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The object of the GENERAL APPENDIX to the Annual Report of the Smithsonian Institution is to furnish brief accounts of scientific discovery in particular directions; reports of investigations made by collaborators of the Institution and memoirs of a general character or on special topics that are of interest or value to the numerous correspondents of the Institution.
It has been a prominent object of the Board of Regents of the Smithsonian Institution, from a very early date, to enrich the annual report required of them by law with memoirs illustrating the more remarkable and important developments in physical and biological discovery, as well as showing the general character of the operations of the Institution; and this purpose has, during the greater part of its history, been carried out largely by the publication of such papers as would possess an interest to all attracted by scientific progress.
In 1880 the Secretary, induced in part by the discontinuance of an annual summary of progress which for thirty years previous had been issued by well-known private publishing firms, had prepared by competent collaborators a series of abstracts, showing concisely the prominent features of recent scientific progress in astronomy, geology, meteorology, physics, chemistry, mineralogy, botany, zoology, and anthropology. This latter plan was continued, though not altogether satisfactorily, down to and including the year 1888.
In the report for 1889 a return was made to the earlier method of presenting a miscellaneous selection of papers (some of them original) embracing a considerable range of scientific investigation and discussion. This method has been continued in the present report for 1905.
NEW MEASUREMENTS OF THE DISTANCE OF THE SUN.
By A. R. HINKS, M. A.
When I received the honor of an invitation to lecture at the School of Military Engineering on some astronomical subject, I had little difficulty in making my choice of a topic. There is just one subject on which I may speak with some little first-hand knowledge; and by great good fortune that subject is concerned with a problem which has both in its nature and its history a connection with the Corps of Royal Engineers.
The problem of the determination of the distance of the sun is, in some respects at least, the most fundamental in the whole
of astronomy, for the number which represents it is involved in almost any calculation of distances and masses, of sizes and densities, either of planets or their satellites or of the stars. The distance of the sun bears somewhat the same relation to other problems of celestial surveying as the size and shape of the earth bear to terrestrial. It may not always appear on the surface, but it is generally concealed somewhere in the depths of the calculations. And I am compelled to confess that in one respect the earth measurers have the advantage over astronomers. The utmost that the astronomer can do is to show that the distance of the sun is so many times the radius of the farth. But ask him to put it into miles and he is powerless to do so until the geodesists have told him how large the earth is; and it is there that, in the very nature of the case, we are compelled to depend in the end upon the scientific labors of your corps.
Lecture delivered at the Royal Engineers’ Institute on February 9, 1905. Reprinted, by permission, from the Royal Engineers' Journal, Volume II, No. 1, July, 1905. Chatham, England, Royal Engineers’ Institute.
Distance of sun corresponding to different values of the solar parallar and
Clarke's figure of 1880.
A difference of 0.01" in the parallax is equivalent to 106,000 miles, or 170,000 kilometers.
Let us look at the matter for a moment as a problem in pure surveying. To measure the distance of the sun we have as a base a chord somewhat less than the diameter of the earth, since observations can not be made on a heavenly body when it is actually on the horizon. Suppose we put our base line at nine-tenths of the diameter. Our problem is to determine the distance of a body so far away that the whole diameter of the earth subtends at it an angle of only about 17.6 seconds of arc; and with our somewhat diminished base this angle is reduced to a little less than 15 seconds. I believe that the length of your base upon the great lines of Chatham is about 1,730 feet. Imagine that from that base you had to determine with an accuracy greater than one in a thousand the distance of an intersected point about 4,500 miles away, as far away as Chicago, and you have a problem which is by comparison simplicity itself. For the ends of our 7,000-mile base are not visible from each other, being on opposite sides of the world, and our angles at the base must be determined by a complicated reference to the zenith, with all the well-known impossibilities of determining absolute places in the sky increased by the special difliculties that arise when the object to be observed is the sun. You will readily grant that to determine the distance of the sun by direct observation of that body is impossible, unless you are content with an accuracy of about 1 in 10.
Now, it is a curious fact that there is a way of obtaining the distance of the sun with an accuracy of 10 per cent with no other instrument than a clock keeping accurate time. You do it by observing the times of minima of the variable star Algol. Every two days twenty-one hours Algol drops more than a magnitude, and does this with a regularity which would be unfailing were it not for the fact that at one season of the year we are nearer the star by nearly the whole diameter of the earth's orbit then we are at the opposite