How to safely watch a solar eclipse | Science News for Students

How to safely watch a solar eclipse

Certified eyeware will protect against permanent harm to the eyes
Apr 27, 2017 — 7:05 am EST
Schoolgirls eclipse

Students keep their eyes safe with certified glasses when looking up at the sun.

American Paper Optics

Some people have been willing to travel halfway round the world to see a solar eclipse. But for many in the United States, that won’t be necessary on August 21, 2017. They’ll only need to walk outside to gaze at a partial or full eclipse of the sun. However, to view such a spectacle safely, they will need to take some special steps.

People have “watched eclipses in all history,” says Jarita Holbrook, a fellow at the National Science Foundation in Washington, D.C. (This physicist usually works at the University of the Western Cape in Cape Town, South Africa.) Ancient people “knew that the moon was covering up the sun,” she says. And they made up myths to describe why this happened. Some described the sun and moon as fighting. Others talked about a marriage between the sun and moon.

But just gazing up at the sun is a bad idea. Its intense light can easily damage eyes. “You have to wear your protective glasses before totality — before the moon totally blocks the sun,” warns Shadia Habbal. She’s an astronomer at the University of Hawaii at Manoa. Once totality happens, you must put the glasses back on just before the moon moves to let light through again. And people watching a partial eclipse must keep their glasses on throughout the entire event.

The eyes have it

The sun’s electromagnetic energy includes several types of radiation. Some of that is visible light. It’s why you see the sun. But there’s also ultraviolet light. It has wavelengths shorter than those of visible light, close to violet on the spectrum. You can’t see this radiation, but it can cause sunburn, DNA damage and eye injury. The sun also emits infrared radiation. It has longer wavelengths than red light and transfers heat.

If someone looks directly at the sun, its energy can harm light-sensitive cells called rods and cones. They’re found on the retina at the back of the eye. Harm to those cells can happen without warning. That part of the eye has no pain cells.

“The primary danger is photochemical,” says Rick Fienberg. He’s an astronomer in Boston, Mass., with the American Astronomical Society. “Intense sunlight can cause chemical reactions to occur in the retina that damage or destroy the rods and cones.” That harm could lead to partial or total blindness. And such damage can be permanent.

“The light interacts with materials within the cell,” explains Ralph Chou. He’s a vision scientist at the University of Waterloo in Ontario, Canada. These reactions produce molecules such as peroxide (Puhr-OX-ide) and hydroxyl (Hy-DROX-uhl) radicals. Those chemicals can easily swipe electrons from nearby atoms or molecules. And resulting cell-killing free-radical reactions can “very rapidly chew up the internal organelles of the cell.” Organelles are parts of a cell that do specific jobs. Depending on the harm, damage to them can be either temporary or permanent.

eclipse Japan
To view an eclipse safely, you will need eye protection. And your camera doesn’t count.
Takeshi Kuboki/Flickr (CC BY 2.0)

But that’s not all.

“The intense light and heat can actually cook the retina,” Feinberg notes. That could lead to a permanent blind spot. And looking at the sun through binoculars or a telescope could worsen the risk. Their lenses focus the sun’s energy on the eye. The resulting harm could occur almost instantly.

The danger is no less when the moon starts to block out the sun. “Whatever is not covered up by the moon is every bit as intense as it is outside of an eclipse,” says Chou. Indeed, as the visible light gets less bright, the eye’s muscles respond to let more light in. This can actually boost the danger.

Get certified

Regular sunglasses can’t protect eclipse viewers, even if their lenses are very dark. Homemade filters can’t guarantee safety either. Fortunately, there are ways to see a solar eclipse and keep eyes safe. The key is to get glasses or shields that are certified to meet an international standard known as ISO 12312-2. Chou helped develop it.

“The lens material is specially made to filter out 100 percent of harmful ultraviolet rays, 100 percent of harmful infrared rays and 99.999 percent of intense visible light,” explains Paulo Aur. Aur heads up operations at American Paper Optics in Bartlett, Tenn. That company and several others make glasses and viewers that have been tested by independent labs to meet the international standard.

The filter must be dense enough to provide protection, explains Chou. Yet viewers want a sharp solar image. So the filter must allow very little scattering of whatever light comes through. Otherwise, you’d notice “a wash-out of the image,” he says.

Certified eclipse glasses have plastic filters with light-absorbing material inside the plastic. Or, they can have a metal coating on the plastic that provides the protection. “They are both safe if they have been ISO-certified and are used properly,” Aur says. Glasses from his company have black polymer lenses with a silver coating outside. That gives an orange tint to the sun’s image and reflects more heat from your face, he says. Mylar lenses offered by some other companies give the sun a white tint.

Either way, the glasses are inexpensive. Schools and other groups can get fun designs for less than $1 per unit in orders of 100 or more. They can be bought online in smaller quantities for only a couple of dollars each. Some local science museums will give them away for free.

“Plan well in advance,” says Holbrook. “You need to have your eclipse viewing glasses at least two weeks before.” Otherwise, you may be disappointed if supplies run out.

Wherever you get your eclipse glasses, check them out before using them. Make sure the back shows that they are certified to meet the ISO standard. And make sure there are no scratches or holes.

Story continues below image. 

ISO certification
Make sure any eclipse glasses you use have no scratches and are certified to meet the standard ISO 12312-2 from the International Organization for Standardization.
K. M. Kowalski

Use common sense when you use the glasses. Turn away from the sun before putting them on or taking them off, says Habbal. And because they’ll make it very hard to see anything other than the sun, don’t walk around while wearing them!

Watch for other changes that also happen during an eclipse, says Holbrook. “It’s like nothing you’ve ever seen before in your life.” Some flowers will fold up as the sun’s light dims. Animals will scurry as if night is falling. At the same time, light filtering through trees will cast images of little crescent suns on the ground. The spaces between leaves will act like a pinhole camera. (A pinhole camera is another safe option for viewing the eclipse, because you never look directly at the sun.)

And be thankful you won’t have to take the same precautions that Habbal’s group needed in March 2015. That year, her science team went to Spitsbergen to study an eclipse. (That’s an island east of Greenland and north of the Arctic Circle.) “We had to wear warm clothing,” says Habbal. “But the biggest danger was the polar bears. The people of the town require that any group has some people who are trained to carry rifles in case of a bear attack.”

Power Words

(more about Power Words)

Arctic Circle     The northernmost point at which the sun is visible on the northern winter solstice and the southernmost point at which the midnight sun can be seen on the northern summer solstice.

atom     The basic unit of a chemical element. Atoms are made up of a dense nucleus that contains positively charged protons and uncharged neutrons. The nucleus is orbited by a cloud of negatively charged electrons.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Some organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

chemical     A substance formed from two or more atoms that unite (bond) in a fixed proportion and structure. For example, water is a chemical made when two hydrogen atoms bond to one oxygen atom. Its chemical formula is H2O. Chemical can also be used as an adjective to describe properties of materials that are the result of various reactions between different compounds.

chemical reaction     A process that involves the rearrangement of the molecules or structure of a substance, as opposed to a change in physical form (as from a solid to a gas).

cones    (in biology) A type of eye cell that is part of the retina inside the back of the eye. These cells can sense red, green or blue light. And recent research has uncovered evidence that some can sense white light — but only white light. 

DNA     (short for deoxyribonucleic acid) A long, double-stranded and spiral-shaped molecule inside most living cells that carries genetic instructions. It is built on a backbone of phosphorus, oxygen, and carbon atoms. In all living things, from plants and animals to microbes, these instructions tell cells which molecules to make.

eclipse     This occurs when two celestial bodies line up in space so that one totally or partially obscures the other. In a solar eclipse, the sun, moon and Earth line up in that order. The moon casts its shadow on the Earth. From Earth, it looks like the moon is blocking out the sun. In a lunar eclipse, the three bodies line up in a different order — sun, Earth, moon — and the Earth casts its shadow on the moon, turning the moon a deep red.

electromagnetic     An adjective referring to light radiation, to magnetism or to both.

electron     A negatively charged particle, usually found orbiting the outer regions of an atom; also, the carrier of electricity within solids.

filter     (in physics) A screen, plate or layer of a substance that absorbs light or other radiation or selectively prevents the transmission of some of its components.

free radical    A charged molecule (typically highly reactive and short-lived) having one or more unpaired outer electrons. It will attempt to steal electrons to make itself whole again through a process known as oxidation.

Greenland     The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. Indeed, Vikings arrived in Greenland around the 10th century, and for a time the island was a colony of Denmark. In June 2009, Greenland became an independent nation.

lens     (in optics) A curved piece of transparent material (such as glass) that bends incoming light in such a way as to focus it at a particular point in space. Or something, such as gravity, that can mimic some of the light bending attributes of a physical lens.

metal     Something that conducts electricity well, tends to be shiny (reflective) and malleable (meaning it can be reshaped with heat and not too much force or pressure). 

molecule     An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).

moon     The natural satellite of any planet.

muscle     A type of tissue used to produce movement by contracting its cells, known as muscle fibers. Muscle is rich in protein, which is why predatory species seek prey containing lots of this tissue.

National Science Foundation     The U.S. Congress created this independent federal agency in 1950 to promote the advancement of science; national health, prosperity and welfare; and the nation’s defense. This agency funds nearly one-fourth of all federally supported basic research in U.S. colleges and universities. In many fields such as mathematics, computer science and the social sciences, NSF is the major source of federal funding.

online     (n.) On the internet. (adj.) A term for what can be found or accessed on the internet.

optics     Having to do with vision or what can be seen.

organelle     Specialized structures, such as mitochondria, found within a cell.

peroxide     A group of chemicals that contain a “bivalent” pair of oxygen atoms. Each oxygen atom has an unpaired electron orbiting it that is available to form bonds (attachments) with other atoms. Peroxides are oxidizing agents, meaning that they can react vigorously at room temperatures. Some are used as bleaches.

photochemistry      (adj. photochemical) A branch of science that deals with the chemical reactions that are triggered by exposure to light.

physicist     A scientist who studies the nature and properties of matter and energy.

plastic     Any of a series of materials that are easily deformable; or synthetic materials that have been made from polymers (long strings of some building-block molecule) that tend to be lightweight, inexpensive and resistant to degradation.

polymer     A substance made from long chains of repeating groups of atoms. Manufactured polymers include nylon, polyvinyl chloride (better known as PVC) and many types of plastics. Natural polymers include rubber, silk and cellulose (found in plants and used to make paper, for example).

radiation     (in physics) One of the three major ways that energy is transferred. (The other two are conduction and convection.) In radiation, electromagnetic waves carry energy from one place to another. Unlike conduction and convection, which need material to help transfer the energy, radiation can transfer energy across empty space.

radical     A charged molecule having one or more unpaired outer electrons. Radicals readily take part in chemical reactions. The body is capable of making radicals as one means to kill cells, and thereby rid itself of damaged cells or infectious microbes.

retina     A layer at the back of the eyeball containing cells that are sensitive to light and that trigger nerve impulses that travel along the optic nerve to the brain, where a visual image is formed.

rods    (in biology) A type of eye cell that is part of the retina inside the back of the eye. These cells are rod shaped and sensitive to light. Although more sensitive to light than cone cells are, rods can not tell what color something is.

solar eclipse     An event in which the moon passes between the Earth and sun and obscures the sun, at least partially. In a total solar eclipse, the moon appears to cover the entire sun, revealing on the outer layer, the corona. If you were to view an eclipse from space, you would see the moon’s shadow traveling in a line across the surface of the Earth.

spectrum     (plural: spectra) A range of related things that appear in some order. (in light and energy) The range of electromagnetic radiation types; they span from gamma rays to X rays, ultraviolet light, visible light, infrared energy, microwaves and radio waves.

sun     The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Also a term for any sunlike star.

telescope     Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.

thermal     Of or relating to heat. (in meteorology) A relatively small-scale, rising air current produced when Earth’s surface is heated. Thermals are a common source of low level turbulence for aircraft.

totality     (in astronomy) The brief period during an eclipse when one object totally obscures another. For a solar eclipse (when viewed from Earth), this would be when the moon appears to completely block out the sun’s light.

ultraviolet light     A type of electromagnetic radiation with a wavelength from 10 nanometers to 380 nanometers. The wavelengths are shorter than that of visible light but longer than X-rays.

wavelength     The distance between one peak and the next in a series of waves, or the distance between one trough and the next. Visible light — which, like all electromagnetic radiation, travels in waves — includes wavelengths between about 380 nanometers (violet) and about 740 nanometers (red). Radiation with wavelengths shorter than visible light includes gamma rays, X-rays and ultraviolet light. Longer-wavelength radiation includes infrared light, microwaves and radio waves.


Meeting: S. Habbal et al. Dynamics and thermodynamics of the solar corona as inferred from total solar eclipse observations. Trienniel Earth Sun-Summit. April 27-30, 2015. Indianapolis, Ind.

Further Reading

National Aeronautics and Space Administration. How to make a pinhole camera.