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The Great American Eclipse of 2017

Mike Isley
Product Developer

May 2017


The next total eclipse of the Sun occurs on August 21, 2017. This is the first eclipse to be seen only in the continental US since the country was founded. Thus, it is being billed as the “Great American Eclipse.”

The Moon’s shadow will begin in the Pacific, enter the state of Oregon, and create a 70-mile-wide corridor of totality (total eclipse) moving through portions of Oregon, Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina, and exiting off the coast of South Carolina. Everyone in the US will see at least a partial eclipse, but those in the corridor of totality as shown in Fig. 1 will see a total eclipse.


Figure 1  Path of the Great American Eclipse.
 Illustration by www.GreatAmericanEclipse.com. 


What Causes an Eclipse?

Even though the Sun is 400 times larger than the Moon, it is also 400 times farther away. This coincidence makes both bodies the same size when viewed from Earth, enabling the Moon to cover the Sun when their orbits intersect. A total eclipse occurs only during a new moon, when the Moon moves between Earth and the Sun creating a shadow on Earth’s surface.

This shadow consists of 2 parts. The larger, lighter part is called the penumbra, and can span entire continents. In the penumbra, only partial solar eclipses can be seen. Within the penumbra is the smaller, darker part called the umbra. The umbra is an ellipse of 150 miles or less in which a total solar eclipse can been seen. See Fig. 2 showing the geometry of this phenomenon.

Not every new moon creates an eclipse because the Moon’s orbit tilts 5º relative to Earth’s, which usually places the Sun too high or too low in the sky for an eclipse alignment. Therefore, the number of eclipses seen on Earth per year is small, from 2 to 5, with no more than 1 or 2 of them being total eclipses.


Figure 2  Geometry of a total solar eclipse.


Facts about the Great American Eclipse of 2017

  • The eclipse shadow will be traveling at about 1,800 mph (2,897 kmph) as it enters Oregon. At that rate, it will take 1 hour, 33 minutes to complete its journey across the continental US.
  • For observers located on the centerline of the umbra’s 70-mile-wide corridor, totality can last for up to 2 minutes, 40 seconds. Remember that the duration of totality decreases as you move north or south of the corridor’s centerline. Consult these resources for detailed totality maps for each state.
  • EclipseWise.com
  • NationalEclipse.com


Safety Tips for Viewing an Eclipse

  • When viewing a partial eclipse, and the time just before and after a total eclipse, you must wear eclipse glasses or filters that are ISO 12312-2:2015 approved for eliminating all harmful ultraviolet rays. 


Figure 3: Eclipse glasses in use.

Sunglasses and smoked glass are NOT OK. Even if 99% of the Sun is blocked by the Moon, you can still suffer permanent eye damage by staring at a partial eclipse without approved eyewear. NEVER view a partial eclipse through binoculars or a telescope unless the front (objective) lenses of those instruments are covered with sun filters that meet ISO standards.


What to Expect During a Total Eclipse

  • As the Moon covers a large portion of the Sun, the temperature starts dropping, the sky gets darker around the Sun, and you may be able to see the umbra rapidly advancing toward you from the west. Wind patterns change and it starts getting very quiet as birds and other animals think it is night.
  • Just before totality, you may see through your eclipse glasses the last bit of sunlight dancing through the valleys on the Moon, creating beads of light. These are Baily’s beads (Fig. 4). As they fade, one spectacular bead forms, creating a diamond ring effect lasting 2 to 3 seconds as the last burst of sunlight goes through a valley on the Moon’s edge (Fig. 5). 
 
Figure 4  Baily’s beads.   Figure 5  Diamond ring effect.

  • Only during totality can you view the eclipse with your naked eyes. You will know when this occurs because you will not be able to see anything through your eclipse glasses. At this moment, if you remove your glasses you can see the faint flickering glow of the Sun’s outer atmosphere known as the corona (Fig. 6).


Figure 6  The Sun’s corona.

  • You might be able to see prominences (pink or red tongues of hot gases) projecting from the Sun’s surface. It would be wise to know the duration of totality at your location beforehand and set a timer to remind you to put on your glasses maybe 15 seconds prior to the end of totality.
  • With your glasses back on, you should see another diamond ring form as the Moon begins to move away from the Sun.


Indirect Viewing Methods

Indirect viewing is always one of the safest methods of viewing the Sun or an eclipse. The following methods can be used:

  • Make a pinhole projector from 2 white paper plates (see Fig. 7). Insert a hole in the center of 1 plate with a straight pin. Hold this plate in sunlight with the Sun at your back. The second plate is placed in front of the first as a projection screen. This works better if this second plate is shaded. Moving the plates farther apart makes a larger but fainter image.

           
Figure 7  Paper plate pinhole projector.

  • A pinhole projector box (see Fig. 8) makes a darker viewing area for the projected image.


Figure 8  Pinhole projector box.

  • Indirect viewing with binoculars or a telescope (see Fig. 9) projects the Sun’s image onto a white screen just like the pinhole projectors, except that the image is magnified. Never look into the eyepiece(s) of the binoculars or telescope when using this method. Serious eye damage or blindness can result. With the Sun at your back, point the instrument’s front (objective) lens(es) at the Sun and while not looking into the eyepiece(s), move the binoculars or telescope around until the Sun’s image appears on the stationary white screen. Adjust the instrument’s focus and experiment with screen distance until there is a sharp and fairly large image. If possible, keep the screen in the shadows to aid in viewing. You can also view sunspots with this setup.


Figure 9  Indirect viewing with binoculars or telescope. Telescope illustration by Sky & Telescope.


When Is the Next Total Eclipse in the United States?

The next total eclipse in the US will be on April 8, 2024. It will travel northeast from Mexico through the states of Texas, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, New York, and Vermont.


Resources for More Information

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