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NOTES ON OBSERVATIONS OF TOTAL SOLAR
By R. C. JOHNSON, F.R.A.S. The infrequency of total eclipses of the sun at any given locality is somewhat remarkable.
Taking London, as an example, the last eclipse total there dates back to the year 1715, and its immediate predecessor was considered for a long time, upon the authority of the celebrated Halley (a former AstronomerRoyal), to have taken place in the year 1140; this date has, however, subsequently proved to be incorrect, and it is necessary to go back to A.D. 878, in the reign of King Alfred, to find an eclipse total at London.
On the 22nd May, 1724, the last eclipse total in England occurred, but on this occasion complete obscuration did not extend to the metropolis, but passed a little to the north of that city.
During the nineteenth century no eclipse has been total in England, and if we look to the future the interval is still immense, for, during the twentieth century, there will only be two notable eclipses, of which the one in 1999 will be total in the west of England, but not at London.
In the twenty-first century there will occur, in the year 2090, a total eclipse in the south-west of England, visible only for fifteen minutes before sunset, and it has been calculated there cannot be a total solar eclipse visible at London any time before the end of the twenty-fourth century.
As the apparent magnitude of the lunar disc is often equal to, and sometimes greater than, that of the sun, it would at first appear that there should be a total eclipse of the sun as often as one of the moon. It is owing, however, to the small actual size of the moon as compared with that of the sun (the ratio of their respective diameters being as 1 to 500) that only those places on the earth which happen to lie almost directly under an imaginary axial line, joining the centres of the two bodies, can be covered by the moon's shadow; and in no case can the width of a strip of the earth's surface swept by the shadow of totality exceed 180 miles,
The limit of duration of totality is governed by the same circumstances; this cannot exceed eight minutes, and may be anything less.
It has been calculated that if it had been possible for an indefatigable observer to occupy the best positions at every total eclipse of the sun during the last fifty years, he might have been able (clear skies being granted) to have averaged a period of one minute per annum of totality.
The cause of science has, however, suffered little, for the numbers of observers who have concentrated themselves upon the narrow track of totality during the eclipses of the last fifty years have done much to neutralize this lack of opportunity.
The eclipse of 1842 attracted both Mr. Airey (the Astronomer-Royal) and Mr. F. Baily from this country to the South of France; and the much more arduous journey of M. Louville, who travelled from Paris to London for the express purpose of seeing the total eclipse of 1715 must not be forgotten. A description of this eclipse appeared in Mémoires de l'Académie des Sciences for that year.
These were pioneers who opened out the way for the crowded expeditions which are carried out in our day under such comparatively advantageous circumstances.
At the eclipse of 1851 a large number of observers attended ; and in the year 1860 there was a government expedition to Spain, and 40 observers went there from this country alone, while Norway, France, and the United States all sent expeditions; and from this time forward, no opportunity, however remote, has been neglected.
An eclipse has always been regarded with popular favour as affording ocular evidence of the accuracy of the calculation of the time of conjunction of the sun and moon, but in the period under review the interest has been transferred from the mathematical to the physical side of the question, owing to the desire to understand something of the nature of the centre of life of our system.
Hofrath Schwabe, of Dessau, was the originator of this line of research, for which he was awarded the gold medal of the Royal Astronomical Society in the year 1857.
Imbued with the idea of discovering an intra-Mercurial planet, this indefatigable observer, beginning in 1826, and continuing for 43 years, rarely missed an opportunity of examining the sun on every day when he was visible.
No success attended this pursuit, but as has frequently happened in the history of science, his critical inspection of the sun's spots led to a discovery of superior value, viz., that of their periodicity—as he himself quaintly observes, “like Saul, in seeking his father's asses, I found a kingdom."
The work thus inaugurated has been continued to the present time, and auto-photographic records of the sun's appearance are made at several places on each day. The collation of these, with independent records of magnetic variation which are continuously being tabulated, may, at some future time, lead to valuable results.
By far the most important discovery in solar physics is that achieved by Kirchoff in 1859, which, by showing the meaning of the absorption bands (known as Fraunhofer lines) in the solar spectrum, opened the door to an infinity of research upon the nature of the elements of which the photosphere of the sun is composed; and since that day spectrum analysis has been the determining factor in the results attained by eclipse observations.
The use of photography, by which observations have not only been multiplied one hundred-fold, but rendered unbiassed by personal equation, has also been of the utmost importance; and a third method, which was first practised during the eclipse of 1898, in India, is that by studious concerted action of amateur observers, long prior to the actual observations on the spot, of diagrams made to resemble various eclipse phases, a wonderful improvement has been found in the fidelity of sketches, made by hand, of the form and extent of the coronal rays when compared with similar drawings made in the absence of such systematic training.
The phenomena observed about the time of, and during totality, occur in the following order :
Firstly, the formation of Baily's Beads, seen only for a few seconds just as the moon is completely covering with her disc the body of the sun at the time of second contact (which is the commencement of totality), and, again, as totality is broken at the time of third contact. These “beads," which move rapidly along the edge of the two discs, are probably due to irradiation of the sun's intense light, possibly some diffraction effects being also mixed up with it. They are named after Mr. F. Baily, who first investigated them at the annular eclipse at Jedburgh in 1836.
Secondly, just at the moment of totality the chromosphere flashes out, as a narrow ring of brilliant rose coloured light round a portion of the edges of the sun and moon. The angular extent of this arc of light depends upon the relative diameters (apparent) of the sun and moon, for, as the depth of illuminated stratum does not exceed a few seconds of arc, it is manifest that if the eclipse were just central for a second, this coloured layer might be seen all round the moon's edge, and, conversely, if the totality is of long duration, it might scarcely appear at all. There are always some parts of the chromosphere in a state of great commotion, visible as red prominences, which vary in number and shape on every such occasionthese prominences assume the most grotesque forms, and frequently flame out to a height of over 100,000 miles above the chromosphere.
The third feature of the display is the appearance of the corona coincidentally with totality - this is a faint pearly effulgence, which varies in shape and extent at every eclipse, and is by far the most conspicuous of eclipse phenomena.
A combination is thus presented to the bewildered spectator of unwonted weirdness and grandeur which renders the vision of a total eclipse of the sun the most exciting spectacle afforded by the magnificence of nature, one never to fade from the memory of the favoured observer. It is in the midst of such a scene that science has calmly to perform her duties, and to beware lest one precious moment of time be wasted.
With this general review, we may now pass to a consideration of the order of discoveries by which our present knowledge of the physical condition of the sun has been attained.
It was at the eclipse of 1851 that the red prominences, which were very active and appeared at a great height above the sun's circumference, were believed to be Solar and not Lunar phenomena, and this opinion was proved