by Gabrielle Bais
On April 8th 2024, a total solar eclipse was visible across the continental United States. This is a rare point in time in which the moon either partially or totally blocks the sun. The next total solar eclipse visible in the United States is predicted for March 30, 2033, so if given the opportunity, you should try to see the eclipse! It is well known that it is dangerous to stare directly at the sun during an eclipse. Relatedly, Google Trends showed searches for “hurt eyes” rapidly spiked after the April 8th solar eclipse, suggesting that people may have hurt their eyes by attempting to look directly at the solar eclipse. But if the moon is covering some or all of the sun during a solar eclipse, why is it dangerous to watch the eclipse with your naked eyes?
Figure 1. The path of totality for the April 8th, 2024 solar eclipse. The locations marked in black delineate the regions and respective times where a total eclipse was visible. Note that areas outside of the path of totality may have seen a partial solar eclipse. Source: NASA’s Scientific Visualization Studio.
First and foremost, looking directly at the sun is always damaging to our eyes. Most of the time we will instinctively squint and look away to avoid looking directly at the sun for too long because the human eye is too sensitive to handle direct sunlight. Aside from the bright visible light, the sun emits invisible ultraviolet (UV) rays. Short-term exposure to UV rays can be beneficial since it helps us produce vitamin D which is crucial for our overall and bone health. To get these benefits the World Health Organization recommends 5-15 minutes of sun exposure 2-3 times per week. Too much exposure to UV rays from the sun can lead to sunburn or, in more extreme cases, skin cancer or blindness. To reduce the risk of developing these conditions, the CDC recommends wearing sunscreen with sun protection factor (SPF) 15 or higher and UV-protective sunglasses to protect our skin and eyes. However in the case of solar eclipses, tinted glass or plastic sunglasses are not sufficient protection for looking directly at the sun.
Figure 2. Diagram showing how vision works. Light enters your eye through the cornea and pupil (an opening in the iris). The light is then focused by the lens and hits the photoreceptors on your retina. The photoreceptors generate a signal that is sent down the optic nerve which sends the visual message to the brain. Source: National Keratoconus Foundation
If you were to look directly at the sun, UV rays would be absorbed by photoreceptors in the retina of your eyes. Photoreceptors are specialized cells that transmit visual cues to our brains and allow us to see light and color. The UV rays from the sun are too intense for the naked human eye and will result in degradation of the retina merely seconds or minutes after viewing the sun directly. Since the retina is not connected to any nerve endings we cannot feel pain from UV damage to our retinas. This process of retina degradation is referred to as solar retinopathy. This condition can cause temporary or permanent loss or distortion of vision. If you at all suspect you may have changes in vision after looking directly at the sun during a solar eclipse or otherwise, you should consult an eye doctor.
During rare events like solar eclipses it is easier for us to look at the sun without our instincts kicking in to turn away, because the moon is blocking a portion of the sun. Similar to how a large cloud, tall tree, or building will shade your surroundings when blocking the sun, the moon will fully or partially block sunlight during a solar eclipse. But even with the moon blocking the sun, damaging UV rays are still transmitted and we are at risk of damaging our retinas if we attempt to look at the sun during a partial eclipse. As long as you are located in the path of totality it is completely safe to look at the sun during the few minutes of totality because the moon will completely cover the sun at this time. In fact, if you tried looking at the sun with eclipse glasses during totality you wouldn’t be able to see anything! But since the complete totality phase only lasts for a few minutes, you should put your glasses back on as soon as you start to see sun peeking through from behind the moon again as looking at a partial eclipse without proper eye protection is never safe.
Figure 3. Timelapse of the progression of a total solar eclipse taken in Madras, Oregon on August 21st, 2017. Source: NASA/Aubrey Geminagi
In short, it is always a bad idea to stare directly at the sun (even during a solar eclipse when the sun looks especially cool). But fear not, there is still a way to safely witness a solar eclipse firsthand. Protective eyewear like solar eclipse glasses or solar viewers are specifically designed to filter out enough direct sunlight for safe viewing of a solar eclipse. These eyewear protect our eyes using filters made of either polyester film coated in aluminum or black polymer that are opaque enough that you can only see the sun shining through. Make sure that any protective eyewear you plan to use for the next eclipse has International Organization for Standardization (ISO) certification to guarantee that they will protect your eyes from the sun. After news that certain manufacturers were selling counterfeit glasses that are not eclipse-safe, the American Astronomical Society has compiled a helpful list of reputable suppliers of solar eclipse glasses.
Figure 4. Image of people properly using solar eclipse glasses to look at the eclipsed sun. Source: NASA/Mamta Patel Nagaraja
To summarize, the reason why we cannot look directly at the sun during a solar eclipse is because harmful UV rays from the sun are powerful enough to damage our retinas which allow us to see. Depending on the length and duration of direct sun exposure, the effects of UV light on our eyes can cause temporary or permanent blindness. However, this fact should not scare you away from experiencing rare solar eclipses. As long as you use certified protective eyewear when looking at the sun directly during the eclipse, you are free to enjoy this perhaps once in a lifetime experience!
