A Guide to the Total Solar Eclipse of August 21, 2017: Part 2

July 2017  :  Dave Huestis

Part II: Within the Path of Totality

You’ve decided on an expedition to position yourself within the path of totality. Please read Part I of my eclipse primer (http://www.theskyscrapers.org/2017-solar-eclipse-guide) you haven’t already done so. It will provide some important background information about solar eclipses in general and how to safely view the partial phases.

In choosing your observing location you should reference this interactive eclipse map: https://eclipse2017.nasa.gov/sites/default/files/interactive_map/index.html?zoom=1. See further details in Part I. The closer to the centerline as possible will provide the maximum duration of totality for that area.

As the eclipse begins (called 1^st contact) the Moon will begin to cover the solar disk from right to left. As for any partial phase, you will require eye protection until just before totality. As more and more of the Sun is eclipsed look under nearby trees to see images of the partial phase. These images are created in much the same way the shoebox eclipse viewer works. Spaces between the leaves act as a pinhole camera, resulting in a myriad of eclipse images projected upon the ground or onto any surface.

The Moon continues to cover the Sun. As totality nears, everything seems to progress much faster. This is merely an illusion. Look for the approaching lunar shadow out of the west-northwest. It will soon be upon you. About two minutes before the Sun is completely obscured, look around to see if you can spot shadow bands. The shadow band phenomena are the result of cells of atmospheric turbulence above the observer in the tropopause that are accentuated due to the decreasing solar crescent as totality draws near. They are best observed in contrast against white or bland backgrounds, like sandy ground or the sides of buildings. They also move at varying speeds and in many directions, dependant on the upper air currents. Check out this video: https://youtu.be/2tiI6VWXqj4. They have been described as A) undulating B) wavy C) shimmering D) fluctuating E) scintillating F)  indistinct G) almost imperceptible shadings H) fluctuating I) diffuse smoky bands J) all of the above. They are fairly elusive to capture with a camera due to their low contrast and irregular motion.

While waiting for 2nd contact (totality begins) in Tanzania in 1980, shadow bands became quite pronounced. Unlike many of the descriptions you have just read, these bands couldn't be missed. It seemed like we were immersed in them, not just on the ground but surrounding us as well. All of a sudden they were there, 85 seconds before 2nd contact, moving in the opposite direction as the moon's shadow cone. They were two and one half to three inches wide, approximately 14 - 18 inches apart, and very fast. It was like watching an old-time movie where you can see the frames flashing by. During totality they disappeared. After 3rd contact (totality ends) they reappeared and had the same characteristics as those before 2nd contact. They also moved in the opposite direction as the shadow cone. I was quite impressed with this phenomenon, especially since they were so prominent. My description: rapidly moving striations of light and dark shadows.

Don’t get too absorbed by the shadow bands, as they occur just up to the beginning of the grand event – totality (2^nd contact). Seconds before the Moon completely covers the Sun a phenomenon known as Bailey’s Beads occurs. Along the Moon’s limb (edge) sunlight is rapidly disappearing behind the lunar disk. However, since the Moon’s limb is irregular, while some light is being block by lunar mountain ranges, valleys still allow some light to reach us. We see these glimpses of sunlight as beads. Then just a second or two before totality we experience the Diamond Ring Effect, the last of the Bailey’s Beads.

At Diamond Ring time you will most likely begin to see the Sun’s pearly white and very hot atmosphere called the corona, as well as the red glow of hydrogen in the Sun’s chromospheres. Solar prominences may also extend beyond the disk of the Moon.  Catching prominences will depend on whether an active solar region is close to the solar limb. Solar activity is currently on the downside toward solar minimum, projected for 2019-2020. Therefore, prominences may not be prominent.

Once totality begins you will see the corona completely encircle the Moon. With solar activity so low, an observer should see white coronal streamers outline the Sun’s magnetic fields. One should note the Sun’s north and south magnetic poles, as the magnetic field lines, and therefore the corona, will curve up and away from the poles, like iron filings oriented to a magnet. The corona is quite bright, about the same brightness as a Full Moon.

If you don’t already have enough activity to absorb, take some time to look at your surrounding horizon. You should see the rosy red glow of sunset colors all around you. Also, look immediately around your observing site. It may not be as dark as you expected. Depending upon how extensive and bright the corona is will determine that. However, your surroundings should be about as bright as if a full moon was present.

But wait!! There’s still more to see. The sky will be dark enough to see a couple of planets and a bright star or two with your naked-eye. Brilliant Venus will be west of the Sun, while bright Jupiter will be towards the east. The bright star Sirius will be low in the southwestern sky. Depending upon local sky conditions you might be able to detect Mercury to the southeast of the Sun and Mars to the northwest. See this sky map for the details: https://dyer.vanderbilt.edu/wp-content/uploads/sites/63/Eclipse-Sky.png.

Duration of totality will depend where you are along the eclipse path. Once the Moon begins to uncover the Sun (3rd contact) you will see the Diamond Ring Effect again, followed by Bailey’s Beads. It will be time to protect your eyes once again as more and more of the solar disk is uncovered. Then it’s time to catch your breath, because it may be possible you didn’t breathe during totality! Regardless of the duration of totality from your location, it will be the fastest minutes of your life. Some astronomers view and image the eclipse right up until the Moon “moves off” the solar disk (4^th contact) and the Sun returns to normal. I do have to say though, after totality anything else is extremely anticlimactic!! 

I wish everyone the best of luck with experiencing this fantastic event. Let’s hope Mother Nature cooperates and provides everyone clear skies on eclipse day for the Great American Total Solar Eclipse of August 21, 2017.