Eclipse glasses, done right

The standard

ISO 12312-2

Filters on

09:41 – 12:01

Naked eye safe

10:45:33 – 10:50:01

Verify sellers at

eclipse.aas.org

Real eclipse glasses conform to ISO 12312-2, the international standard for solar viewers, and say so on the frame. The filter cuts visible light to a sliver and blocks the ultraviolet and infrared that do the silent damage. Certification is the entire test. A cheap certified pair protects you exactly as well as an expensive one; an uncertified pair is a liability at any price.

The catch is that a counterfeit prints the same words on the same cardboard. The American Astronomical Society keeps a list of manufacturers and resellers whose filters have been independently tested, and buying from that list is the only reliable defence. It costs nothing to check and takes a minute.

1. 01 Inspect the filter

   Scratches, pinholes, creases, or film lifting from the frame mean the pair goes in the bin. Filters are cheap; retinas are not.

2. 02 Test them indoors

   Through an intact solar filter you should see almost nothing — only the very brightest lights, faintly. If you can make out furniture, the filter is not safe for the Sun.

3. 03 Fit them to children properly

   Small faces leak light around adult-sized frames. The partial phases run for over two hours, so plan on supervising for the duration, not just at the start.

• Sunglasses are not filters, however dark and however many pairs you stack. They pass thousands of times more light than a solar filter.
• A phone screen does not make the Sun safe, and pointing an unfiltered camera at it can destroy the sensor. Looking through an unfiltered optical viewfinder is worse than the naked eye.
• Smoked glass, exposed film, CDs and space blankets belong to folklore. The one household alternative the safety bodies accept is welder's glass of shade 12 or darker.

During totality the Moon covers the Sun completely and the corona shining around it is about as bright as a full moon. For those 4 minutes 28 seconds — and only those — the naked eye is safe, and glasses would actually rob you of the sight. Off at second contact, when the last bead of sunlight breaks. Back on at third, the moment the diamond ring flashes on the far rim: the returning crescent is dangerous from its first instant.

A pinhole in a piece of card, held with the Sun behind you, throws a crisp little crescent onto any pale surface. During the partial phases the gaps between tree leaves do the same for free, scattering hundreds of crescent suns across the ground. Projection needs no equipment and carries no risk, because you never look at the Sun at all.