On August 21, 2017, beginning at 10:15 a.m. PDT, a total solar eclipse will sweep across the United States. The whole continent will experience a partial eclipse lasting 2 to 3 hours. Halfway through the event, anyone within a roughly 70-mile-wide path from Oregon to South Carolina will experience a brief total eclipse. At that point, the moon will completely cover the face of the sun for up to 2 minutes 40 seconds.
How Often Do Solar Eclipses Occur?
There are between 2 and 5 solar eclipses every year, each one visible only in a limited area. The maximum number of solar eclipses that can take place in the same year is 5, but this is rare.
What Are Solar Eclipses?
A solar eclipse is when the moon blocks any part of the sun from our view. The bright face of the sun is covered gradually by the moon during a partial eclipse, lasting a few hours. During the brief period of a total eclipse when the moon fully covers the sun (only a couple of minutes), the light of day gives way to a deep twilight sky. The sun’s outer atmosphere (called the solar corona) gradually appears, glowing like a halo around the moon in front of it. Bright stars and planets become more visible in the sky.
Types of Solar Eclipses
There are 4 different types of solar eclipses.
1. Partial solar eclipses occur when the Moon only partly obscures the Sun's disk and casts only its penumbra on Earth.
2. Annular solar eclipses take place when the Moon's disk is not big enough to cover the entire disk of the Sun, and the Sun's outer edges remain visible to form a ring of fire in the sky. An annular eclipse of the Sun takes place when the Moon is near apogee, and the Moon's antumbra falls on Earth.
3. Total solar eclipses happen when the Moon completely covers the Sun, and it can only take place when the Moon is near perigee, the point of the Moon's orbit closest to Earth. You can only see a total solar eclipse if you're in the path where the Moon's casts its darkest shadow, the umbra.
4. Hybrid Solar Eclipses, also known as annular-total eclipses, are the rarest type. They occur when the same eclipse changes from an annular to a total solar eclipse, and/or vice versa, along the eclipse's path.
Solar Eclipse Eye Safety
Never look directly at the Sun, eclipse or otherwise, without any protective eyewear. The Sun’s UV radiation can burn the retinas in your eyes leading to permanent damage or even blindness.
Keep in mind that ordinary sunglasses, even very dark ones, or homemade filters are not safe for looking at the sun.
Make sure your eclipse glasses are safe and meet a very specific worldwide standard known as ISO 12312-2, or the Sun's UV radiation can seriously injure the retinas in the eyes. Use a solar filter if you are using binoculars or a telescope.
Steps to follow for safely watching a solar eclipse:
Carefully look at your solar filter or eclipse glasses before using them. If you see any scratches or damage, do not use them.
Always read and follow all directions that come with the solar filter or eclipse glasses. Help children to be sure they use handheld solar viewers and eclipse glasses correctly.
Before looking up at the bright sun, stand still and cover your eyes with your eclipse glasses or solar viewer. After glancing at the sun, turn away and remove your filter—do not remove it while looking at the sun.
The only time that you can look at the sun without a solar viewer is during a total eclipse. When the moon completely covers the sun’s bright face and it suddenly gets dark, you can remove your solar filter to watch this unique experience. Then, as soon as the bright sun begins to reappear very slightly, immediately use your solar viewer again to watch the remaining partial phase of the eclipse.
Never look at the uneclipsed or partially eclipsed sun through an unfiltered camera, telescope, binoculars or other similar devices. This is important even if you are wearing eclipse glasses or holding a solar viewer at the same time. The intense solar rays coming through these devices will damage the solar filter and your eyes.
Talk with an expert astronomer if you want to use a special solar filter with a camera, a telescope, binoculars or any other optical device.
Eclipse Resources Downloadables
Eclipse Fact Sheet
Eclipse Safety Information
For information check out the American Astronomical Society.
For more information on eye safety visit the American Academy of Ophthalmology.