We see the Sun because it emits visible light, but it also emits ultraviolet and infrared radiation too and looking at the Sun allows this radiation to enter the eye. The light sensitive part of the eye, the retina, has no pain sensors or nerves. This means that the infrared light from the Sun can literally burn the retina and as this is totally painless you won’t even feel it! This could cause blindness, or in the least black spots, and once the retina has been damaged it cannot be repaired. The ultraviolet radiation also damages the eye and can contribute to the formation of cataracts.
So, when you are watching either a transit of a planet or an eclipse there are certain safety rules that must be followed. The Sun should certainly never be viewed directly through a telescope, which collects large amounts of light, or binoculars.
- Never look directly at the Sun with the naked unprotected eye as this could cause blindness within seconds
- Never look directly at the Sun through binoculars or a telescope which collects a large amount of sunlight
- If you are using a telescope with a solar filter make sure that you have checked the safety guidelines with your supplier and know how to use it correctly
- Dedicated solar filters on a telescope should always be fitted at the front end and not into the eye piece
- Never look at the Sun using sunglasses, photographic film or smoked glass
There are filters available which can block most of the visible, ultraviolet and infrared light and allow safe viewing of the Sun. It is this type of filter which is used in eclipse viewers. If a filter is being used:
- Read and follow the manufacturer’s instructions carefully
- Make sure the filter has a ‘CE’ mark and that the filter is approved for direct solar viewing
- Check the filter carefully for any damage
- Do not use the filter if it is scuffed, scratched or has holes
You might remember the solar eclipse in 1999 when, for several seconds, the light from the Sun was blocked by the Moon and it was safe to look directly at the Sun’s atmosphere. This is generally not the case, so at all times transits and eclipses must be viewed by methods which reduce the light reaching the eyes, such as special safety glasses or filters, or by indirect viewing methods such as projecting an image of the Sun. It might sound obvious, but don’t forget – it’s better to be safe than sorry.
For more information please view this leaflet created by the Royal Astronomical Society.