What to Expect and How to Safely View the Total Eclipse of 2017

LEILANI ARTHURS (larthurs2@unl.edu) is an assistant professor in the Department of Earth and Atmospheric Sciences at the University of Nebraska-Lincoln.

A total solar eclipse is one of the most beautiful cosmic phenomena that we can see with our own eyes. Although a total eclipse happens somewhere in the world about every eighteen months, it is rare for one to happen close to home. So, mark your calendars! On August 21, 2017, the Moon will cast its shadow across the contiguous United States. Weather permitting, people along the path of the shadow will have the opportunity to see a partial eclipse of the Sun, and those who find themselves anywhere in the darkest part of the Moon's shadow will also have the opportunity to witness a total eclipse of the Sun.

The eclipse provides a unique opportunity to engage learners of all ages in both the sciences and the liberal arts. Such engagement can be planned for the day of the eclipse and/or for some time during the coming academic year. For those planning activities on the day of the event, you will need to protect your eyes when looking directly at the Sun's position. The safest way to do this is by wearing certified protective eclipse-viewing glasses.

To conceptualize how a total eclipse happens, we need to know some key points about the Sun-EarthMoon system. Although the Sun is about 400 times larger in diameter than the Moon, it is also 400 times further away from the Earth than the Moon is. This coincidence of size and distance means that, viewed from Earth, the apparent sizes of the Sun and Moon are equal — cool! This is what makes it possible for our small Moon to totally eclipse our Sun.

During a total solar eclipse, the Moon is positioned between the Sun and the Earth (Figure 1). Even though the Moon passes between the Sun and the Earth every month (i.e., new Moon), the Moon does not cast a shadow on the Earth every month. A total eclipse happens only when the 5-degree tilt of the Moon's orbit is positioned such that the fulcrum of its orbital tilt is pointed toward the Sun. Backlit by the Sun, the Moon casts its shadow upon the Earth's surface (Figure 2). The shadow is comprised of two parts — the umbra (the darkest part) and the penumbra (Figure 2).

At locations where the umbra falls, the apparent size of the Sun and the Moon are equal; thus, the Moon blocks the entire disk of the Sun, producing a total solar eclipse. The umbral shadow of the eclipse on August 21 will be 70 miles (112.65 km) wide. It will make landfall in Oregon, track across parts of fourteen states, and then move out to sea from South Carolina (see back cover). This is the path of totality.

Depending on your specific location and how close you are to the centerline, you may experience up to 2 minutes and 40 seconds of totality. This fleeting moment is the only time that you can safely look directly at the position of the Sun without your protective eclipse-viewing glasses, because the bright light emanating from the Sun's disk is blocked by the Moon.

What you will see during totality is the Sun's corona, its hot outer atmosphere that extends hundreds of thousands of miles (millions of km) into space. The corona is usually masked from the view of Earth-based observers by the intense light of the Sun, so a total eclipse provides an opportunity to see that which is usually not seen. Scientists and engineers will take this opportunity to address a variety of scientific questions and engineering problems (https://eclipse2017.nasa.gov/science). While viewing the corona, you may also see solar prominences. These are long, bright, ribbon-shaped features that are anchored to the Sun's surface and extend out into the Sun's corona. They are composed of hot gas that erupts from the Sun and is composed of electrically charged hydrogen and helium.

As you gaze up and experience the total eclipse, be sure to take a moment to look around you and notice how the lighting has changed, giving a perhaps unworldly appearance to the objects around you. Also, mark the drop in temperature in your vicinity. You might notice wildlife coming out to "observe" the total eclipse as well! The approximate two minutes of totality will go by quickly, so be sure to soak in the experience.

The start and end of the total eclipse are announced by the presence of a "diamond ring" around the Moon. A moment before the start of the total eclipse, the appearance of the first diamond ring is your signal that it is safe to take off your protective eclipse-viewing glasses. A moment after the end of the total eclipse, the second diamond ring is your signal that it is time to again put on your protective glasses. Figure 3 summarizes what you can expect to see during the total eclipse.

Wearing approved/certified eclipse-viewing glasses is the most affordable way to safely look directly at the Sun when it is not blocked by the Moon or when it is even partially blocked by the Moon. The lenses in these glasses are made of special-purpose solar filters that are orders of magnitude darker than regular sunglasses. Do NOT wear regular sunglasses to look directly at the Sun! They will not protect your eyes if you look at the Sun.

If you are planning an eclipse-viewing activity to witness either the total eclipse or the partial eclipse, then you and everyone there will need protective glasses. In addition to providing necessary protection, these can also make for wonderful memory-laden souvenirs of eclipse day experiences. If you are an educator, you can receive a discount on your purchase of these glasses from American Paper Optics. Additionally, many libraries have free eclipse glasses and eclipse information, made available through a project of the Space Science Institute's National Center for Interactive Learning.

In addition to ordering protective glasses in advance, you may also find it helpful to order the Road Atlas for the Total Solar Eclipse of 2017 by Fred Espenak. This will aid you in making back-up plans in the case your first-choice viewing site is forecast to have inclement weather on the day of the total eclipse.

For most people, viewing the August 21 total eclipse will be a once-in-a-lifetime event. So be prepared to safely view this celestial event and make lasting memories of an unearthly beauty seldom beheld by the human eye.