Something strange will happen on Monday August 21, 2017. Daylight will turn into darkness, temperatures will drop, the sound of crickets will replace birds chirping, and millions of people will be staring up at the sky. For the first time in 99 years, a total solar eclipse will sweep across the entire United States along a narrow corridor from Oregon to South Carolina. The “Great American Eclipse” will be one of the most beautiful natural phenomena you can experience.
The last total solar eclipse in the United States occurred in 1979 and was only visible from states in the northwest corner of the country (Washington, Oregon, Idaho, Montana, and North Dakota).
What is a total solar eclipse?
A total solar eclipse occurs when the moon completely blocks out the sun. As the moon passes between the Earth and sun, it casts two types of shadows on the Earth. The umbra is the moon’s innermost and darkest shadow. If you’re in the path of the moon’s umbral shadow you will see the sun’s entire disk obscured and the glowing appearance of the sun’s corona, its outer atmosphere. This phase of the eclipse is called totality. The narrow track of the umbra moving across the U.S. is known as the path of totality and will be only 71 miles at its widest point.
The penumbra is the moon’s faint outer shadow. If you’re within the moon’s penumbral shadow, you will see a partial solar eclipse. The penumbra covers a much larger area on Earth allowing the partial eclipse to be visible throughout North America including Canada as well as the northern part of South America, and the western parts of Europe and Africa.
A total solar eclipse occurs somewhere on Earth about once every 18 months. This may sound like a more common occurrence than one would think but due to the narrow path of totality, the odds of viewing a total solar eclipse from a particular location on Earth are rare.
There are other types of solar eclipses but nothing compares to a total solar eclipse. An annular solar eclipse occurs when the Earth, moon and sun are properly aligned but the moon is too far away from Earth due to its elliptical orbit. The moon’s smaller apparent size is unable to completely cover the sun and it appears as a very bright ring. If the alignment of the Earth, moon and sun is off slightly, only a portion of the sun’s disk will be covered by the moon. This is a partial solar eclipse and it is not nearly as dramatic or spectacular as witnessing totality even if it covers 99 percent of the sun.
What you will see during a total solar eclipse
Several notable events occur just prior to totality. As the moon almost completely covers the sun, beads of light appear along the edge of the moon as sunlight shines through the lunar mountains, valleys and craters. This effect is known as Baily’s Beads. When only a single large bead of light is visible, it takes on the appearance of a shining diamond ring.
Once totality is reached, the sun’s normally hidden corona becomes visible. The corona is an aura of plasma that is millions of degrees hotter than the surface of the sun and extends millions of miles into space. Witnessing the corona is a profoundly intense experience that is simply beyond words.
As the sky grows dark, the blue skies began to dull and turn deeper blue to twilight blue to blue-black. The darkened skies allow the brighter stars and planets to become visible including Venus and Jupiter. Looking to the horizon you’ll see orange and yellow twilight in all directions. You’re actually seeing light from beyond the umbral shadow in areas where the eclipse is not in totality.
How long totality will last varies depending on your location and how close you are to the center of the umbra. The longest duration of totality will be 2 minutes and 40 seconds.
Where to see the total solar eclipse
The “Great American Eclipse” will first appear in Oregon and cross Idaho, Wyoming, Nebraska, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, North Carolina and South Carolina. Very tiny corners of Montana and Iowa are also just inside the path of totality.
NASA’s Total Solar Eclipse Interactive Map plots the path of totality on Google Maps. Zoom in and click anywhere on the map to see the start and end times, duration, and magnitude of the solar eclipse at a specific location.
A large portion of the U.S. will be within a day’s drive of seeing the total solar eclipse. Several cities and towns have been preparing for this day for several years. Millions of people are expected to witness this event and populations are anticipated to at least double in some locations. Many hotels along the path of totality have been booked for over a year in advance. Expect major traffic getting to and leaving your viewing site.
The best locations to view the longest duration of totality are in Missouri, Illinois and Kentucky along the middle of the path of totality. Just outside of Carbondale, Illinois will be the point of Greatest Duration and an area near Hopkinsville, Kentucky will be the point of Greatest Eclipse. Both are expected to see up to 100,000 visitors. Hopkinsville has been promoting itself as “Eclipseville”.
How to safely view the total solar eclipse
Never look directly at the sun without proper eye protection. You can severely damage your eyes and go blind. Even staring at 1 percent of the sun (a 99 percent partial eclipse) or Baily’s Beads is enough to damage your eyes. Sunglasses are not a sufficient or acceptable form of eye protection, not even wearing two pairs.
Direct observation of the sun during a total solar eclipse can be done safely if using a certified solar filter that meets ISO Standard 12312-2. A solar filter will protect your eyes from the sun’s ultraviolet and infrared radiation, and intense visible light. Certified solar filters can be found in glasses, goggles and hand-held solar shields or viewers. Low cost solar eclipse glasses with cardboard frames can be purchased in packs of 5 or 10. More durable plastic frame versions are also available.