Yesterday, a transit of Venus occurred, the last until 2117. The pictures are stunning. And I got to see it in the brief moments that the clouds cleared in New York City!
You need special glasses or a filter on your telescope to be able to look directly at the sun. Why? Can looking at the sun really cause blindness? And is it more dangerous to look at the sun during an astronomical event?
“Exposure of the retina to intense visible light causes damage to its light-sensitive rod and cone cells.” [Rods perceive brightness and cones perceive color.] “The light triggers a series of complex chemical reactions within the cells which damages their ability to respond to a visual stimulus, and in extreme cases, can destroy them. The result is a loss of visual function which may be either temporary or permanent, depending on the severity of the damage. When a person looks repeatedly or for a long time at the Sun without proper protection for the eyes, this photochemical retinal damage may be accompanied by a thermal injury - the high level of visible and near-infrared radiation causes heating that literally cooks the exposed tissue. This thermal injury or photocoagulation destroys the rods and cones, creating a small blind area. The danger to vision is significant because photic retinal injuries occur without any feeling of pain (there are no pain receptors in the retina), and the visual effects do not occur for at least several hours after the damage is done.”
The only safe time to look at the sun is when it is completely covered during a total eclipse. At all other times, we need protective eyewear. And when we stare for long periods of time, as during a planetary transit or solar eclipse, that eyewear needs to filter out even more visible light and near-infrared radiation than day-to-day sunglasses.
Now you’re prepared for 2117 (or sooner).