Authors: Carl Sagan
Ramsay had recently discovered the element krypton, which was said to have, among fourteen detectable spectral lines, one at 5570 Å, coincident with “the principal line of the aurora.” E. B. Frost concluded: “Thus it seems that the true origin of that hitherto perplexing line has been discovered.” We now know it is due to oxygen.
There were a great many papers on instrumental design, one of the more interesting being by Hale. In January 1897 he suggested that both refracting and reflecting telescopes were needed, but noted that there was a clear movement toward reflectors, especially equatorial coude telescopes. In a historical memoir, Hale mentions that the 40-inch lens was available to the Yerkes Observatory only because a previous plan to build a large refractor near Pasadena, California, had fallen through. What, I wonder, would the history of astronomy have been like if the plan had succeeded? Curiously enough, Pasadena seems to have made an offer to the University of Chicago to have the Yerkes Observatory situated there. It would have been a long commute for 1897.
AT THE END
of the nineteenth century, solar system studies displayed the same mixture of future promise and current confusion that the stellar work did. One of the
most notable papers of the period, by Henry Norris Russell, is called “The Atmosphere of Venus.” It is a discussion of the extension of the cusps of the crescent Venus, based in part on the author’s observations with the 5-inch
finder
telescope of the “great equatorial” of the Halsted Observatory at Princeton. Perhaps the young Russell was not yet considered fully reliable operating larger telescopes at Princeton. The essence of the analysis is correct by present standards. Russell concluded that refraction of sunlight was not responsible for the extension of the cusps, and that the cause was to be found in the scattering of sunlight: “… the atmosphere of Venus, like our own, contains suspended particles of dust or fog of some sort, and … what we see is the upper part of this hazy atmosphere, illuminated by rays that have passed close to the planet’s surface.” He later says that the apparent surface may be a dense cloud layer. The height of the haze is calculated as about 1 kilometer above what we would now call the main cloud deck, a number that is just consistent with limb photography by the Mariner 10 spacecraft. Russell thought, from the work of others, that there was some spectroscopic evidence for water vapor and oxygen in a thin Venus atmosphere. But the essence of his argument has stood the test of time remarkably well.
William H. Pickering’s discovery of Phoebe, the outermost satellite of Saturn, was announced; and Andrew E. Douglass of the Lowell Observatory published observations that led him to conclude that Jupiter 3 rotates about one hour slower than its period of revolution, a conclusion incorrect by one hour.
Others who estimated periods of rotation were not quite so successful. For example, there was a Leo Brenner who observed from the Manora Observatory in a place called Lussinpiccolo. Brenner severely criticized Percival Lowell’s estimate of the rotation period of Venus. Brenner himself compared two drawings of Venus in white light made by two different people four years apart—from which he deduced a rotation period of 23 hours, 57 minutes and 36.37728 seconds, which
he said agreed well with the mean of his own “most reliable” drawings. Considering this, Brenner found it incomprehensible that there could still be partisans of a 224.7-day rotation period and concluded that “an inexperienced observer, an unsuitable telescope, an unhappily chosen eyepiece, a very small diameter of the planet, observed with an insufficient power, and a low declination, all together explained Mr. Lowell’s peculiar drawings.” The truth, of course, lies not between the extremes of Lowell and Brenner, but rather at the other end of the scale, with a minus sign, a retrograde period of 243 days.
In another communication Herr Brenner begins: “Gentlemen: I have the honor to inform you that Mrs. Manora has discovered a new division in the Saturnian ring system”—from which
we
discover that there is a
Mrs.
Manora at the Manora Observatory in Lussinpiccolo and that she performs observations along with Herr Brenner. Then follows a description of how the Encke, Cassini, Antoniadi, Strove and Manora divisions are all to be kept straight. Only the first two have stood the test of time. Herr Brenner seems to have faded into the mists of the nineteenth century.
AT THE SECOND CONFERENCE
of Astronomers and Astrophysicists at Cambridge, there was a paper on the “suggestion” that asteroid rotation, if any, might be deduced from a light curve. But no variation of the brightness with time was found, and Henry Parkhurst concluded: “I think it is safe to dismiss the theory.” It is now a cornerstone of asteroid studies.
In a discussion of the thermal properties of the Moon, made independently of the one-dimensional equation of heat conduction but based on laboratory emissivity measurements, Frank Very concluded that a typical lunar daytime temperature is about 100°C—exactly the right answer. His conclusion is worth quoting: “Only the most terrible of Earth’s deserts where the burning sands blister the skin, and the men, beasts, and birds drop dead, can approach noontide on the cloudless surface of our satellite. Only the extreme polar latitudes of the
Moon can have an endurable temperature by day, to say nothing of the night, when we should have to become troglodytes to preserve ourselves from such intense cold.” The expository styles were often fine.
Earlier in the decade, Maurice Loewy and Pierre Puiseux at the Paris Observatory had published an atlas of lunar photographs, the theoretical consequences of which were discussed in
Ap. J.
(5:51). The Paris group proposed a modified volcanic theory for the origin of the lunar craters, rills and other topographic forms, which was later criticized by E. E. Barnard after he examined the planet with the 40-inch telescope. Barnard was then criticized by the Royal Astronomical Society for his criticism, and so on. One of the arguments in this debate had a deceptive simplicity: volcanoes produce water; there is no water on the moon; therefore the lunar craters are not volcanic. While most of the lunar craters are not volcanic, this is not a convincing argument because it neglects the problem of possible repositories for water. Very’s conclusions on the temperature of the lunar poles could have been read with some profit. Water there freezes out as frost. The other possibility is that water might escape from the Moon to space.
This was recognized by Stoney in a remarkable paper called “Of Atmospheres upon Planets and Satellites.” He deduced that there should be no lunar atmosphere because of the very rapid escape to space of gases from the low lunar gravity, or any large build-up of the lightest gases, hydrogen and helium, on Earth. He believed that there was no water vapor in the Martian atmosphere and that Mars’ atmosphere and caps were probably carbon dioxide. He implied that hydrogen and helium were to be expected on Jupiter, and that Triton, the largest moon of Neptune, might have an atmosphere. Each of these conclusions is in accord with present-day findings or opinions. He also concluded that Titan should be airless, a prediction with which some modern theorists agree—although Titan seems to have another view of the matter (see
Chapter 13
).
In this period there are also a few breath-taking
speculations, such as one by the Rev. J. M. Bacon that it would be a good idea to perform astronomical observations from high altitudes—from, for example, a free balloon. He suggested that there would be at least two advantages: better seeing and ultraviolet spectroscopy. Goddard later made similar proposals for rocket-launched observatories (
Chapter 18
).
Hermann Vogel had previously found, by eyeball spectroscopy, an absorption band at 6183 Å in the body of Saturn. Subsequently the International Color Photo Company of Chicago made photographic plates, which were so good that wavelengths as long as H Alpha in the red could be detected for a fifth-magnitude star. This new emulsion was used at Yerkes, and Hale reported that there was no sign of the 6183 Å band for the rings of Saturn. The band is now known to be at 6190 Å and is 6
v
3
of methane.
Another reaction to Percival Lowell’s writings can be gleaned from the address of James Keeler at the dedication of the Yerkes Observatory:
It is to be regretted that the habitability of the planets, a subject of which astronomers profess to know little, has been chosen as a theme for exploitation by the romancer, to whom the step from habitability to inhabitants is a very short one. The result of his ingenuity is that fact and fancy become inextricably tangled in the mind of the layman, who learns to regard communication with the inhabitants of Mars as a project deserving serious consideration (for which he may even wish to give money to scientific societies), and who does not know that it is condemned as a vagary by the very men whose labors have excited the imagination of the novelist. When he is made to understand the true state of our knowledge of these subjects, he is much disappointed and feels a certain resentment towards science, as if it had imposed upon him. Science is not responsible for these erroneous ideas, which, having no solid basis, gradually die out and are forgotten.
The address of Simon Newcomb on this occasion contains some remarks which apply generally, if a little idealistically, to the scientific endeavor:
Is the man thus moved into the exploration of nature by an unconquerable passion more to be envied or pitied? In no other pursuit does such certainty come to him who deserves it No life is so enjoyable as that whose energies are devoted to following out the inborn impulses of one’s nature. The investigator of truth is little subject to the disappointments which await the ambitious man in other fields of activity. It is pleasant to be one of a brotherhood extending over the world in which no rivalry exists except that which comes out of trying to do better work than anyone else, while mutual admiration stifles jealousy … As the great captain of industry is amoved by the love of wealth and the politician by the love of power, so the astronomer is moved by the love of knowledge for its own sake and not for the sake of its application. Yet he is proud to know that his science has been worth more to mankind than it has cost … He feels that man does not live by bread alone. If it is not more than bread to know the place we occupy in the universe, it is certainly something that we should place not far behind the means of subsistence.
AFTER READING
through the publications of astronomers three-quarters of a century ago, I felt an irresistible temptation to imagine the 150th Anniversary Meeting of the American Astronomical Society—or whatever name it will have metamorphosed into by then—and guess how our present endeavors will be viewed.
In examining the late-nineteenth-century literature, we are amused at some of the debates on sunspots, and impressed that the Zeeman effect was not considered a laboratory curiosity but something to which astronomers should devote considerable attention. These two threads intertwined, as if prefigured, a few years later in G. E. Hale’s discovery of large magnetic field strengths in sunspots.
Likewise we find innumerable papers in which the existence of a stellar evolution is assumed but its nature remains hidden; in which the Kelvin-Helmholtz gravitational contraction was considered the only possible stellar energy source, and nuclear energy remained entirely unanticipated. But at the same time, and sometimes in the same volume of the
Astrophysical Journal
, there is acknowledgment of curious work being done on radioactivity by a man named Becquerel in France. Here again we see the two apparently unrelated threads moving through our few-years snapshot of late-nineteenth-century astronomy and destined to intertwine forty years later.
There are many related examples—for instance, in the interpretation of series spectra of nonhydrogenic elements obtained at the telescope and pursued in the laboratory. New physics and new astronomy were the complementary sides of the emerging science of astrophysics.
Accordingly, it is difficult not to wonder how many of the deepest present debates—for example, on the nature of quasars, or the properties of black holes, or the emission geometry of pulsars—must await an intertwining with new developments in physics. If the experience of seventy-five years ago is any guide, there will already be people today who dimly guess which physics will join with which astronomy. And a few years later, the connection will be considered obvious.
We also see in the nineteenth-century material a number of cases where the observational methods or their interpretations are clearly in default by present standards. Planetary periods deduced to ten significant figures by the comparison of two drawings made by different people of features we now know to be unreal to begin with is one of the worst examples. But there are many others, including a plethora of “double-star measurements” of widely separated objects, which are mainly physically unconnected stars; a fascination with pressure and other effects on the frequencies of spectral lines when no one is paying any attention to curve of growth analysis; and acrimonious debates on the presence or
absence of some substance based solely upon naked-eye spectroscopy.
Also curious is the sparseness of the physics in late-Victorian astrophysics. Reasonably sophisticated physics is almost exclusively the province of geometrical and physical optics, the photographic process, and celestial mechanics. To make theories of stellar evolution based on stellar spectra without wondering much about the dependence of excitation and ionization on temperature, or attempting to calculate the subsurface temperature of the Moon without ever solving Fourier’s equation of heat conduction seems to me to be less than quaint. In seeing elaborate empirical representations of laboratory spectra, the modern reader becomes impatient for Bohr and Schrödinger and their successors to come along and develop quantum mechanics.