Why staring at the sun is a bad idea

The burned-out pig's eye (YouTube/Mark Thompson)
The burned-out pig's eye (YouTube/Mark Thompson)

Don't stare at the sun. It's advice you shouldn't need after the age of three, but just in case, there's now video showing why it's a bad idea.

British astronomer and TV host Mark Thompson took a pig's eyeball and held it up to a telescope, as you would were you looking through it yourself.

Within 20 seconds it began smouldering, burning a hole right through the cornea and into the retina, where light is turned into electronic signals for the brain to interpret.

"If this was a human eye, there would almost certainly be some pretty severe damage," he explains, holding the charred, dissected eye.

"That is why it's a very, very bad idea to look at the sun through the eyepiece of a telescope."

Telescopes work by focusing light on a small area. If all that light is coming directly from the sun, as anyone who's ever burned an ant with a magnifying glass knows, it can be extremely hot.

Looking at the sun with the naked eye won't be quite so dramatic. It takes about 100 seconds of constant exposure to cause permanent damage this way.

The warning comes ahead of the transit of Mercury, which takes place on the morning of May 10 (NZ time). It won't be visible from New Zealand.