SPACE

Rare total eclipse of the sun with easy US viewing coming Aug 21, 2017

This will be the 1st time in 40 years that a total solar eclipse will be visible from the continental United States.


Details on the forthcoming Aug 21 eclipse

The Diamond Ring appears just before the beginning of totality, when a single bright point of sunlight (the diamond) shines through a deep valley on the Moon's limb (edge) and the inner corona (the ring) becomes visible. As the diamond vanishes, it's safe to remove your solar filters. A 2nd diamond appears at the end of totality when a deep valley on the opposite side of the retreating Moon exposes a single bead of bright sunlight that begins to wash out the corona. Put your filters back on!

The path of totality (where a total eclipse can be observed) for the Aug 21 eclipse passes from Lincoln Beach OR across the US to Charleston SC. This will be the 1st eclipse to cross both the US Pacific Coast and Atlantic Coast since 1918. The path of totality is exclusively within the United States, making it the 1st such eclipse since the founding of the US. The last equivalent was in 1257.

Totality will begin over land at 1716 UT on the coast of Oregon. It will move off of the continent a little over an hour and a half later at the coast of South Carolina. The path of totality is approximately 100 km wide.

Right before totality begins, the only parts of the Sun's bright face not yet covered by the Moon are those that peek through deep valleys on the advancing lunar limb. Seen through safe solar filters, they look like an arc of bright points. These are called Baily's Beads, after English astronomer Francis Baily, who first described them during a solar eclipse in 1836. They disappear one after another as the Moon continues its advance. Their subsequent reappearance, as the Moon begins its retreat, signals the end of totality. Also visible at the beginning and end of totality are brilliant red arcs, loops and jets of hot gas. Called prominences, they originate in the chromosphere (Sun's atmosphere thin middle layer) and shine in the red light of hydrogen heated to more than 20,000° C (36,000° F).

Anyone within this corridor will see a total eclipse – weather permitting. But those anywhere in North America as well as the northern part of South America or Western Europe can witness a partial solar eclipse. Timing of the eclipse, whether partial or total, will depend on your location. For those along the path of totality, the total eclipse will last approximately 2 minutes, with the maximum duration being around 2 minutes and 40 seconds in southern Illinois. The partial eclipse, on the other hand, will run from just about anywhere in North America, and it will last for 2 to 3 hours.

Solar eclipse safety

Solar eclipses are spectacular events, but one must follow appropriate procedures to observe them safely. Looking directly at the Sun without proper filters can cause permanent eye damage. The exception is during the brief period of complete totality during the 2 minutes or so in which the Moon is completely blocking the Sun, except for its corona. But during all other times, one needs to use appropriate safety glasses or filters. Sunglasses are definitely not sufficient.

One can easily order eclipse glasses online prior to the event. They range from simple cardboard frames to sturdier versions. Welder's glass may be used but it must be #14 or darker, otherwise it is not safe. It is very dangerous to look at the Sun through an unfiltered telescope, binoculars, camera or other optical device. And, once again, the filters used with optical devices must be the proper filters designed for direct viewing of the Sun.

One can also view the partial eclipse by projecting images of the Sun using simple pinhole or equivalent devices. You can put a small hole in a piece of cardboard and project an image of the Sun onto a sidewalk or a piece paper. Or try the fun example of putting one hand with fingers partially spread over the other hand with fingers partially spread forming a waffle pattern. Now put your back to the Sun and look at the shadow of your hands on the ground. The numerous holes between your fingers will form a series of images of the partial eclipse showing bright circles with dark "bites" taken out of them.

Here's what you will see

Once you are safely observing, you will gradually see the Moon cross in front of the Sun during the partial eclipse phase. It will look like larger and larger bites being taken out of the Sun. Then the Moon will begin to move off of the disk of the Sun. The farther you are from the path of totality, the smaller the amount of the Sun that will be blocked out at the maximum point. As the Moon slowly moves across the Sun over 2 to 3 hours, don't forget to take a little time to ponder this rare celestial event: You're seeing the effect of the Moon 400,000 km away passing in front of the gigantic Sun which is 150 million km away.

If you are along the path of totality, the show will be even more spectacular for the 2 minutes or so that the Moon completely covers the main disk of the Sun. Just before the Moon completely covers the Sun, there is a moment referred to as The Diamond Ring, which is when the last rays of the Sun illuminate just 1 part of the circle. And at the beginning and end of the total eclipse, one also may see a sight known as Baily's Beads, where sunlight streams through topographic gaps in the edge of the Moon such as canyons.

But what about totality itself? It sounds like the trailer to a horror movie, but much more impressive and considerably less dangerous: Day will turn into night, birds may stop singing, sunset will appear all around you, stars and planets will become visible in the darkened sky, and the otherwise hidden solar corona will become visible as white rays streaming out from around the dark lunar disk. A total solar eclipse is truly one of nature's most amazing sights. As you get swept away, don't forget that once the sunlight starts appearing from around the Moon, you need to put solar glasses back on.

Let me emphasize that, in terms of viewing, there is a huge difference between being where the Moon blocks 100% of the Sun, the path of totality, or being where the Moon blocks 99% of the Sun. That little bit of sunlight will wipe out the more profound effects that would be seen in the path of totality.

Other opportunities

Eclipses can be predicted very far in advance very accurately, but weather cannot. The specter of disappointment wields the weapon of clouds. Just a forewarning that if you do everything else right, weather can still block out an eclipse. So what are your opportunities if weather defeats you this time? Or if you just love it so much that you want to watch as many eclipses as you can? For every total solar eclipse, there are tours that will take you to the locations you need to go even if those are viewing from cruise ships in the middle of an ocean. There are people that love the experience so much that they will hunt down every possible solar eclipse.

The next total solar eclipse is Jul 2, 2019 and it will be visible from portions of Argentina and Chile. Those of us in North America also don't need to wait too long for another total solar eclipse, at least as these things go because on Apr 8, 2024, a total solar eclipse will cross the central United States as well as portions of Mexico and Canada. Also, keep in mind that there will also be numerous online streaming events that you can follow live during the solar eclipse.

Conclusion

Solar eclipses, particularly total solar eclipses, are spectacular displays put on by the cosmos. It may be challenging to get reservations along the path of totality at this point, but it's well worth the effort. Depending on their situation, pilots may also have the option of flying into an airport along the path of totality and just watching from the airport itself. Whether you are able to watch the partial eclipse, the total eclipse, or have to watch online, I hope you enjoy this cosmic spectacle.

Bruce Betts, PhD, is a planetary scientist with degrees from Stanford and Caltech. He is Director of Science and Technology at The Planetary Society and has done research focused on infrared studies of planetary surfaces. He also managed planetary instrument development programs at NASA Headquarters.


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