion around the candle cause variations in the atmosphere which affect the light projected from the flashlight. Shadow bands are not always seen during an eclipse (it depends on the atmosphere).

As totality is almost reached, the last sliver of sun is slipping away in silence, darkness is almost surrounding you. One can see bright spots around the edge of the moon, where the valleys and mountains on the moon allow sunlight through in some places but not in others. This effect is often called “Baily’s Beads” after the first person to explain the effect, Francis Baily. Right before totality, the last bead is left with a final glimpse of the sun peering out, making a beautiful “diamond ring” affect. Encompassing the diamond is the faint glow that surrounds the nearly perfectly darkened sun. This glow is the corona (Latin for “crown”), which is an aura of plasma that always surrounds the sun but cannot be seen over the brightness of the undarkened sun.

Then, quietly, calmly, and almost instantaneously the sun is swallowed by the moon. Boom, it is totality! It hits you suddenly, even though you were expecting it. A marvel of nature: a perfectly dark disk outlined with the outstanding corona (which is best viewed by the naked eye – it’s safe now and as bright as a full moon) shines in your eyes. An eerie quiet covers the landscape, birds and other animals seem confused and quiet down to rest. Light breezes stop. The temperature drops by 10 to 15 degrees Fahrenheit. A closer look at the sun may reveal little wisps of red, which are solar prominences, giant clouds of hot gas and plasma. This is it. What we have been preparing for. A special moment of calm and of beauty. You stand in the shadow of the moon!

Look around, it is not pitch dark. But it is an uniquely odd darkness. At the horizon, you can still see areas around you where the eclipse is only partial. It should be dark enough to see nearly all of the planets. To the west is Mars and farther out is Venus. To the east is Mercury and Jupiter. Closer to the aun, about two solar diameters away to the east, is a bright star called Regulus, and maybe you will see Sirius, the brightest star in the sky, in the southwest sky.

Live in this moment. You will try to look everywhere at once. But all too soon you feel the shadow rushing away. The western sky is beginning to brighten. You were in the moment, but the moment is passing. The shadow silently moves away from you into the east, providing the experience to others – your time is up (in Johnson County this is 1:23 p.m.). The diamond ring returns, but this time it is somehow not as beautiful – pale compared to the experience of just seconds ago. You will breathlessly ask, “When can I do this again?”

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