Abstract

There are several simple ways to view solar eclipses safely using little or no special equipment.

1. Introduction

A solar eclipse occurs when the new Moon obscures part or all of the Sun’s photosphere and casts a shadow on Earth. Between 2017 and 2024 three major solar eclipses favored the Americas, ending a decades-long drought. On 21 August 2017, a total solar eclipse crossed the continental U.S. from coast to coast. On 14 October 2023, an annular (“ring”) solar eclipse was visible from parts of North, Central, and South America. On 8 April 2024, a total solar eclipse crosses Mexico, the U.S. from Texas to Maine, and parts of eastern Canada. In each case, some half a billion people experienced (or will experience) at least a partial solar eclipse.

During any solar eclipse, eye safety is paramount. The American Astronomical Society (AAS) Solar Eclipse Task Force, charged with helping to ensure safe and enjoyable viewing of the 2017, 2023, and 2024 American solar eclipses, promoted the following safety messaging through its website (eclipse.aas.org), press conferences at AAS meetings, press releases, and social media. It was developed by task force members and colleagues including experienced eclipse chasers, NASA heliophysicists, and eye doctors.

2. Direct Viewing with Eyes Alone

Except during the brief total phase (“totality”) of a total solar eclipse, looking directly at the Sun without proper eye protection is unsafe. Even a tiny sliver of the photosphere is bright enough to cause serious retinal injury if glanced at for more than a split second (Chou, 2023). In contrast, the solar corona is roughly the same brightness as a full Moon, so it’s just as safe to stare at during the few minutes when the Moon entirely covers the Sun’s bright face. This happens only within the narrow path of the Moon’s umbral shadow during a total solar eclipse.

The only safe way to look directly at the uneclipsed, partially eclipsed, or annularly eclipsed Sun is through special-purpose “eclipse glasses” (available with cardboard or plastic frames) or handheld solar viewers (Figure 1). Ordinary sunglasses, even very dark ones, are not safe for looking at the Sun; they transmit at least 1,000 times more sunlight than is safe for direct viewing.

A welding filter with a shade number of 13 or 14 is safe for solar viewing, but you should not use adjustable and/or auto-darkening welding helmets or similar products to view the Sun. Many don’t go as dark as shade 13 or 14, and even those that do post a grave risk to your eyesight, either because you accidentally adjust them to an unsafe setting or because they don’t auto-darken fast enough when you look at the Sun with them.

In 2015 the International Organization for Standardization adopted the ISO 12312-2 standard for filters for direct observation of the uneclipsed, partially eclipsed, or annularly eclipsed Sun (ISO, 2015). Safe solar viewers are those with maximum ultraviolet, visible, and infrared transmittances no higher than the values specified in ISO 12312-2. At visible wavelengths, safe filters that provide comfortable unmagnified views of the Sun generally transmit between 1 part in 100,000 and 1 part in 2.5 million of its light.

The AAS Solar Eclipse Task Force has compiled a list of vendors of safe solar viewers. For every seller on the list, we’ve confirmed three things: the identity of the manufacturer, that the manufacturer’s viewers have been tested against the ISO 12312-2 standard by a lab properly accredited to do so, and that the viewers meet the standard’s transmittance requirements across the parts of the spectrum to which our eyes are at risk from overexposure.

3. Instructions for the Safe Use of Solar Viewers

The following safety tips apply to partial solar eclipses, annular solar eclipses, the partial phases of total solar eclipses, and times when no eclipse is occurring at all:

• Always inspect your solar viewer before use; if scratched, punctured, torn, or otherwise damaged, discard it. Read and follow any instructions printed on or packaged with the viewer.

• Always supervise children using solar filters.

•  If you normally wear eyeglasses, keep them on. Put your eclipse glasses on over them or hold your handheld viewer in front of them.

•  Stand still and cover your eyes with your eclipse glasses or solar viewer before looking up at the bright Sun. After looking at the Sun, turn away and remove your viewer — do not remove it while looking at the Sun.

•  Do not look at the Sun through an unfiltered camera, telescope, binoculars, or other optical device.

•  Similarly, do not look at the Sun through an unfiltered camera, telescope, binoculars, or any other optical device while using your eclipse glasses or handheld solar viewer in front of your eyes — the concentrated solar rays could damage the filter lenses, enter your eyes, and cause serious injury.

•  Seek expert advice before using a solar filter with a camera, telescope, binoculars, or any other optical device; note that solar filters must be attached securely to the front of any telescope, binoculars, camera lens, or other optics.

4. Additional Instructions for a Total Solar Eclipse

• If you are inside the path of totality, remove your solar viewer only when the Moon completely covers the Sun’s bright face and it suddenly gets quite dark. Experience totality, then, as soon as the bright Sun begins to reappear, replace your solar viewer to look at the remaining partial phases. Note that this applies only to viewing without optical aid (other than ordinary eyeglasses). Different rules apply when viewing or imaging the Sun through camera lenses, binoculars, or telescopes; consult an expert before using a solar filter with any type of magnifying optics.

• Outside the path of totality, and throughout a partial solar eclipse, there is no time when it is safe to look directly at the Sun without using a special-purpose solar viewer that complies with the transmittance requirements of the ISO 12312-2 standard.

Not everyone in a family or other group of observers needs their own eclipse glasses or handheld solar viewer. A partial or annular solar eclipse, and the partial phases of any eclipse, progress quite slowly; there’s no point in watching continuously. If you instead take a brief glance every few minutes, the motion of the Moon across the Sun’s face will be readily apparent. There’s plenty of time to share a small number of solar viewers among all the members of your group.

4.1 Pinhole Projection

Another convenient method for safe viewing of the partially eclipsed Sun is indirectly via pinhole projection. With the Sun at your back, pass sunlight through a small opening (for example, a hole punched in an index card) to project an image of the Sun onto a nearby surface (for example, another card, a wall, or the ground). During the partial phases of a solar eclipse, the image will reveal the Sun's crescent shape. A colander makes a terrific pinhole projector, as does a straw hat, a perforated spoon, or anything else with lots of small holes in it. Do not look at the Sun through the pinhole(s)!

You don't need any special equipment to create a pinhole projector: Just cross the outstretched, slightly open fingers of one hand over the outstretched, slightly open fingers of the other in a waffle pattern (Figure 3). With your back to the Sun, look at your hands’ shadow on the ground. The little spaces between your fingers will project a grid of small images of the Sun on the ground. If your observing site has leafy trees, look at the shadows of leaves on the ground. During the partial phases of any solar eclipse, the tiny spaces between the leaves will act as pinhole projectors, dappling the ground with images of the crescent Sun. During an annular eclipse, the projected images will be little rings.

Pinhole projection is not useful for observing the total phase of a total solar eclipse; the projected image will be too faint to see. Again, during totality, it is perfectly safe to look directly at the eclipsed Sun.

4.2 Optical Projection

You can also use a telescope or binoculars to project images of the uneclipsed, partially eclipsed, or annularly eclipsed Sun onto a surface for convenient viewing. This is called optical projection. Compared with pinhole projection, optical projection generally provides bigger, brighter, sharper images.

Because passing unfiltered sunlight through binoculars or a telescope can damage the device, and because of the danger that someone might look at the Sun through the device and injure their eyes, you should not attempt optical projection unless you are an experienced observer, are using your own equipment, and can remain with your equipment at all times to supervise its use, especially when children are around.

A solar eclipse is one of nature’s grandest spectacles. By following these simple rules, you can safely enjoy the view and be rewarded with memories to last a lifetime.

Acknowledgement

This article is adapted from “How to View a Solar Eclipse Safely” at eclipse.aas.org. It does not constitute medical advice. Readers with medical questions should contact a qualified eye-care professional.