The most important consideration for any solar eclipse is eye safety. While it's perfectly safe to view a solar eclipse if you follow safe eclipse viewing procedures, you can cause permanent eye damage or even blindness if you attempt to view an eclipse incorrectly.
You must use special eclipse safety glasses or viewers to view a partial eclipse, an annular or "ring of fire" eclipse, and the partial phases of a total eclipse. (For the record, a total eclipse is partial most of the time within the path of totality and partial all of the time outside the path of totality.) Although it may be tempting to look directly at an eclipse with unprotected eyes when so much of the Sun is obscured, the small amount of light emitted from even a 99.9 percent partially eclipsed Sun is still dangerous. The only time it's safe to look at a total eclipse without proper eye protection is during the very brief period of "totality" when the Sun is 100 percent blocked by the Moon. If you're not located within the narrow path of totality where the eclipse will become total for a very brief period of time, there is never a time when it's safe to look with unprotected eyes.
Make sure that your eclipse safety glasses or viewers are certified as meeting international standards for safe solar viewing. The current standard for safe solar viewing is ISO 12312-2; your eclipse safety glasses or viewers should have this designation printed on them. Take care to purchase your glasses or viewers directly from a recognized manufacturer of certified safe eclipse glasses or a reputable seller that purchased from a recognized manufacturer. Be wary of products that claim to be safe but aren't. Be very careful and don't use any product unless claims of safety can be verified. For the 2017 eclipse, there were many reports of unsafe eclipse glasses being distributed as well as reports of counterfeit eclipse glasses printed with names of reputable manufacturers, including many that were sold on Amazon and coming from overseas. Leading up to the 2017 eclipse, the American Astronomical Society provided guidance on how to tell if your eclipse glasses or viewers are safe. Before using your glasses or viewers, make sure that they are not damaged in any way (lenses shouldn't have scratches or wrinkles) and that you read all of the safety instructions that came with them. Children should always be supervised by a responsible adult when using eclipse safety glasses or viewers.
Unless a product has been specifically designed for safe solar viewing and has been certified as meeting international standards for such products, it's best to assume that a device, method, or instrument is unsafe. Don't risk it! Unfortunately, the media doesn't always get it right and there is a great deal of misinformation in print and online about what's safe and what isn't. Items such as regular or polarized sunglasses, smoked glass, exposed film, medical x-rays, homemade filters, and many others are all unsafe. You can use welder's glass to view an eclipse, but it must be #14 welder's glass; any rating below #14 is not safe. It's also safe to view an eclipse using indirect methods, such as projecting an image of the eclipsed Sun onto a white screen. To make an indirect viewer, search online for "pinhole projector" and follow the instructions provided by a trusted organization like NASA.
If the eclipse will become total in your location (i.e., if you're in the narrow path of totality), don't remove your eclipse glasses until the very last bit of the Sun is gone and you can no longer see any light through them. Again, it's only safe to look with unprotected eyes when the Sun is 100 percent blocked by the Moon and only the soft wisps of the solar corona are visible. Once totality begins, it's important to know precisely when totality will be ending in your exact location so that you can once again put on your eclipse glasses before the first brightness of the exposed Sun is revealed. To determine local circumstance details for any total solar eclipse, including the precise start time, end time, and duration of totality for your exact location, use the interactive Google eclipse maps developed by Xavier Jubier. You should also allow for a very generous margin of error to ensure that you are no longer looking with unprotected eyes when totality ends (and keep in mind that your clock or watch may not be in sync with astronomical time!). Additionally, NASA advises that you should pay careful attention to the edge of the Moon opposite of where the Sun last appeared. When you start to notice a very slight crescent-shaped brightening, you’ll know that totality is coming to an end. This is your signal to look away or put your eclipse glasses back on before the first flash of exposed sunlight. As mentioned earlier, children should always be supervised by an adult who fully and clearly understands safe eclipse viewing procedures.
Note that attempting to view an eclipse using cameras, binoculars, telescopes, or other optical devices without proper solar filters is extremely hazardous and can permanently damage the eyes in an instant. These devices need specially designed solar filters that fit snugly on the front end (the Sun side) of the device. Never attempt to view an eclipse through an optical device using eclipse glasses or any type of filter that attaches to the viewing side (as opposed to the Sun side) of the instrument; the focused light will destroy the filter and enter and damage your eyes. Since viewing or photographing a solar eclipse with an optical device requires specialized equipment and knowledge, we recommend consulting with a qualified astronomer or just enjoying the eclipse with your own eyes (using safe eclipse viewing procedures, of course).