While telescopes can be adapted to partly simulate an eclipse, a real eclipse offers the best visibility for observing the corona. At eclipse we see the corona as a crown of light around the sun; its shape is sometimes more round, sometimes more elliptical. We see its glow extending a million miles from the edge of the sun. The corona is composed of the same gases as the rest of the sun: 90 percent hydrogen, almost 10 percent helium, and a tiny quantity of the other elements. The corona flows outward into the solar system as the solar wind_streams of charged particles. These travel 93 million miles to earth and even pass beyond the outer planets.
Eclipses are a time-tested tool of solar astronomy. Observers pursuing the 1868 eclipse determined through spectroscopy the existence of the gas helium, from helios, the Greek word for the sun; only later was it found on earth. Other eclipses showed over time that the corona changes shape in step with the 11 year sunspot cycle.
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