Popular Science Monthly/Volume 10/February 1877/An American Astronomical Achievement
AN American astronomer, Prof. Young, of Dartmouth College, Hanover, New Hampshire, has recently achieved a victory over a problem which has for many years foiled the skill of the best European observers; and, in so doing, he may be said to have added the keystone to an arch of no small importance in the edifice of modern astronomical science. It will be in the knowledge of most of my readers that astronomers have succeeded, during the last eight years, in measuring the rate at which some of the stars travel from or toward us, employing for the purpose what is called the spectroscopic method. I do not mean here spectroscopic analysis simply, but a special application of this now familiar analysis to measure the rate at which luminous bodies are approaching us or receding from us. The principle of the method is very readily explained. Light comes to us from the heavenly bodies, as from other luminous bodies, in waves, which sweep through the ether of space at the rate of about 185,000 miles per second. The whole of that region over which astronomers have extended their survey, and doubtless a region many millions of millions of times more extended, may be compared to a wave-tossed sea, only that instead of a wave-tossed surface there is wave-tossed space. At every point, through every point, along every line, athwart every line, myriads of light-waves are at all times rushing with the inconceivable velocity just mentioned. It is from such waves that we have learned all we know about the universe outside our own earth. They bring to our shores news from other worlds, though the news is not always easy to decipher.
All the celestial bodies are in motion amid the multitudinous waves of space. Something can be learned respecting their motions by studying the waves. If a strong swimmer were stemming a series of long, rolling waves, their crests would pass him in more rapid succession than if he were at rest; if, on the other hand, he reversed his course, so that waves overtook instead of meeting him, their crests would pass in slower succession. One can easily conceive how, if he knew the exact rate at which the crests would pass him—so many exactly per minute—were he at rest, their slower or more rapid succession might indicate how fast he himself was moving, either from or toward them. If he were quite unconscious of his own motion, the effect would be simply that the distance from crest to crest would seem to be diminished in one case, lengthened in the other—that is, the waves narrowed or widened. Similarly with the aërial waves which produce sound. They are seemingly shortened when the source of sound is approaching, whether by its own motion or the hearer's, and lengthened when the source of sound is receding. In the former case the tone of the sound is made more acute, in the latter graver than it really is. This is strikingly illustrated when a swift train rushes past a station, the whistle sounding all the time, for there is a perceptible lowering of the whistle's note as the. engine passes a nearer on the platform. While the train ishim, he hears a note somewhat sharper than the true note of the whistle; after it has passed he hears a note somewhat flatter than the true note. Still more obvious, even to non-musical ears, is the corresponding change when two trains pass each other. In America, where a hideously-clanging engine-bell is used, the change is very remarkable, insomuch that a person unfamiliar with the arrangement actually adopted would suppose a different bell was rung the moment the engine passed the hearer.
Light traveling also in waves, it is obvious that a similar effect must be produced by approach or recession, if only the rate of motion is sufficiently rapid. The swimmer of my first illustration must have a velocity comparable with that of the water-waves, or no change will be observed. The trains of the second illustration must have a velocity comparable with that of the aërial waves producing sound, or no change of tone will be produced. And in like manner a star or other celestial body must be approaching us, or receding from us, with a velocity comparable with that of the ethereal waves producing light, or no change of color will be produced, the color of light corresponding with the tone of sound. Unfortunately for this purpose, though most fortunately in other respects, light travels at so enormous a rate that even the swiftest motions of the heavenly bodies seem rest by comparison. What, for instance, is the rush of even Newton's comet past its point of nearest approach to the sun, though at the rate of more than 300 miles per second, to the flight of light over nearly 200,000 miles in the same time? "Very much as the movement of a person taking only six steps a minute, each less than half a yard long, to the rush of the swiftest express-train. Yet astronomers have undertaken to measure the approach and recession of stars, moving—in some cases—with less than a tenth part of that comet's motion, and whose velocity, therefore, sinks into still more utter insignificance by comparison with that of light.
Secchi claims (but not justly) to have first invented and applied the method used for this purpose, which consists in noting whether some known line of the spectrum of a heavenly body changes in position—either by moving toward the violet end of the spectrum, which would imply approach, or by moving toward the red end, which would imply recession. Of course, the method is exceedingly delicate and difficult, involving a number of details which would be quite unsuited to these pages; but that, in principle, is its nature. Secchi tried the method, and failed to get any results from it, announcing his unsuccessful attempt in March, 1868. "Then," he says, "Mr. Huggins retried (reprit) the method, announcing in April, 1868, the discovery that Sirius is receding at the rate of twenty miles per second." Secchi should know well, however, that our great spectroscopist did not achieve this success in the few weeks between Secchi's announcement of failure and Huggins's announcement of success. Months had elapsed, during which Huggins had been struggling with this difficult problem. If the enunciation of the method gave claim to the credit of its successful application, I myself could advance a stronger claim than Secchi's, for in an essay in Fraser's Magazine for January, 1868, I definitely indicated the nature and value of the method. But I would rather refer to the circumstance as enabling me to support Huggins's assertion that he was observing by this method for months before Secchi announced his own failure, for immediately on the appearance of my essay I received a letter from Dr. Huggins, mentioning (in confidence, until his paper should be published) that he had been for some time striving for success by the method I had described. This was nearly three months before Secchi's paper appeared. Subsequently Huggins observed by this method a number of stars, some of which he found to be receding from us, others approaching us.
Recently, however, the method itself has been called in question—first, by Van der Willigen, for reasons professedly mathematical, but unsound; secondly, by Secchi, because of his failure to see what Huggins has observed. Secchi had once based his attack on his failure to detect by this method the effects of the sun's rotation. As the sun's equator is spinning swiftly round, it is necessarily approaching on one side and receding on the other. By some amazing miscalculation, never yet explained, Secchi made the rate of this motion many times larger than it really is—so large, in fact, that the method we have described should have exhibited the sun's whirling motion. Small as the effect really is—amounting, in fact, only to a relative motion of about two and a half miles per second—Huggins did not despair of recognizing it; but he failed, though he used a double and twice-acting battery of prisms (the invention of the present writer—so far, at least, as its duplex character was concerned) made by Mr. Browning. Mr. Christie, of Greenwich, after resolutely grappling with the work of determining star-motions by this method, and in the main confirming Huggins's results, succeeded in recognizing by its means the known motions of Venus toward or from the earth, in various parts of their respective orbits. This was a great triumph, and more than met Secchi's objections. But Prof. Young has gone, for the present, ahead of all other observers by this method. Availing himself of a beautiful extension of spectroscopic powers, due to Dr. Rutherfurd, of New York, he has succeeded in unmistakably recognizing the effects of the sun's motion of rotation by the spectroscopic method. Young has made the observations so satisfactorily that he relies even upon the difference between his results and the measured rate of the sun's rotation. He finds the sun's atmosphere (whence, of course, the spectral lines come) to be traveling faster than the sun's visible surface. To use his own words, "The solar atmosphere really sweeps forward over the underlying surface, in the same way that the equatorial regions outstrip the other parts of the sun's surface." The difference of rate is about ten miles per minute. For my own part, I doubt very much whether so small a difference can be indicated by this method. But even if we regard this part of Young's work as not yet proved—nay, even if we go further, and accept nothing more than the bare recognition of the sun's rotation by the new method—he must be congratulated on having effected the most delicate piece of spectroscopic observation yet achieved by man. He has placed beyond doubt or cavil a method of motion-measuring the most remarkable yet invented, and likely, as instrumental means improve, to be most fruitful in results of astronomical interest and importance.