Issue link: https://wardsworld.wardsci.com/i/1379459
During the diamond-ring effect, the solar chromosphere becomes visible around the limb of the Moon, glowing pinkish because most of its radiation is in the form of emission lines of hydrogen, mostly the red hydrogen-alpha line. Its emission-line spectrum apparently flashes into view for a few seconds, and is called the flash spectrum. As the advancing limb of the Moon covers the chromosphere, the corona becomes fully visible (Fig. 4a). Its shape is governed by the solar magnetic field; common are equatorial streamers and polar tufts. At the maximum of the solar activity cycle, so many streamers exist that the corona appears round when it is seen in projection, as viewed from Earth. At the minimum of the solar activity cycle, only a few streamers exist so that the corona appears more elongated in projection. Totality (Fig. 5) lasts from an instant up through somewhat over 7 min, 30 s. At its end, the phenomena repeat in reverse order, including chromosphere, diamond ring, Baily's beads, shadow bands, and the partial phases. The corona is now monitored continuously in x-rays and in ultraviolet and visible light by orbiting spacecraft such as the Solar and Heliospheric Observatory (SOHO; ultraviolet and visible light), the GOES Solar X-ray Images, and, since 2010, the Solar Dynamics Observatory (SDO; ultraviolet). The spacecraft Hinode, a successor to the Yohkoh spacecraft, was launched in 2006 to continue such x-ray studies as well as visible-light and ultraviolet observations. Eclipse observations can provide ground truth for comparison with these missions on the days of eclipses and image parts of the corona that cannot be observed with spacecraft, whose coronagraphs must occult the lowest corona to limit scattering of light in the telescope systems. Positions and timing The paths of the Sun and Moon in the sky intersect at two points, the ascending node and the descending node. Only when both the Sun and the Moon are near a node can an eclipse occur. Thus eclipse seasons take place each year, whenever the Sun is near enough to the node so that an eclipse is possible. Each eclipse season is 38 days long. Because the Sun's gravity causes the orientation of the Moon's elliptical orbit to change with an 18.6-year cycle, the nodes slide along the ecliptic and a cycle of two eclipse seasons—an eclipse year— has a period of 346.6 days, shorter than a solar year. There must be at least one solar eclipse each eclipse season, so there are at least two each year. There may be as many as five solar eclipses in a calendar year (technically, a year), though most of these will be partial. Some of the partials will occur near the ends of the eclipse seasons, and it is also possible for there to be three eclipse seasons during a solar year. Adding lunar eclipses (including penumbral lunar eclipses, which may not be noticeable), there may be seven eclipses in a year. Eclipse (continued) + ward ' s science Fig. 4 Total solar eclipse of July 11, 2010, observed from Easter Island in the Pacific Ocean. (a) Image composited from several exposures using radial filters in order to show a wide dynamic range of intensity of the corona and to bring out the structures of the streamers (Credit: Copyright © 2010, Wendy Carlos, Jay M. Pasachoff, and Jonathan Kern. All rights reserved). (b) Merger of inner and outer images from spacecraft sandwiching an eclipse image from the ground. An image of the Sun's outer corona was taken by the Large Angle Spectro- metric Coronagraph (LASCO) on the SOHO spacecraft and is shown in red false color. An image of the face of the Sun was taken in extreme ultraviolet light by the Atmospheric Imaging Assembly on the Solar Dynamics Observatory (SDO). Though spacecraft can observe both these regions, the "doughnut" between these images is not visible from Earth except during total solar eclipses. Merging the eclipse and spacecraft images allows tracing of coronal features from their bases on the Sun's surface until the gas escapes into inter- planetary space as the solar wind (Credit: Williams College Eclipse Expedition-Jay M. Pasachoff, Muzhou Lu, and Craig Mal- amut; SOHO's LASCO image courtesy of NASA/ESA; solar disk image from NASA's SDO; compositing by Steele Hill, NASA's Goddard Space Flight Center)