When it comes to total solar eclipses, nothing can be more confusing than eye safety — because you need it for the partial phases but not for the total phase. Despite global efforts to build awareness as to when you should and shouldn't wear eye protection, the concept still isn't universally clear. So some people opt to use eye protection throughout totality for fear of damaging or losing their eyesight. But that defeats the purpose, because you cannot see totality using eye protection.
So let's wipe the slate clean and look at a total solar eclipse in a different way. Think this: Sun (bad); Moon (good). Every total solar eclipse has two parts: the partial phases, when part of the Sun is visible (bad); and totality, when the Moon is fully visible and the Sun is not (good)!
If you watch only the total phase a solar eclipse, you don't have to worry about eye safety because you are looking at the Moon — so you do not need eye protection. Just as you don't fear looking at the Moon at night, you shouldn't fear looking at the Moon during totality — because, literally, what you don't see (the Sun, which the Moon is blocking) can't hurt you.
Eye safety is only for those who wish to watch the partial phases of the eclipse, which last for roughly 90 minutes before and after totality.
Warning: Looking directly at the Sun without proper protection may result in retinal burns or thermal injury, which could cause or lead to blindness. Never look directly through binoculars or a telescope unless they are properly covered with specifically designed filters — filters that cover the front end of the binoculars or telescope and let only about one part in 100,000 through, reflecting the rest. If in doubt about the filter, don't use it!
Eclipse Phases and Eye Safety
We've all learned not to touch a stove when it is on, because it is hot and we may burn ourselves. Well, imagine the Sun as a stove. Anytime the Sun is visible, the stove is on, and we need eye protection. During totality, however, we cannot see the Sun, so the stove is off (and cold). Let's look at each of these examples more closely and how they pertain to a solar eclipse.
Eclipse Begins: Stove On (eye protection required). An eclipse begins at first contact — the instant the Moon first "touches" the Sun. To see this event, which appears as a tiny black notch in the Sun's western limb, you will need eye protection, such as safe eclipse viewing glasses or solar filters on your binoculars or telescope. First contact is followed by the partial phases — as the Moon's silhouette moves across the face of the Sun, it blocks more and more (but not all) of its illuminated disk. As parts of the Sun remain visible during the partial phases (bad), we need proper eye protection to view them. Even if the Moon covers 99 percent of the Sun, the remaining 1 percent of sunlight is enough to do retinal harm.
Totality: Stove Off (remove eye protection). About 15 seconds before totality, an anemic solar crescent splits like quicksilver into beads of sunlight. Second by second, these beads "dry up" ... until one last bead of sunlight forms on the eastern limb of the Moon; this last event, called the Diamond Ring, signals the start of second contact ? the instant the Moon fully covers the Sun and the beginning of totality (good). At this point it is completely safe to remove your eclipse viewing glasses and see the Moon as a dark silhouette crowned by a white and gossamer glow called the corona. The corona is about as bright as the full Moon, so it too is a lovely sight to watch throughout the duration of totality.
Eclipse Wanes: Stove On Again (eye protection required). Third contact marks the end of totality. A second Diamond Ring swelling into view on the western limb of the Sun announces the return of the Sun (bad), so we must immediately put our safe viewing glasses or instrument solar filters back on and keep them on for the remainder of the eclipse, which ends at fourth contact, when the last bit of the Moon slips off the Sun's disk.
What is proper eye protection?
Inexpensive "eclipse glasses" or "solar glasses" featuring solar polymer film and cardboard frames can be found from a variety of stores and online sources. It is important to only use glasses from reputable sources, featuring solar film that has been rigorously tested for safety. Another common filter for safe naked-eye solar viewing is a shade number 14 welder's glass, which is available for a few dollars from welder supply shops; a 10-centimeter-square (4-inch square) glass will cover both eyes. Do not use sunglasses, smoked glass, CDs, DVDs, CD-ROMs, film negatives, or polarizing filters to look at the Sun — they are not safe for this purpose! Again, if in doubt about a filter's safety ... do not use it!
CAUTION: Never look at the Sun, either directly or through a telescope or binocular, without a professionally made protective solar filter installed that completely covers the front of the instrument, or permanent eye damage could result. When using a truss tube telescope to view the Sun, both a properly fitting solar filter and light shroud are required.
Stephen James O'Meara is an award-winning visual observer, whose writings, lectures, and numerous books on amateur astronomy have inspired observers across the globe to see the sky in new and wonderful ways. A contributing editor for Astronomy magazine, Stephen is an avid "eclipse chaser", having witnessed a dozen total solar eclipses dating back to 1959 (when he was 3 years old).