How Eclipses Work
On Monday, August 21, 2017, a solar eclipse will be visible (weather permitting) across all of North America. 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, when the moon completely blocks the sun’s bright face for up to 2 minutes 40 seconds, turning day into night and making visible the otherwise hidden solar corona — the sun’s outer atmosphere — one of nature’s most awesome sights. Bright stars and planets will become visible as well.
Eclipses, whether solar or lunar, occur because of the periodic alignments of the sun, Earth, and moon. These three bodies, orbit in space in very predictable paths (yes, the sun orbits too. It orbits the galaxy once every 200 million years!). Ever since the days of Kepler and Newton, we have been able to predict the motion of planetary bodies with great precision. So, why do eclipses happen?
Solar Eclipses Happen when the moon moves between Earth and the sun.
When the moon does eclipse the sun, it produces two types of shadows on Earth (seen above). The umbral shadow is the relatively small in diameter point on Earth where an observer would see a total eclipse. The penumbral shadow is the much larger area on Earth where an observer will see a partial eclipse. Here, the sun is not completely covered by the moon.
There are Four Types of Eclipses
Depending on your location and on the specific geometry of the sun-Earth-moon system, you may experience one of four types of solar eclipses; total, partial, annular and hybrid.
A TOTAL ECLIPSE happens when the moon completely covers the sun. Here, the observer is standing under the umbral shadow of the moon. In a total solar eclipse, the sun’s outer atmosphere can be seen.
The brighter stars and the planets come out. Animals change their behavior. Birds and squirrels nest. Cows return to the barn. Crickets chirp. There is a noticeable drop in both light level and air temperature. It is an eerie feeling. Totality can last for no more than about seven and a half minutes but is usually less than three minutes long.
A PARTIAL ECLIPSE occurs when the moon passes in front of the sun, off center and only a portion of the sun’s disk is obscured. Here, the observer is standing in the penumbral shadow of the moon.
AN ANNULAR ECLIPSE occurs when the moon passes dead center in front of the sun but, because the moon’s orbit is elliptical and so is sometimes closer and sometimes further from Earth, it appears too small to fully cover the disk of the sun.
Here, a bright ring called the “ring of fire” appears around the dark disk of the moon. In an annular eclipse, the moon’s umbral shadow comes to focus – to a point – above the Earth’s surface.
A HYBRID ECLIPSE is a combination of total and annular eclipses. The eclipse begins as one type and ends as another.
How to View the 2017 Solar Eclipse Safely
Looking directly at the sun is unsafe except during the brief total phase of a solar eclipse (“totality”), when the moon entirely blocks the sun’s bright face, which will happen only within the narrow path of totality.
The only safe way to look directly at the uneclipsed or partially eclipsed sun is through special-purpose solar filters, such as “eclipse glasses” or hand-held solar viewers. Homemade filters or ordinary sunglasses, even very dark ones, are not safe for looking at the sun. To date, four manufacturers have certified that their eclipse glasses and handheld solar viewers meet the ISO 12312-2 international standard for such products: Rainbow Symphony, American Paper Optics, Thousand Oaks Optical, and TSE 17.
- Always inspect your solar filter before use; if scratched or damaged, discard it. Read and follow any instructions printed on or packaged with the filter. Always supervise children using solar filters.
- Stand still and cover your eyes with your eclipse glasses or solar viewer before looking up at the bright sun. After glancing at the sun, turn away and remove your filter — do not remove it while looking at the sun.
- Do not look at the uneclipsed or partially eclipsed sun through an unfiltered camera, telescope, binoculars, or other optical device. Similarly, do not look at the sun through a camera, a telescope, binoculars, or any other optical device while using your eclipse glasses or hand-held solar viewer — the concentrated solar rays will damage the filter and enter your eye(s), causing serious injury. Seek expert advice from an astronomer before using a solar filter with a camera, a telescope, binoculars, or any other optical device.
- If you are within the path of totality, remove your solar filter only when the Moon completely covers the sun’s bright face and it suddenly gets quite dark.
An alternative method for safe viewing of the partially eclipsed sun is pinhole projection. For example, cross the outstretched, slightly open fingers of one hand over the outstretched, slightly open fingers of the other. With your back to the sun, look at your hands’ shadow on the ground. The little spaces between your fingers will project a grid of small images on the ground, showing the sun as a crescent during the partial phases of the eclipse.
A solar eclipse is one of nature’s grandest spectacles. By following these simple rules, you can safely enjoy the view and be rewarded with memories to last a lifetime.
Courtesy of NASA