« ПредыдущаяПродолжить »
and the unwearying sun, and the moon waxing to the full, and the signs everyone wherewith the heavens are crowned, Pleiads and Hyads and Orion's might, and the Bear that men call also the Wain, her that turneth in her place and watcheth Orion, and alone hath no part in the baths of Ocean.”
About four centuries before Christ, Eudoxus brought from Egypt to Athens an improved celestial sphere, and a century and a half later the poet Aratus, who was then the court poet at Macedonia, sang of the stories of the stars and of their positions in the heavens. A few years after Christ, between 125 and 150 A.D., Ptolemy made the first accurate star charts, and propounded his philosophy that our little world was the center of the universe, and that all creation was for our edification. This explanation of the universe had been the regular belief of men long before Ptolemy, and continued to be so long after him. As late as the seventeenth century it was adopted by Milton (there is reason to think he did not really believe in it) as the basis for “ Paradise Lost," although a century before his time Copernicus had conceived the simple truth that the world turned on its axis. At the time of Milton the wonderful Galileo had made his name immortal by the invention of the telescope, an instrument which has made our greatly advanced astronomy of to-day possible.
At last truth came into her own, and the past two centuries have seen stupendous strides in astronomy. Observatories have been established all over the world. Brilliant and skilled men work in them; and by their labors benefit beyond all estimate has been rendered to the men of the sea and of the land. The great saving of life at sea can never be known. And it is highly probable that the advanced astronomy of to-day saves more money in the transactions of business than it costs to run all our observatories.
Telescopes were made larger and larger, with better and better lenses, until they reached almost the acme of perfection. It began to look as if the limits of progress had been nearly reached: there was no way to learn those secrets of the stars that the telescope failed to penetrate. But human ingenuity, not to be balked, goes on finding new instruments to supplement the telescope or the eye. The photographer has given us a sort of new eye. A photographic apparatus attached to a telescope records many things that escape the eye, because the plate can be exposed for hours and hours, while the eye becomes tired and dull in a very short time. To mention but a single example, the vast nebula surrounding the Pleiades has been shown to us. At Harvard Observatory in Cambridge, Mass., Professor Pickering is having the whole sky photographed on long time exposures. Very careful records are kept of all the plates, and they are examined with minute care in the search for new stars. These pictures of the sky will make by far the most perfect star charts that the world has ever known, and will be a wonderful gift to pass on to the generations who will follow us.
Another discovery which we will mention will be a little harder to understand, but more interesting. And while you cannot understand the principles perfectly until you have studied physics and chemistry, you can still enjoy the revelations. You, perhaps, know that our light comes to us in waves like the water waves on the seashore, or the little waves that come rolling in when you cast a stone into a brook. The length of the tiny
light waves is so extremely small that we can scarcely conceive of them. Yet an instrument has been made that can measure them. It is called the spectroscope, and the fundamental part of it is a prism. It may be that you have looked through prisms. “I shall never forget the pleasure I had with them when I was a small child. My grandmother had a swinging lamp hung from the parlor ceiling; its shade was decorated with dozens of prisms dangling from the edges. If one slipped from its chain, I secured it with glee; and when I looked at things through it, every object seemed to be laved about with a rainbow.” The picture of colored bands that the light makes when passing through the spectroscope is called the spectrum of the object from which the light comes. This picture is like our rainbow, which is a genuine spectrum. Now, different solids and gases, at white heat, have different spectra; so, by examining the spectrum of the light from a certain star, the scientist can tell of what material the star is composed.
Nor is this all that the spectroscope tells us. While these many-banded pictures were being examined, it was detected that the spectra of some stars seemed compressed, as if the light waves were beating down upon them; of other stars the spectra seemed to be dispersed, as if something was retarding the light waves. Analysis of this condition revealed that the spectrum seemed to be compressed when the star was moving towards us. and that the spectrum seemed to be dispersed when the star was moving away from us. So at last science had, in a sense, compassed the vast inconceivable distances of space, and was able almost directly to measure the motion of the stars as they approach us and leave us. By comparing the spectrum of a star with the spectrum
of an object at rest, and making some mathematical calculations, it can be determined with what speed the star which gives the compressed spectrum is approaching us, and with what speed the star which gives the dispersed spectrum is leaving us.
In the last four or five years of the nineteenth century, it was found that the Pole Star is approaching us at a great speed that changes every four days. These changes show that Polaris is whirling around some dark object-dark, else we could see it—as our moon goes around the earth, but at a tremendous speed, making its circuit in four days. But all the while this system is coming on through space at a still greater speed. When Polaris is on her backward voyage around this dark companion, her motion towards us is the difference between her motion around it, and the motion of the system towards us, just as if you were running backwards through a swiftly moving train. But when Polaris is coming on around, her speed towards us is the sum of the motion of the system towards us and her motion around her companion, just as if you were running forward in a swiftly moving train.
The distance of a star from the earth makes no difference in its treatment with a spectroscope. The light from Polaris that reaches us to-night started on its journey over fifty years ago, and has been traveling 186,400 miles a second during all these years. Yet it comes with all its inherent qualities, and tells us the secrets of its creator.
AN ADDRESS DELIVERED IN 1856
THE object of an observatory, erected and supplied with instruments of admirable construction, and at proportionate expense, is, as I have already intimated, to provide for an accurate and systematic survey of the heavenly bodies, with a view to a more correct and extensive acquaintance with those already known, and, as instrumental power and skill in using it increase, to the discovery of bodies hitherto invisible, and in both classes to the determination of their distances, their relations to each other, and the laws which govern their movements.
Why should we wish to obtain this knowledge? What inducement is there to expend large sums of money in the erection of observatories, and in furnishing them with costly instruments, and in the support of the men of science employed in making, discussing, and recording, for successive generations, these minute observations of the heavenly bodies?
What is the use of an observatory, and what benefit may be expected from the operations of such an establishment in a community like ours?
In the first place, then, we derive from the observations of the heavenly bodies which are made at an observatory, our only adequate measures of time, and our only means of comparing the time of one place with the time of another. Our artificial time-keepers-clocks,