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Eclipse (continued) derivations of certain basic quantities must be carried out to provide trustworthy results; there have been cases in which data reduced from space observations had to be restudied because the need for different calibrations had shown up in eclipse work. In Fig. 4b, a merger of an eclipse image and spacecraft images from SDO and SOHO shows how ground-based images during a total solar eclipse fill a doughnut-shaped region between the inner and outer spacecraft images that is not visible from Earth at other times. The central image from SDO shows the Sun's disk at temperatures around 2,000,000 K (2,000,000°C or 4,000,000°F) through a filter showing extreme ultraviolet radiation from highly ionized iron gas. The outer im- age, from SOHO, shows gas at the millions of degrees typical of the Sun's corona scattering photospheric light toward us, with one of LASCO's coronagraphs hiding the bright inner corona. Filling in the doughnut ring allows coronal streamers to be traced from their roots on the Sun's surface, through the eclipse corona, and into the solar wind, which is the expanding outer corona. Related eclipse studies include use of the advancing edge of the Moon to provide high spatial resolution for radio observa- tions of the Sun and, historically, of celestial radio sources. The atmospheric effects of the removal of incident sunlight from the Earth's atmosphere have also been studied at eclipses. Historically, the test of the deflection of starlight carried out at the eclipse of 1919 and repeated in 1922 was the first verification of German-born U.S. theoretical physicist Albert Einstein's general theory of relativity. These results were restudied for the 1979 Einstein centennial, and their accuracy improved. The experiment is a very difficult one, and has been attempted at some eclipses, notably 1970 in Mexico and 1973 in Africa, without improving on the early results. But the effect has been verified to higher accuracy by studies in the radio part of the spectrum and by the observation of gravitational lenses, so optical eclipse tests are no longer necessary. Annular eclipses Central eclipses in which the Moon is sufficiently far from the Earth that it does not cover the solar photosphere are annular. The Moon's umbra is about 374,000 ± 6400 km (232,000 ± 4000 mi) in length while the Moon's distance is 382,000 ± 25,000 km (237,000 ± 16,000 mi), so the umbra sometimes falls short of reaching the Earth's surface. Annular eclipses, like total eclipses, occur about every 18 months on average. Since the corona is 1,000,000 times fainter than the pho- tosphere, if even 1% of photosphere is showing the corona is overwhelmed by the blue sky and cannot be seen. So most annular eclipses are of limited scientific use. The annular eclipse of 1984, visible from the southeastern United States, provided 99.8% coverage. The major scientific work carried out involved detailed timing of the Baily's beads in order to assess the size of the Sun. Such work has also been carried out at total eclipses. Some of the results, compared with solar diameters deduced from historical eclipse paths, seem to show a possible shrinking of the Sun by a measurable amount in a time of decades or centuries, which would lead to impossibly large effects on geological time scales; this has led to the suggestion that the Sun could be oscillating in size. But the question of whether any real effect is present has not been settled. The 1984 annular eclipse was so close to total that the corona could even be briefly seen and photographed, although no scientific studies of the corona were made. In the May 10, 1994, annular eclipse, widely viewed across the United States, only 94% of the Sun's diameter was covered, more typical of annular eclipses. The 1999 annular eclipse, whose path crossed Australia, provided 99% coverage, but since the Sun is about a million times brighter than the full moon, that still left about 10,000 times more light than on a moonlit night. The January 15, 2010, annular eclipse was viewed not only in vis- ible light but also with the Giant Metre-wave Radio Telescope in India, to measure fine details in solar active regions as they were occulted by the Moon. The May 20, 2012 annular eclipse was similarly observed, because it was visible from large radio telescopes in California and from the National Radio Astronomy Observatory's Very Large Array in New Mexico. The April 8, 2005, hybrid eclipse was annular at the ends of its path and total in the middle, over the Pacific Ocean, where the umbral cone reached the Earth's surface (Fig. 5). Recent and future eclipses Notable total eclipses in terms of duration and favorable weather were on June 21, 2001, in southern Africa; on December 4, 2002, in southern Africa and south Australia; and on March 29, 2006, in Africa, Turkey, and into Asia (Table 1; Fig. 6). More recent total solar eclipses were on August 1, 2008, in Siberia, Mongolia, and China; on July 22, 2009, in China; and on July 11, 2010, from the Cook Islands and atolls in French Polynesia, from Easter Island, and from southernmost South America. Major scientific expeditions carry out research on such occasions. The last total eclipse to cross the continental United States was on August 21, 2017 (Fig. 1), and the next eclipse to cross Canada (it also will be visible from the United States) will be in 2024. + ward ' s science