The only safe way to look at the Sun: a certified film or glass fixed securely over the front of the instrument, blocking more than 99.99% of the light. Never use a filter that screws onto the eyepiece: it can crack from the heat. A single glance through an unfiltered instrument causes blindness in a fraction of a second.
Related terms
- Sunspot
A dark area on the Sun’s surface, cooler (about 4,000 °C against 5,500 °C) because of an intense magnetic field. Some are larger than the Earth. Their number follows a cycle of about 11 years; observe them only with a proper solar filter.
- Solar eclipse
The Moon passes in front of the Sun, at new Moon. It is total if the Moon hides it completely, annular if the Moon is too far to cover it (a ring stays visible), partial elsewhere. Never look at the Sun without certified eclipse glasses, except during the brief total phase.
- Transit
A small object passing in front of a larger one without hiding it: Mercury across the Sun (next on 13 November 2032), one of Jupiter’s moons across the planet, or the ISS across the Sun or the Moon. For the Sun, a solar filter is essential.
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