There are some persistent “urban legends” about viewing eclipses. Some people are under the impression that it is dangerous to view any sort of eclipse, no matter what. Let’s clear this up right off the bat….
It is TOTALLY SAFE to view a TOTAL SOLAR ECLIPSE. During the phase of totality, the Sun is completely covered by the Moon, and there is no portion of the solar disc visible. Also, it is TOTALLY SAFE to view a lunar eclipse in any phase.
The dangerous part is looking at the phases of partial solar eclipse that lead up to totality and after totality ends. The whole danger is in looking at the blindingly bright disc of the Sun, which is dangerous to view at any time. During the partial stages, you must practice safe viewing, as explained in detail below. So rest assured that there are no magical “eclipse rays” that shine off of an eclipsed Sun or Moon to cause blindness!
The bright part of the Sun is the photosphere, the incandescent gas on the Sun’s surface which shines light down on to the Earth and the Moon, along with all the other planets of the solar system. The disc of the Sun is 1,000,000 times brighter than a Full Moon! This means that each “pixel” on the Sun is 1,000,000 times brighter than a similar “pixel” on the Moon! So it’s very dangerous to view any of the “pixels” of the Sun’s surface prior to the full commencement of totality.
To emphasize, even if only 1% of the Sun is visible just prior to totality, it’s still harmful “sunlight” that can damage the eyes. But during totality, when the entire photosphere is covered, it’s TOTALLY SAFE to view the Sun! At this time, the elusive corona is seen, the mysterious “crown” of rays that are dim enough to safely view directly with the unaided eye.
To protect your eyes during totality, you can use a properly-filtered solar viewer, or else a projected image.
Welding glass — The Sun can be safely viewed through #14 welding glass, which dims the Sun’s light to a level where most of the light is filtered. You must make sure it is #14, because the lower numbered filters do not provide adequate filtering to safely view the Sun.
Eclipse Shades — These are inexpensive, commercially-available paper glasses with a special “black mylar” film specially created for viewing the Sun. Eclipse Shades are designed to block out more than 99% of the Sun’s visible light, and 100% of the harmful ultraviolet and infrared solar radiation. In fact, Eclipse Shades are so dark that you really can’t see anything else but the Sun through them! You can also view the Sun through Eclipse Shades at any other time to view sunspots or to otherwise look at the Sun. Though many brands of solar viewers are available, we recommend our own Commemorative Special Edition American Eclipse USA Eclipse Shades, sold through our online store.
WARNING! There are some incorrect misconceptions about solar viewing that are NOT safe and WILL NOT protect your eyesight! In particular, do not use smoked glass which has been blackened by a candle. Also, do not use x-ray film or any other darkened photographic film. Additionally, do not use a common aluminized mylar balloon. None of these have enough filter density to absorb enough sunlight to protect your eyes. In general, DO NOT USE any solar filter not specifically designed to filter the Sun or other similarly bright light.
For those who are not sure whether to trust their eyes to a solar filter, the absolutely safest method of viewing the Sun is through image projection, in which the light of the eclipsed Sun shines on a screen, like a movie projector.
Pinhole camera — The camera obscura is a completely safe method, based on a principle of optics that a focused image of the Sun can be shone through a pinhole onto a projection surface. With the basic “shoebox camera,” a hole poked in the side of an ordinary shoebox will project an image of the eclipsed Sun onto the opposite interior sidewall of the shoebox. Point the pinhole at the Sun and peek through an opening in the mostly-closed lid to make it dark inside the box. Though simple in concept, the image is of low quality.
Tree branches — This is the same principle as the camera obscura, where rays of sunlight that shine through tree branches form images of the Sun through the shadows on the ground, Normally, the images are circles, and not noticeable. But during a partial eclipse, the tree branches shine dozens of little crescent Sun images all over the ground! And during an annular eclipse, dozens of little solar rings shine all over the ground!
Telescope projection — Many commercial telescopes include a projection screen for shining a focused image of the Sun. Only experienced telescope owners well-versed in safe solar viewing should attempt this, to safely point the scope at the Sun and avoid the very dangerous concentrated sunlight at the viewing end of the scope.
Sun Funnel — Probably the coolest design for viewing a projected solar image is the Sun Funnel, a nifty homemade telescope attachment. Made of commonly available parts, the Sun Funnel projects a high-contrast solar image out of the eyepiece end of a telescope, completely safe. The Sun Funnel was designed by well-known astronomy popularizers Chuck Bueter and Rick Fienberg, and Lou Mayo from NASA. You can find directions for making your own Sun Funnel at this site and at this site.