A striking aspect of photographs of the Sun is the darkening of the solar image toward the limb (see Fig. 28.1). The decrease in brightness, which is accompanied by a reddening of the solar image in color photographs, is not uniform. Near the center of the disk the change in brightness is small but within a few arc seconds of the limb the brightness falls off so steeply that the edge of the solar disk appears sharp. This is the limb darkening of the Sun, sometimes called the center-to-limb variation. An incandescent solid, on the other hand, will have a constant surface brightness. A glowing tungsten or platinum wire, for example, shows no limb darkening. Limb darkening is the most visible sign of the gaseous nature of the outer layers of the Sun.
KeywordsOptical Depth Solar Atmosphere Solar Disk Solar Eclipse Disk Center
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