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.
Related terms
- Solar filter
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.
- 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.
← The whole glossary