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Viewing the Sun SafelySunspots
Sunspots are cooler, relatively dark areas on the Sun’s bright surface (the photosphere). Most have a central dark umbra and a lighter gray penumbra. At high resolution the penumbra is seen to have thin fibrils radiating from the umbra. Only the smallest of sunspots, called pores, lack penumbras.
Spots tend to form in pairs, marking the “feet” of magnetic-field loops that arch above the surface. A pore appearing in a clear area is a sign that a spot group may soon be born. Pores often die out in a day or two, but sometimes one grows and develops a penumbra. More spots may appear nearby, and within about 10 days a complex active region is in full bloom, strung out between two major spots lined up east-west. The eastern spot is usually the first to die out; the western one becomes round and smaller, sometimes persisting alone for weeks. The end of a sunspot is often signaled by a large light bridge, a finger of the photosphere that intrudes into the spot and appears to split it apart.
If you make drawings of spot positions, the Sun’s rotation quickly becomes evident. A spot takes about two weeks to appear from behind the eastern limb, travel across the disk, and depart around the western limb. Different regions of the Sun rotate at different rates. The synodic rotation period (apparent period as seen from the moving Earth) is 27 ¼ days at the Sun’s equator but about 30 days at solar latitude 40°. If you get a good run of daily drawings, you can align them and flip through the stack with your thumb to watch the Sun rotate in an animated cartoon.
A standard index of solar activity is the relative sunspot number. Naturally this number depends heavily on the instrument, seeing conditions, and on observer judgment. Standardized daily sunspot numbers range from zero on some days near the time of minimum activity to more than 200 near solar maximum. (For more on sunspot numbers, see S&T: November 1984, page 475.)
