document: `` It 's maybe the greatest natural occurrence one can witness , '' said Larry Toy , a Chabot College astronomy professor . `` The phenomenon is just awe-inspiring . '' ; Solar eclipses occur twice a year , when the sun , moon and earth line up . -LRB- This line-up would occur at every new moon , except that the moon 's orbit is tilted so it only crosses the sun 's path every six months . -RRB- ; Because of variations in the moon 's orbit , fewer than a third of the solar eclipses are total eclipses . In these cases the moon 's shadow , up to 200 miles wide on the earth 's surface , races west to east along the ground at 1,000 to 5,000 miles per hour for a few thousand miles. ; Most total eclipses get little public attention . They occur over water , over the poles or in difficult-to-reach terrain . On average , any point on Earth is eclipsed just once every 360 years. ; But when the shadow 's narrow path does cross inhabited areas , people flock there to experience `` totality '' -- to watch the light snap off and the stars snap on , and to gaze at the sun 's shaggy corona hanging in the dark sky for a few minutes before daylight abruptly returns. ; In the United States , the opportunity has come twice in the past couple of decades -- along the East Coast in 1970 and across the Northwest and Northern Plains in 1979. ; This Thursday the moon 's shadow will be back for another performance , one of the century 's best. ; It 's unusual for several reasons . At its peak , observers will see one of the longest-lasting eclipses this century -- six minutes , 53 seconds . Also , its 150-mile-wide path across Hawaii , Baja California , Mexico City and four Central American capital cities `` puts more people in the moon 's shadow than any other eclipse in history , '' says Alan Dyer , an editor of Astronomy magazine. ; Finally , since the center of the shadow will pass right across the world 's largest cluster of astronomical observatories , the audience will include several dozen solar scientists intent on getting the best view they 've ever had. ; `` It 's going to be great from up there on the mountain , '' said Harold Zirin , a California Institute of Technology astronomer who will be at one of the observatories atop Mauna Kea , a dormant volcano that makes up a large piece of the island of Hawaii. ; Historic events ; For science , two eclipses have proved especially historic : In 1868 , spectroscopic study of the sun 's halo turned up evidence of a new element , which was named helium , from Helios , the Greek sun god . And in 1919 , astronomers spotted a star , which should have been behind the sun , instead peeking around the edge of the sun . That was the first confirmation of Albert Einstein 's prediction that a star 's gravity will bend the path of light. ; But in recent years solar eclipses have declined in significance for researchers. ; For one thing , chasing eclipses by trekking to remote regions with delicate astronomical instruments to capture a few minutes of data has lost much of its glamour , especially in times of tight budgets. ; For another , some of the information now can be gathered any day of the year by using instruments that simulate a solar eclipse. ; But this time , the eclipse is coming to them , and they ca n't resist a look . At 13,800 feet , Mauna Kea is the world 's tallest island mountain . The air above it is still , dry and clean , making it one of the world 's choicest spots for viewing the cosmos. ; It 's so high that oxygen deprivation is a serious concern . And , unlike most people headed for Hawaii , astronomers will be packing long underwear and down jackets in case , as sometimes happens even in July , the temperature at the summit drops below freezing. ; 7 observatories in use ; Seven of the nine observatories atop Mauna Kea will be used Thursday morning . -LRB- The Keck Observatory , being built by the University of California and the California Institute of Technology , is behind its original construction schedule and will not be completed in time for the eclipse . -RRB- ; What they want to know is more about the temperature and composition of the sun 's upper atmosphere -- layers called the chromosphere and the corona -- which normally is difficult to examine against the glare of the sun. ; They hope the information will provide clues to what causes the sun 's 11-year cycle of sunspots and flares -LRB- now near its peak -RRB- , what propels the solar wind and why the sun 's outer atmosphere is , paradoxically , so much hotter than its lower atmosphere. ; Solar wind studies ; At some observatories , astronomers will take repeated photographs of the corona during the eclipse to study it close up and to watch for material accelerating outward to become solar wind. ; Others will use telescopes to study the sun 's emissions at microwave frequencies to learn more details about how its temperature varies with altitude. ; But , frankly , the astronomers also want to watch the lights go out. ; They 'll be fighting the temptation to abandon their instruments and go outside to watch. ; `` The biggest problem , '' Zirin said , `` is whether we 're going to see the eclipse . 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extractive_keyphrases: solar eclipses sun shadow total eclipses solar scientists mexico city hawaii
abstractive_keyphrases: eclipse studies