According to NASA, a total coast-to-coast eclipse of the sun hasn’t been seen in the U.S. in 99 years, so the Aug. 21 event will be a can’t-miss for many of us. A total solar eclipse is caused by a unique set of circumstances. Oddly enough, the sun is 400 times larger than the moon — but it’s also 400 times farther away. So when the moon’s orbit takes it across the ecliptic — the apparent path of the sun through the sky — it can fit across it exactly. It happens somewhere around the world roughly once every 18 months.
Whether you’re in the “path of totality” – running from Oregon to South Carolina, where the moon will block the sun’s surface entirely – or getting a partial view of the eclipse elsewhere, it will be tempting to glance skyward, especially because the sun will appear to be dimmer than usual. But don’t let the eclipse fool you: It’s a critical time to protect your eyes.
Looking directly at the sun is unsafe except during the brief total phase of a solar eclipse (“totality”), when the moon entirely blocks the sun’s bright face, which will happen only within the narrow path of totality.
The only safe way to look directly at the uneclipsed or partially eclipsed sun is through special-purpose solar filters, such as “eclipse glasses” (example shown at left) or hand-held solar viewers. Homemade filters or ordinary sunglasses, even very dark ones, are not safe for looking at the sun; they transmit thousands of times too much sunlight. Refer to the American Astronomical Society (AAS) Reputable Vendors of Solar Filters & Viewers) page for a list of manufacturers and authorized dealers of eclipse glasses and handheld solar viewers verified to be compliant with the ISO 12312-2 international safety standard for such products.
- Always inspect your solar filter before use; if scratched or damaged, discard it. Read and follow any instructions printed on or packaged with the filter.
- Always supervise children using solar filters.
- 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 filter — do not remove it while looking at the sun.
- Do not look at the uneclipsed or partially eclipsed sun through an unfiltered camera, telescope, binoculars, or other optical device.
- Similarly, do not look at the sun through a camera, a telescope, binoculars, or any other optical device while using your eclipse glasses or hand-held solar viewer — the concentrated solar rays will damage the filter and enter your eye(s), causing serious injury.
- Seek expert advice from an astronomer before using a solar filter with a camera, a telescope, binoculars, or any other optical device. Note that solar filters must be attached to the front of any telescope, binoculars, camera lens, or other optics.
- If you are within the path of totality (https://go.nasa.gov/2pC0lhe), remove your solar filter 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.
- Outside the path of totality, you must always use a safe solar filter to view the sun directly.
- If you normally wear eyeglasses, keep them on. Put your eclipse glasses on over them, or hold your handheld viewer in front of them.
Remember to be cautious but also enjoy this unique experience. Since the moon is drifting away from Earth at 3.8cm every year, such perfect total solar eclipses will not always occur. So we’re born lucky, in an age of totality, but don’t feel too blessed: The moon will one day be too far away from Earth to totally eclipse the sun (but that will take 538 million years). The last partial solar eclipse in North America was 38 years ago in 1979. But we are in luck as another total solar eclipse will streak across North America in just over 6.5 years on April 8, 2024. It will have a completely different path, starting in Mexico and finishing in Canada after busting through Texas, Arkansas, Missouri, Illinois, Kentucky, Indiana, Ohio, New York, and Vermont.