Cooler-than-average region of gas on the Sun's surface associated with strong local magnetic activity. Sunspots appear as dark spots, but only in contrast with the surrounding photosphere, which is several thousand degrees hotter. Spots several times larger than Earth are visible to the unaided eye (viewed through a filter); very small ones are hard to see even with a telescope. They come and go as part of the solar cycle, usually in pairs or groups, and may last for months; their cause appears to be related to the magnetic field reversals that occur every 11 years. The reality of these apparent flaws in the Sun was generally accepted only c. 1611. Periods of high sunspot activity are associated on Earth with brighter auroras and interference with radio signals.