The Evolution of Worlds

by Percival Lowell

The planet’s rotation time was to be investigated by means of the motion it brought about in the line of sight. Visual observation, telescopically, reveals motion thwart-wise by the displacement it produces in the field of view; spectroscopic observation discloses motion to or from the observer by the shift it causes in the spectral lines due to a stretching or shortening of their wave-lengths.

The spectroscope is an instrument for analyzing light. Ordinary light consists of light of various wave-lengths. By means of a prism or grating these are dispersed into a colored ribbon or band, the longer waves lying at the red end of the spectrum, as the ribbon is called, the shorter at the violet. Now the spectroscope is primarily such a prism or grating placed between the image and the observer, by means of which a series of colored images of the object are produced. In order that these may not overlap and so confuse one another, the light is allowed to enter the prism only through a narrow slit placed across the telescopic image of the object to be examined. Thus successive images of what is contained by the slit are presented arranged according to their wave-lengths. In practice the rays of light from the slit enter a small telescope called the collimator, and are there rendered parallel, in which condition they fall upon the prism. This spreads them out into the spectrum and another small telescope focusses them, each according to its kind, into a spectral image band which may then be viewed by the eye or caught upon a photographic plate.

Now, if an object be coming toward the observer, emitting or reflecting light as it does so, each wave-length of its spectrum will be shortened in proportion to the relative speed of its approach as compared with the speed of light, because each new wave is given out by so much nearer the observer and in reflection the body may also meet it. Reversely it will be lengthened if the object be receding from the observer or he from it. This would change the color of the object were it not that while each hue moves into the place of the next, like the guests at Alice’s tea-party in Wonderland, some red rays pass off the visible spectrum, but new violet rays come up from the infraviolet and the spectrum is as complete as before. Fortunately, however, in all spectra are gaps where individual wave-lengths are absorbed or omitted, and these, the lines in the spectrum, tell the tale of shift. Now if a body be rotating, one side of it will be approaching the observer, while the opposite side is receding from him, and if the slit be placed perpendicular to the axis about which the spin takes place, each spectral line will appear not straight across the spectrum of the object, but skewed, the approaching side being tilted to the violet end, the receding side to the red.

This was to be the procedure adopted for the rotation of Venus. By placing the slit parallel to the ecliptic, or, more properly, to the orbit of Venus, which is practically the same thing, it found itself along what we have reason to suppose the equator of the planet. Even a considerable error on this point would make little difference in the rotational result. In order that there might be no question of illusion or personal bias, photographs instead of eye observations of the spectrum were made. For reference and check side by side with that of Venus were taken on either hand the spectra of iron, made by sparking a tube containing the vapor of that metal. The vapor, of course, had no motion with regard to the observer, and therefore its spectral lines could have no tilt, but must represent motional verticality.

Dr. Slipher chose his time astutely. He selected the occasion when Venus was passing through superior conjunction, or the point in her orbit as regards us directly beyond the sun. At first sight this might seem to be the worst as well as the most impracticable of epochs, inasmuch as the planet is then not only at her farthest from the Earth, but in a line with the Sun, and so drowned in his glare. But in point of fact any tilt of the spectral lines is then, owing to phase, twice what it is at elongation, and exceeds still more what it is when Venus has her greatest lustre [[see NOTE 3]]. In his purpose he was abetted by the Flagstaff air, which enabled the planet to be spectrographed much nearer the sun than would otherwise have been the case. He thus selected the best possible opportunity. To guard against any subsequent bias on the part of the examiner of the plates, after the spectroscope had taken a plate it was then reversed, and the process repeated on another one, the iron being sparked as before. What had been the right side of Venus with regard to the red end of the spectrum thus became the left one, and vice versa. In this manner, when the plates came to be measured for tilt, the measurer would have no indication from the spectrum itself which way the lines might be expected to tilt; he could, therefore, not be influenced either consciously or unconsciously in his decision.

Spectrogram of Venus, showing its long day—V. M. Slipher,
Lowell Observatory, 1903.

Eight plates with their comparison ferric spectra were thus secured; four with the spectroscope direct, four with it reversed. They were then shuffled, their numbers hidden, and given to Dr. Slipher to measure. The spectral lines told their own story, and without prompting. All the plates agreed within the margin of error accordant with their possible precision, a precision some thirty times that of Belopolski’s experiment on the same lines,—a result not derogatory of that investigator, but merely illustrative of superior equipment. They showed conclusively that a rotation of anything like twenty-four hours was out of the question. They yielded, indeed, testimony to a negative rotation of three months, which, interpreted, means that so slow a spin as this was beyond their power to precise.

For Dr. Slipher was at no less care to determine just what precision was possible in the case, although a speed corresponding to a spin of twenty-four hours on a globe the size of Venus is well known to be spectroscopically measurable. It would mean a motion toward us of one thousand miles an hour, or about a third of a mile a second. The tilt occasioned by this speed is well within the spectroscope’s ability to disclose. Not content with this, however, by two special investigations, he proved the spectroscope’s actual limits of performance to be far within the quantity concerned. One of them was the determination by the same means and in like manner of the rotation time of Mars, the length of that planet’s day, which in other ways we know to the hundredth of a second, and which is 24ʰ 37ᵐ 23.66ˢ Now Mars offers a test nearly twice as difficult as Venus, even supposing the apparent disks of the two the same, because his diameter being less in the proportion roughly of one-half, the actual speed of a particle at his edge is less for the same time of rotation in the like proportion, and it is only with the speed in miles, not in angular amount, that the spectroscope is concerned. Nevertheless, when a like number of plates were tried on him, they indicated on measurement a rotation time within an hour of the true. This corresponds to half an hour on Venus. We see, therefore, that had Venus’ day been anywhere in the neighborhood of twenty-four hours, Dr. Slipher’s investigation would have disclosed it to within thirty-one minutes.