More Solar Features

Solar limb darkening is another feature readily observed. Note how the edge of the Sun’s disk is darker than the center. Here is proof that the Sun has an atmosphere. At the center of the disk our vision extends straight down into a deeper, hotter layer of the photosphere, while near the limb our line of sight enters obliquely and penetrates only to a higher, cooler layer.

Often seen near the limb are faculae, large bright patches that float above active areas. Faculae in an unspotted region mean either that spots will break out within about a day, or, more likely, that a spot group has died there sometime in the past few weeks. Keep an eye especially on the eastern limb. Faculae rotating into view often mean sunspots will follow.

Granulation is visible only in excellent seeing with high power and at least a 4-inch aperture. Sometimes called rice grain or lemon peel, this textured appearance is due to hot gas continually boiling up to the Sun’s surface, cooling, and sinking down again. Each granule is a convection cell of upwelling gas about an arc second (500 miles) across and persisting 5 or 10 minutes.

A very rare but dramatic event is a white-light flare, a small but brilliantly incandescent eruption that lasts just a few minutes. Flares are believed to result from a sudden release of energy in the tangled magnetic fields above a sunspot group. The first ever witnessed was in 1859 by the English solar observers Richard Carrington and Richard Hodgson. The latter described “a very brilliant star of light, much brighter than the Sun’s surface, most dazzling to the protected eye” that hugged the adjacent spot formations like “the edging of the clouds at sunset.”

Other rare sightings include dark faculae, which are occasionally reported toward the middle of the Sun’s disk, and ill-defined spots, sunspots that are unaccountably vaguer than normal ones visible at the same time. The nature and significance of these features remains obscure. Regular observation is even more essential for enjoying the Sun than other areas of astronomy. Follow the Sun’s many changes, become familiar with the habits of sunspots and the slow pulse of the solar cycle, and the time will come when every view will be filled with meaning.

The Solar Section of the Association of Lunar and Planetary Observers (ALPO) welcomes inquiries from novices as well as experienced observers. It also publishes a guide to making useful drawings and photographs.