With a total solar eclipse set to sweep across parts of the Arctic, Greenland, Iceland and Spain on August 12, interest in do-it-yourself eclipse viewing tools has surged — but safety experts and space agencies are cautioning that not every homemade option is safe for the eyes.

According to NASA and the National Solar Observatory, looking directly at the sun without proper protection can cause permanent eye damage, and that risk applies during a solar eclipse just as it does on any ordinary day, except during the brief window of total darkness known as totality. Outside of that narrow period, any glimpse of the sun's bright surface — even a thin sliver during a partial eclipse — can harm the retina, since the eye lacks pain receptors that would otherwise warn of the damage occurring in real time.

The August 12 eclipse will be total along a path crossing a remote stretch of northern Russia, Greenland, Iceland, northern Spain and a small corner of Portugal. Much of Europe, North America and northwestern Africa will see only a partial eclipse, meaning the moon will cover just a portion of the sun's disk rather than blocking it entirely. For viewers along the path of totality, NASA says it is safe to remove eye protection and look directly at the sun only during the short period when the moon fully covers it — and protection must go back on the instant any sliver of sunlight reappears.

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