The Chinese observed sunspots in ancient times
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The Chinese observed sunspots in ancient times and undoubtedly considered them evil omens. For once the portents were right. The turbulent condition of the Sun does have an effect on terrestrial weather, but the correlation is very general and no sunspot will tell us when our next local storm is due. Strictly speaking, the sunspots are not the cause of our bad weather, any more than our storms are the cause of sunspots. Both are the result of some hidden disturbance in the Sun itself. They are--so to speak-but the outward and visible signs of an inner and spirited fire.
One evil result which the Chinese could not have foreseen is the effect on our radio reception. Our own weather, we could somehow forgive, but it does seem rather highhanded of the Sun to interfere with our radio when we most want to hear a brilliant speech from the House of Commons.
Tabulations of sunspots over a great many years show that they correspond to our wet and dry seasons with general uniformity over large areas of the Earth. Some years there are a great many sunspots, and then our weather is at its dismal worst; gradually they decrease until there are practically none, and their absence results in our droughts. Then they begin to increase again. Their occurrences form a cycle, and the maximum number of sunspots comes about every eleven years.
The causes for this cycle are not known, but they appear to affect the Sun and the Earth simultaneously and have given us a wonderful calendar of sunspots for the past centuries. The effect of wet and dry seasons is recorded by the rings in the annual growth of trees. In wet summers the growth is greater than in dry, and a study of tree rings gives a complete account of sunspots at least as far back as the most important date in history since the year one. This discovery is due to Professor A. E. Douglass, Director of Steward Observatory, Tucson, Arizona, who applied sunspots to the chronology of American Indian pueblos.
Professor Douglass compared sunspot records with the tabulations of wet and dry years and with rings on growing trees. The two corresponded very well. Looking over old trees he found a period of seventy years, from 1645 to 1715, when the rings were narrow, as if the trees had been starved for moisture. While he was wondering about them, he received a letter from a London astronomer who said that there had been no sunspots for the same seventy years. Was there any effect of drought visible in the tree rings? Professor Douglass replied at once in the affirmative, and proceeded to apply his method with greatly increased confidence.
He then examined the trees used in American Indian pueblos. Going from the known to the unknown, he constructed a calendar backward. There was one gap for a time which gave two uncorrelated periods, but finally in the Chaco Bonito Pueblo in New Mexico, he discovered in a charred stake the missing link. He could then determine the date of the pueblo, by irrefutable evidence, as 1066!
Sunspots have also served to mark dearly definite places on the Sun. They change continually, but they retain a resemblance to their original appearance, and can be re-identified. We can watch them apparently traveling across the face of the Sun and vanishing and reappearing again, after a tour of the other side. By the movement of sunspots astronomers have been able to gauge the rotation of the Sun. Once in about twenty-six days it makes a full rotation on its own axis.
In spite of the enormous size of all sunspots, only the largest are visible to the naked eye. Before you begin to study them you must equip yourself with a smoked glass such as you see on the rare occasions when an eclipse of the Sun is visible in your neighborhood. You can make the glass yourself, or you can buy one from a man who manufactures them. Pick out the laziest man you know. He won't advertise, for he is too indolent; but, if he is really the laziest man in the community, he will claim to earn his living by making glasses suitable for an eclipse of the Sun.
If you wish to observe sunspots through a small telescope or a pair of field glasses a dark glass is essential. One man looked at the Sun with his naked eye for twenty seconds. He went blind. Through a telescope the sight is even more dangerous. Our surveyor's transit has a twenty-five power lens, and a dark glass set in the cover of the eyepiece. Sometimes we have forgotten to slip the protection into place but the glare is so terrific that we automatically recoil; our eyes shut and refuse to be placed in the hazardous position.
An easy way of looking for sunspots is by allowing the Sun to project its own image onto a sheet of paper. We cut in a piece of cardboard a hole a little larger than our object lens and fit it neatly into place. Then we hold a notebook about a foot away from the eyepiece end. Except for the image, all the notebook is shadowed, and the circle of bright light is the picture of the Sun. Beginners are inclined to believe that the whole field of the telescope is shining through onto the book; but the Sun is so bright in comparison with the rest of the sky, that its image obscures the rest. By adjusting the focus of the lens we can make the image sharp, and bring any large sunspots into prominence. We have mentioned a notebook as part of the equipment, because our sunspot gazing is usually a by-product of other astronomical hobbies for which notebooks are necessary and indispensable. For simple Sun-gazing any plain surface will do as well. A painted table, a glazed paper plate or the back of an old envelope will give equally satisfactory results.
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