Everyone who saw it will remember the last total eclipse in this part of the world—on June 8, 1918, visible from Oregon to Florida. Many will recall the last total eclipse that was visible before that in the eastern part of the United States, on May 28, 1900, visible in a narrow path from New Orleans to Norfolk. One's father or grandfather will perhaps remember the total eclipse of July 29, 1878, which passed over the United States from Pike's Peak to Texas (it was the writer's maiden eclipse), and another on August 7, 1869, which passed southeasterly over Iowa and Kentucky. On all these occasions the paths of total eclipse were dotted with numerous observing parties, many of them equipped with elaborate apparatus for studying and photographing the solar corona and prominences, together with a multitude of other phenomena which are seen only when total eclipses take place.
Looking forward rather than backward, a striking series, or family, of eclipses happens in the future: it is the series of May, 1901 and 1919, recurring again on June 8, 1937 (over the Pacific Ocean), June 20, 1955 (through India, Siam, and Luzon), and June 30, 1973 (visible in Sahara, Abyssinia, and Somali). Already in 1919 this totality was 6 minutes 50 seconds in duration; in 1937, as already mentioned, it will be 7 minutes 20 seconds, and at the subsequent returns even longer yet, approaching the estimated maximum of 7 minutes 58 seconds which has never been observed. This remarkable series of total eclipses is longer in duration than any others during a thousand years. Its next subsequent return is in 1991, occurring with the eclipsed sun practically at noon in the zenith of Mount Popocatepetl in Mexico.
Whatever may be the progress of solar research during the intervening years, it is impossible to imagine the alert astronomer of that remote day without incentive for further investigation of the sun's corona, in which are concealed no doubt many secrets of the sun's evolution from nebula to star.
CHAPTER XXXI
THE SOLAR CORONA
"And what is the sun's corona?" mildly asked a college professor of a student who might better have answered "Not prepared."
"I did know, Professor, but I have forgotten," was his reply.
"What an incalculable loss to science," returned the professor with a twinkle. "The only man who ever knew what the sun's corona is, and he has forgotten!"
Only in part has the mystery of the corona been cleared by the research of the present day. Our knowledge proceeds but slowly, because the corona has never been seen except during total eclipses of the sun; and astronomers, as a matter of fact, have never had a fair chance at it. Two total eclipses happen on the average of every three years; their average duration is only two or three minutes; totality can be seen only in a narrow path about a hundred miles wide, though it may be several thousand miles long; there is usually about equal chance of cloud with clear skies; and fully three-fourths of the totality areas of the globe are unavailable because covered by water. So that even if we imagine the tracks of eclipses quite thickly populated with astronomers and telescopes, at least one every hundred miles, how much solid watching of the corona would this permit? Only a little more than one week's time in a whole century.