Eclipse Eye Safety
The first thing to remember about observing an eclipse is safety. A lunar eclipse -- an eclipse of the Moon -- is perfectly safe to watch with the naked eye; you're only looking at the Moon, at night, which is quite safe. A solar eclipse is potentially dangerous, however, because viewing a solar eclipse involves looking at the Sun, which can damage your eyesight.
This page therefore contains some information on eye safety during a solar eclipse.
The Danger of the Sun
A solar eclipse can be viewed safely with the naked eye only during the few brief seconds or minutes of a total solar eclipse, when the Sun itself is completely obscured by the Moon. Partial eclipses, annular eclipses, and the partial phases of total eclipses are never safe to watch without taking special precautions.
Even when 99% of the Sun's surface is obscured during the partial phases of a total eclipse, the remaining crescent is intensely bright -- just as intense as at any other time -- and cannot be viewed safely without eye protection. Do not attempt to observe the partial or annular phases of any eclipse with the naked eye. Failure to use appropriate filtration may result in permanent eye damage or blindness!
It is never safe to look directly at the Sun except during a total eclipse; a partial or annular eclipse, even when the Sun is mostly covered, can still cause permanent eye damage, even though you might not feel any discomfort. Looking at the Sun through any kind of optical aid (binoculars, a telescope, or even a camera's viewfinder) is extremely dangerous, and can cause permanent blindness.
Sunglasses do not provide anything like adequate protection, as they do not block the wavelengths of light which are likely to damage your eyes, or reduce the intensity of the visible light sufficiently. Various other ad-hoc solar filters, such as welder's goggles or using fully exposed and developed black-and-white negatives, are sometimes discussed, but unless you know exactly what you are doing, can be extremely dangerous, and so can't be recommended.
Properly designed solar filters, made and certified to appropriate national safety standards, should be safe. A commonly available type uses aluminized mylar, in a dual sandwich with the aluminium on the inside. (This means that you're actually looking through a double layer of metal.) However, there does exist a risk with using viewing glasses if they aren't in perfect condition; if in doubt, throw them away.
Viewing the Sun indirectly, by projecting its image onto a screen, is far safer. You can make a projector with a simple pinhole, or with binoculars or a telescope, as described in Observing Eclipses. However, never look through the projector -- only look at the image on the screen.
Note that a screen refers to a matte surface, such as a white sheet, or a piece of paper, so that the Sun's image can be seen by anyone looking at it from any angle. Looking at a reflection of the Sun in any shiny surface is basically the same as looking directly at the Sun.
There's been such hysteria -- allbeit well-intentioned -- stirred up about eye damage, that many people are convinced that it is specifically solar eclipses that cause eye damage; that is, at any other time of year, looking at the Sun is OK. This is not true. Looking at the Sun at any time for more than a second or two can cause permanent eye damage.
Finally, I've heard some truly daft ideas for eclipse viewing, such as looking through a sheet of Perspex, or in a reflection in a bucket of water. I have no idea where these come from, but these are not safe! If you can see the Sun clearly and brightly, whether directly, in a reflection, or via Perspex, then it's dangerous.
NEVER attempt to look at the Sun through a telescope, camera, binoculars, or anything else!
Copyright (C) 1995-2006 Ian Cameron Smith.
visits since 18Aug05. Last modified: Sat May 3 11:44:21 PDT 2008 ($Revision: 1.2 $)