Most people are aware that it is not safe to stare at the Sun. Humans have an automatic response to look away or close their eyes when attempting to observe the Sun. This is a natural process that helps to protect our eyesight. When an eclipse is known to be occurring, children have a natural curiosity and may be tempted to try to look. They can be less aware than adults of the dangers of staring at the Sun. This is exacerbated when there is a deep partial eclipse and the Sun is reduced to a thin crescent as the "look away" response may be reduced. The message that viewing of the Sun without proper precautions at any time is dangerous needs to be reinforced, especially for young children.
The primary concern over unprotected viewing of the Sun during an eclipse (or at any other time) is the development of "eclipse blindness" or retinal burns. Exposure of the retina to intense visible light causes damage to its light-sensitive rod and cone cells. The light triggers a series of complex chemical reactions within the cells which damage their ability to function and in extreme cases, can destroy them. The result is a loss of vision, which may be either temporary or permanent depending on the severity of the damage.
In addition, the high level of visible and near-infrared radiation can cause heating that literally cooks the exposed tissue. This thermal injury or photocoagulation destroys the rods and cones, creating a blind spot burned into the retina.
There are two effects which compound the danger. Firstly the injuries occur without any feeling of pain (the retina has no pain receptors) and secondly the visual effects may not become apparent for at least several hours after the damage is done. For these reasons, significant damage may occur without the victim being aware of it.
Because optical instruments intensify the light, looking at the Sun through an unfiltered telescope or binoculars is very dangerous as it can cause permanent damage almost immediately.
It is never safe to look directly at a partial or annular eclipse or the partial phases of a total solar eclipse without the proper equipment and techniques. Even when 99% of the Sun's surface (the photosphere) is obscured during the partial phases of a solar eclipse, the remaining crescent Sun is still intense enough to cause a retinal burn, even though illumination levels may be comparable to twilight. Failure to use proper observing methods may result in permanent eye damage and severe visual loss.
Safe viewing techniques must be used for the partial eclipse that will occur across Queensland and for the partial phases that precede and follow the total eclipse.
The Queensland government has issued the following advice for safe viewing of the eclipse.
- Never look directly at the Sun without proper eye protection. It is possible to suffer serious and permanent eye damage by looking at a solar eclipse the wrong way, even for a very short time.
- Always use solar eclipse glasses, or filters that have been made specifically to attach to hand-held glasses, telescopes or binoculars for safe solar eclipse viewing.
- Look for filters that have been appropriately certified against the European Standard for personal eye equipment (EN1836:2005 +A1:2007) or the Australian Standard for welding shields and goggles with a lens category higher than 12 (AS/NZS 1338.2:1992 & AS/NZS 1338.1:1992).
- Before using solar eclipse glasses or filters, check to see if they are scratched or damaged. If so, do not use them as they will not fully protect your eyes.
- Do not use solar eclipse glasses or filters that do not show compliance with the Standards listed above – they may do you more harm than good.
- Do not look directly through binoculars, telescopes or camera optical viewfinders. It is not safe to use regular sunglasses, exposed film or x-ray film to view a solar eclipse.
- Alternative safe viewing methods include using pinhole, binocular or telescope projection. Visit http://sunearthday.nasa.gov/2012/transit/viewing.php for details on how to do this safely. For information on filters for telescopes or binoculars visit http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html.
This advice is available on the following Queensland government website: http://www.fairtrading.qld.gov.au/safe-viewing-of-astronomical-events.htm.
The NASA solar eclipse website also has information describing how to view a solar eclipse safely. They advise that "the total phase of an eclipse can and should be viewed without any filters whatsoever. The naked eye view of totality is not only completely safe, it is truly and overwhelmingly awe-inspiring!" See http://eclipse.gsfc.nasa.gov/SEhelp/safety2.html for the full text.
So, if you are in the path of totality (and you must be certain of this) you must remove your filter (eg eclipse glasses) in order to see the total part of the eclipse, but you must keep the filter on until the Sun is no longer visible (even the tiniest part) and it goes very dark; and you must look away or replace the filter immediately any part of the Sun reappears.
A safe and inexpensive method of observing the Sun during a partial solar eclipse is to view an image of it formed by projection. Use two pieces of card, one with 1 to 2 mm hole in it. Stand facing away from the Sun and project an image of the Sun through the hole in the card onto the other card. Do not look at the Sun through the hole in the card. The crescent shape of the Sun can easily be seen during the partial phases of a solar eclipse. See figure 5-2 and also Activity 10).
Also, during a partial solar eclipse, multiple openings in pegboard, in a loosely woven straw hat, or even between interlaced fingers can be used to cast a pattern of solar images on a screen, on a wall or on the ground. A similar effect is seen on the ground below a broad-leafed tree: the many "pinholes" formed by overlapping leaves create hundreds of crescent-shaped images. Binoculars or a small telescope mounted on a tripod can also be used to project a magnified image of the Sun onto a white card. This method can be effective in showing sunspots (see activity 11).
Commercially manufactured solar projection devices are available. One such example is a Solarscope (www.solarscope.com). See Figure 5-3.
All of these methods can be used to provide a safe view of the partial phases of an eclipse to a group of observers but care must be taken to ensure that no one looks through the pinhole or telescope or binoculars and that any finderscope is covered. The projection method uses indirect viewing with the main advantage being that nobody is looking directly at the Sun.
The potential for skin damage from harmful rays of the Sun are well known. Schools develop sun safety strategies to protect students from the dangers of over-exposure to the Sun, for example by minimising exposure times, requiring the use of protective clothing and encouraging the use of broad spectrum sunscreen. The wearing of sunglasses is normally encouraged to guard against long term UV damage to eyesight.
Messages of protecting eyesight from inappropriate viewing of the Sun and how to observe the Sun safely during an eclipse are natural complements to these initiatives. Students should be encouraged to spread the message of how to view the eclipse safely to their family and friends.
Some of the astronomical societies in Queensland provide support to their local schools. As well as night sky tours or invited talks, some astronomical societies have equipment that is designed for safe viewing of the Sun. Many societies are happy to assist if approached, to arrange astronomical viewing or for advice about the eclipse.
The Astronomical Association of Queensland is very happy to respond to email enquiries about the eclipse or other astronomical phenomena or activities. Send any enquiry using the Contact form. See Activity 14 for more information.