The day has finally come. Whether you’re exhaling from the soon-to-end mania or enthralled by the forthcoming spectacle, there’s one reminder we should all hear again: DO NOT LOOK AT THE SUN. It really is worth saying because it is a big deal.
Why? Isn’t looking at the sun always bad? Yes, but this is different. Because when else does a day call for you to look directly at the sun? Exactly; today is special.
But even though we all know it is bad to look at the sun, many of us don’t know exactly why. Surprisingly, the official response from NASA isn’t stated in big bold letters on its main eclipse safety page, so we parsed it out so that you don’t have to waste any more time googling the eclipse. Here’s what NASA has to say as found on its FAQ page.

Photos courtesy of Thinkstock.

NASA’s Response

The rods and cones in the human retina are very sensitive to light. Even a thin sliver of the sun’s disk covers thousands of these light-sensitive cells. Normally during daylight conditions, the iris contracts so that only a small amount of light passes through the lens and then reaches the retina. This level of indirect sunlight is perfectly OK and the eye has evolved over millions of years to safely see the daylight world under most circumstances. The problem is that the sun’s surface is so bright that if you stare at any portion of it, no matter how small, it produces enough light to damage individual retinal cells.  It takes a few seconds for this to happen, but afterward, you will see a spot as big as the solar surface you glimpsed when you look away from the sun at some other scenery. Depending on how long you gazed at the sun and how badly the retinal cells were damaged,  this spot will either fade away in time or remain permanent.  You should never assume that you can look away quickly enough to avoid eye damage because every person is different in terms of their retinal sensitivity, and you do not want to risk being the one who damages their eyes just to try to look at the sun. If you want to see what the sun looks like, use a properly-equipped telescope. Or why not just go online and view thousands of pictures taken of the sun by telescopes and NASA spacecraft?

Solar retinopathy is a result of too much ultraviolet light flooding the retina. In extreme cases, this can cause blindness but is so painful that it is rare for someone to be able to stare at the sun for that long. Typically, eye damage from staring at the sun results in blurred vision, dark or yellow spots, pain in bright light or loss of vision in the center of the eye (the fovea). Permanent damage to the retina has been shown to occur in ~100 seconds, but the exact time before damage occurs will vary with the intensity of the sun on a particular day and with how much the viewer’s pupil is dilated from decongestants and other drugs they may be taking. 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. Note there are no pain receptors in the retina so your retina can be damaged even before you realize it, and by then it is too late to save your vision!

English, please

Basically, NASA is saying that during a normal day your eyes adjust to take in less light making it safe to see during the day. But if you stare directly at the sun — no matter how big or small the visible portion is — the intensity is too bright and can burn your eyes and cause permanent damage. This can happen within seconds and without you noticing because your retina has no pain receptors. So case in point, don’t take your certified glasses off for any amount of time today or you may end up like this guy.

If you’re planning on watching it from Denver, the eclipse starts at 11:47 a.m. and 92.3 percent of the sun will be obscured. Here are the exact times for other cities in Colorado.