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Total Eclipse of the Sun - 8/21/17

episode number
ED-016
Total Eclipse of the Sun
2006 eclipse over Libyan Desert. Composite image reveals subtle structure in sun's corona. 
Credit: Frank Espenak

Mark your calendars and buy some eye protection, because on August 21, 2017, the U.S. will experience a total eclipse of the sun.

A total eclipse happens when the moon passes perfectly in front of the sun, blocking its view from Earth. Only a small path across the U.S. will see the sun completely obscured, but that “path of totality” stretches from Oregon to South Carolina, passing over nearly 50 million people.

If you were to view the eclipse from space, you’d see a small moon shadow 70 miles wide cast upon the surface of Earth and moving quickly across it at 1500 miles an hour.

From Earth, you can experience this moving shadow, too. If you’re lucky enough to witness the total eclipse from a mountaintop, you’ll see the moon shadow racing toward you from miles away.

Eventually it will engulf you. Darkness will fall. The temperature will drop 10 to 15 degrees. You’ll see what looks like sunset on all horizons. Birds will stop chirping, and crickets may start. But don’t blink—this ethereal scene will last only about 2-and-a-half minutes, before the shadow races on.

The experience can be awe-inspiring, even unnerving. And you’ll understand how early cultures would have given powerful religious significance to a total eclipse.

If seeing one is on your bucket list, head to the path of totality on August 21st, find some high ground, and look to the heavens— wearing the right eye protection, of course.
 


A Total Eclipse of the Sun (August 21, 2017) Background

Synopsis: On August 21, 2017, the United States will be treated to an extraordinary natural event: a total eclipse of the sun that will streak from Oregon to South Carolina at more than 1000 mph—almost Mach 1.5! It has been 38 years since the last total eclipse viewable from the United States.

  • Eclipses are simply the shadow that one celestial body casts on another, as viewed from Earth. For a shadow to be cast, the light source (sun) and the two bodies (Earth and moon) must line up; however, the moon’s orbit is tilted about 5 degrees from Earth’s, so conditions favorable for eclipses only happen when the geometries of the elliptical orbits align.
  • You may have seen lunar eclipses, which are visible from an entire hemisphere as the full moon traverses the large shadow cast by Earth for a couple of hours.
    solar eclipse
    Geometry of the sun, Earth, and moon during an eclipse of the sun. The moon's two shadows are the penumbra and the umbra. (Sizes and distances not to scale.)
    Credit: Fred Espenak, MrEclipse.com
  • Solar eclipses occur when the new moon casts a shadow on Earth. If you are outside of the central umbra shadow, you may see a partial eclipse that will look like a bite being taken out of the sun; the sun will still be almost as bright as usual, so be sure to use eye protection!
  • Total solar eclipses are very special because they completely block the brightness of the sun, revealing the aura of its corona.
    • Total solar eclipses are more difficult to view because the moon casts a relatively small shadow on Earth—over only about 1% of the diameter—ranging from 50 to 125 miles wide, depending on how close the moon is to Earth. You must be in this umbra shadow to see the corona.
    • In 2017, although all of North America will be in the penumbra, the umbra will be a laser-thin 40–70 miles wide, sweeping across Oregon, Idaho, Wyoming, Kansas, Missouri, Illinois, Kentucky, Tennessee, Georgia, and finally South Carolina during its 90-minute traverse of the continent. The maximum length of totality, 2 minutes and 40 seconds, will occur in the center of the path in Illinois.
      paths of solar eclipses
      On map above, yellow curves parallel to path of total solar eclipse mark the degree of maximum partial eclipse. Green lines show times of greatest eclipse as they sweep across the country. Orange curves inside path of total solar eclipse show its duration. More than 50 million people live in the path of totality. Credit: Michael Zeller, Great American Eclipse.com

At this special time in Earth’s history, we enjoy a cosmic coincidence that makes total solar eclipses possible: the sun and moon seem to be about the same size in our sky. The sun has a diameter about 400 times larger than that of the moon, and the moon is about 400 times closer to Earth than the sun, enabling the moon to almost exactly cover the bright disc of the sun to reveal the corona.

earth as seen during solar eclipse
Earth as seen from the Mir space station during a solar eclipse in 1999. Credit: Mir 27 Crew; copyright: CNES
  • What is the experience of a total eclipse?
    • The western sky will darken for more than an hour as the huge shadow approaches; be patient, and always use eye protection to look at the partially eclipsed sun!
    • If you are on high ground, you may see the shadow rush up to you and engulf you; around the horizon, it will look like a 360-degree sunset/sunrise. 
    • Around the disc of the sun, the corona exhibits flares and mass ejections, as well as multispectral effects.
    • Shadows will look different, breezes will calm, temperatures will drop 10–15°F, birds will return to their nests and stop singing, and crickets and frogs will begin their evening songs.
    • In minutes the whole phenomenon reverses: the shadow rushes away from you and the birds sing again.
  • If you miss this one, you will get another chance in 7 years. Another total eclipse will occur April 8, 2024, ranging from Texas to Maine. The umbra will be about twice as wide as in 2017, so viewing will last about twice as long. The 2017 and 2024 paths cross in Carbondale, Illinois.
    Paths of 15 future total eclipses through 2028.
    Paths of 15 future total eclipses through 2028. Credit: Astronomy: Roen Kelly after Fred Espenak, NASA/Goddard Space Flight Center

     

A Total Eclipse of the Sun (August 21, 2017) References

Eclipse 2017 | NASA

The “Great American Total Solar Eclipse” of 2017 (with video) | Space.com

25 Facts About 2017 Total Solar Eclipse | Astronomy Magazine

Solar Eclipses for Beginners | MrEclipse.com

2017 US Eclipse Map | Eclipse2017.org

 

Contributors: Juli Hennings, Harry Lynch