FEB. 20, 1894.
FEB. 21, 1894.
XI.—Photographs of the Sun taken at the Royal Observatory, Greenwich, shewing Sunspots
Wolf’s numbers.
When it became generally established that this periodicity existed, Rudolf Wolf of Zurich collected the facts about sun-spots from the earliest possible date, and represented this history by a series of numbers which are still called Wolf’s Sun-Spot Numbers. You will see from the diagram the obvious rise and fall for eleven years,—not ten years, as Schwabe thought, but just a little over eleven years. The facts are, however, given more completely by the work done at the Royal Observatory at Greenwich. It is part of the regular daily work of that Observatory to photograph the sun at least twice. Many days are of course cloudy or wet, so that photographs cannot be obtained; but there are available photographs similarly taken in India or in Mauritius, where the weather is more favourable, and from these the gaps are so well filled up that very few days, if any, during the whole year are left without some photograph of the sun’s surface.Greenwich areas. On these photographs the positions and the areas of the spots are carefully measured under a microscope, and the results when submitted to certain necessary calculations are published year by year. It is clearly a more accurate estimate of the spottedness of the sun to take the total area of all the spots rather than their mere number, for in the latter case a large spot and a small one count equally. Hence the Greenwich records will perhaps give us an even better idea of the periodicity than Wolf’s numbers. Now, at the same observatory magnetic observations are also made continuously. If a magnet be suspended freely we are accustomed to say that it will point to the North Pole; but this is only very roughly true. In the first place, it is probably well known to you that there is a considerable deviation from due north owing to the fact that the magnetic North Pole is not the same as the geographical North Pole; but this for the present need not concern us.Magnetic fluctuations. What does concern us is, that if the needle is hung up and left long enough to come to rest, it does not then remain steadily at rest, but executes slow and small oscillations backwards and forwards, up and down, throughout the day; repeating nearly the same oscillations on the following day, but at the same time gradually changing its behaviour so as to oscillate differently in summer and winter. These changes are very small, and would pass unnoticed by the naked eye; but when carefully watched through a telescope, or better still, when photographed by some apparatus which will at the same time magnify them, they can be rendered easily visible. When the history of these changes is traced it is seen at once that there is a manifest connection with the cycle of sun-spot changes; for instance, if we measure the range of swing backwards and forwards during the day and take the average for all the days in the year, and then compare this with the average number of sun-spots, we shall see that the averages rise and fall together. Similarly we may take the up and down swing, find the average amount of it throughout the year, and again we shall find that this corresponds very closely with the average number of sun-spots.
Plate XII.