Venus will cross the face of the sun at around 6pm Eastern tonight in a celestial arabesque that will not be repeated until 2117. But in your haste to view the transit, use protective gear -- you could be blinded temporarily or even permanently.
Since Venus will only be covering a tiny portion of the solar disk -- between 2 percent and 3 percent -- watching the event with your naked eye will basically be like staring directly into the sun.
Our eyes work by receiving light that starts off a chain of complex chemical reactions in rod and cone cells. Staring at the sun with your naked eye causes an overwhelming amount of light to enter these tissues and kick those reactions into overdrive, explains B. Ralph Chou, an associate professor of optometry at the University of Waterloo in Canada who penned a guide to eye safety during eclipses and other phenomena posted by NASA.
Essentially what you've got is a runaway chain reaction in these cells, Chou says.
This chain reaction can impair or even kill cells in the eye, leaving retina burns that may not heal for months -- or ever.
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What's especially dangerous is that since there are no pain receptors in the retina, you wouldn't be able to feel your eyes being damaged, and any actual vision impairment wouldn't occur until at least several hours after the damage is done, according to Chou.
Special filters -- not sunglasses! -- can reduce amount of light coming into the eye to a safe level. Another method of viewing the transit is to use a pinhole camera, which is essentially a box with a small hole through which light rays pass, projecting an inverted image on a translucent screen.
Parts of eastern North America will be catching the transit as the sun sets, which may actually afford a chance to see the transit safely without any protective gear.
That's because when the sun is close to the horizon, its light is passing through more of Earth's atmosphere. The atmosphere scatters much of the short wavelengths of blue and green light that can damage the eye. Still, use caution.
If the sun is dark red like at a sunset, that's probably safe to have a quick glance at it. But if it's anything other than a deep red, use protection, Chou says.