Moderator: Mary Anderson Mary  Anderson

2017 Total Solar Eclipse

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On August 21st, 2017, North America will witness one of the greatest eclipses in history. The path of totality will sweep across the US from coast to coast. Every state, including Hawaii and Alaska, will experience at least a partial eclipse. Those lucky enough to be in the 70 mile wide path of totality will experience up to 2 minutes and 40 seconds of darkness while the moon blocks the light of the sun.

Seattle, WA sits near the path of totality and will experience a 92% partial eclipse.  As one of the largest astronomy clubs in the region, the Seattle Astronomical Society is bursting with energy to share the excitement of this phenomenon. This page was created as a resource for safe viewing.

Artwork courtesy of Dr. Tyler Nordgren. This and many other posters may be purchased at www.tyler.nordgren.com

What is a Solar Eclipse?

When the moon passes between the earth and the sun, at just the right angle, a shadow is created on the surface of the earth.  As this happens, the sun becomes totally or partially blocked from view of those on earth.  This is only possible because the sun sits about 400 times farther away from earth than the moon's distance from earth, but the sun is about 400 times larger than the moon.  This creates the optical illusion that the sun and moon are about the same size, or Angular Diameter, in the sky.  

We would have an eclipse monthly, but the moon needs to be within 0.5° of the plane of earth’s orbit around the sun.  The moon’s orbit can move up and down a full 5°, compared to the earth’s orbit around the sun. Since the distance between the Earth, Sun, and Moon also varies slightly, not every eclipse is a total eclipse (even for those observing from along the central path of the eclipse). When the moon is too far from earth to completely cover the Sun, an Annular Eclipse occurs, leaving a red/orange ring of light around the moon. But when the Sun and moon are just the right distance apart, and cross each other at just the right angle, the people of earth, standing in just the right spot, are treated to an incredibly rare and amazing sight: a Total Solar Eclipse.

 

 

What will the Solar Eclipse Look Like?

Those along the (approximately 70-mile-wide) central line of the eclipse, called the Path of Totality, are within the darkest and most complete part of the shadow, the Umbra, and will see a Total Eclipse. Those within around 3000miles on either side of the eclipses path of totality are in the lighter shadow, the Penumbra, and only see a partial eclipse.

The penumbra and umbra of the eclipse move across the earth’s surface at over 1700km per hour, or at Mach 1! Thus, at any given location, totality can be as short as a minute and can’t last longer than about seven minutes.

The moment the moon first nicks the circle of the sun’s intensely bright surface begins an hour of lead up. A steadily growing bite is slowly taken out of the sun, creating a crescent of sunlight and slowly dropping the temperature. This can be observed with solar filters or projection.

For those in the umbra, an amazing experience awaits. About 10-15 minutes before totality, shadows begin to sharpen, daylight dims, the temperature drops more swiftly, and the flora and fauna begin to react.

In the last couple of minutes before the eclipse, you can look to the west and watch as it darkens, while the east illuminates as the shadow passes. On the ground, shadow bands appear like the dancing ripples of light under the water.

At the moment the suns last rays race across the lunar surface, they find valleys along the edge that allow a burst of light through, creating an effect called Baily’s Beads. The final and most intense of Baily’s Beads is created, as the last rays mix with the corona, creating the Diamond Ring Effect.

Once totally covered by the moon, the sun is safe to look at with the naked eye for a precious few moments. As you take in the incredible sight, and the feeling of standing in the shadow of the moon, you will see the eclipse creating a sunset color to in every direction.

The temperature can drop up to 20°F, though it is generally ½ - ¾ of the difference between temperature at that location during the day and the night. The darkness, coupled with this temperature change, causes the local flora and fauna alike to pull their petals in or roost for the ‘night’. Sights otherwise hidden by the incredible luminosity of the sun, the planets, stars, the usually invisible Corona (the suns atmosphere), and possibly the chromosphere, prominences or flares come into view. After a few minutes (anywhere from one to two and half minutes in 2017) the diamond ring appears again on the other side, then Bailys beads, shadow bands, and crescent projections all reappear as the moon slowly unveils its dangerously luminous counterpart.

 

 

Safe Solar Viewing

The light of the sun is intense! The sun emits a range from UV to radio waves (290nm-1+m). The UV Radiation that your eyes are exposed to when you look directly at the sun causes painful short-term damage (Called Solar Retinopathy). This UV radiation can cause corneal sunburns, growths on the surfaces of the eyes, accelerate the ageing of the outer eye (and skin), and leads to development of cataracts (cloudy spots on the eye lens). After about 100 seconds of staring at the sun (either all at once or over the course of your life), depending on the intensity, permeant damage to the Fovea is caused – that is, blindness in the center of the eye.  Since there is a chance for both long term damage over time causing cataracts and long-term damage in under two minutes causing blindness, it’s important to wear sunglasses, and NEVER look directly at the Sun.

Even once the UV light is filtered out (by sunglasses), you can still get retinal burns since wavelengths between 380nm and 1400nm pass through the glasses and through your eye to the retina. Large amounts of blue, green and even IR light triggers harmful and even destructive chemical reactions in the cells of the rods and cones. The resulting photoproducts cause the nerve ending free, darker, epithelium (below the retina) to be heated up and burned. (Mr.eclipse.com,http://scienceline.ucsb.edu/getkey.php?key=3269).

We know what is safe to use for viewing because the eye has a threshold for how much light of each wavelength can be absorbed before damage occurs. It takes less light for the light with shorter wavelengths to reach that threshold, but that doesn’t mean light of longer wavelengths is safe. Retinal damage can occur without pain or even effecting your vision for several hours.  

The Sun can be painful to look at and can damage the eyes, but there are two exceptions to this rule!

  1. At Sunset, the UV radiation is filtered out by the amount of atmosphere it has to pass through (scattering), so it’s safe for a few minutes.
  2. The other exception to this is the corona, which is safe to view without protection, and only comes out during totality – that is, when the sun’s disc is completely covered by the moon and only the dim corona is visible.

There are safe ways to view the build-up and winding down of a total eclipse, and partial eclipses safely: Projection or Filtration.

Projection is achieved by punching a small hole (~1mm) in any piece of tinfoil or cardboard. The spaces between the leaves on a tree can even work sometimes by creating a sea of “pinhole” projections.  There are a variety of filters that work great alone, or combined with a telescope or camera. There are also a lot of filters that DON’T work, including beer bottles or any other variety of smoked glass, neutral density photographic filters, film, sun glasses, and more. Whatever you choose to use, make sure it is designed for solar viewing!

 

 

When and Where can the Solar Eclipse be Seen?

On August 21st, 2017 (THIS YEAR!!!) a total solar eclipse will be traversing a path across the continental United States. Eclipses are on an 18 year and 11 month cycle but there can be up to five solar eclipses and three lunar eclipses in any given year spread out across the globe. While there will be another total solar eclipse in April of 2024, it will be a full 360 years before another eclipse crosses the United States like this (the time between two identical eclipses at the same location). That makes this eclipse is a once-in-a-lifetime event for many, as it is within one day’s drive for almost all of America.

 

Event

UTC Time

Time in Seattle*

First location to see the partial eclipse begin

Aug 21 at 15:46

Aug 21 at 8:46 am

First location to see the full eclipse begin

Aug 21 at 16:48

Aug 21 at 9:48 am

Maximum Eclipse

Aug 21 at 18:21

Aug 21 at 11:21 am

Last location to see the full eclipse end

Aug 21 at 20:02

Aug 21 at 1:02 pm

Last location to see the partial eclipse end

Aug 21 at 21:04

Aug 21 at 2:04 pm

Table courtesy of: https://www.timeanddate.com/eclipse/solar/2017-august-21

 


Resources and Further Reading

Upcoming 2017 Total Solar Eclipse Events

Currently there are no future events planned specifically for 2017 total solar eclipse.

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