212 PUBLICATIONS OF THE SOME PIONEER OBSERVERS1 By Frank Schlesinger In choosing a subject upon which to speak to you this eve- ning, I have had to bear in mind that, although this is a meeting of the Astronomical Society of the Pacific, not many of my audience are astronomers, and I am therefore debarred from speaking on too technical a matter. Under these circumstances I have thought that a historical subject, and one that has been somewhat neglected by the, formal historians of our science, may be of interest. I propose to outline, very briefly of course, the history of the advances that have been made in the accuracy of astronomical measurements. To do this within an hour, I must confine myself to the measurement of the relative places of objects not very close together, neglecting not only measure- ments other than of angles, but also such as can be carried out, for example, by the filar micrometer and the interferometer; these form a somewhat distinct chapter and would be well worth your consideration in an evening by themselves. It is clear to you, I hope, in how restricted a sense I am using the word observer ; Galileo, Herschel, and Barnard were great observers in another sense and they were great pioneers. But of their kind of observing I am not to speak to you tonight. My pioneers are five in number ; they are Hipparchus in the second century b.c., Tycho in the sixteenth century, Bradley in the eighteenth, Bessel in the first half of the nineteenth century and Rüther fur d in the second half. Newton was undoubtedly the greatest astronomer of all time; Hipparchus, in the opinion of many a competent judge, stands next to him. Historians in the field of literature have often lamented the fact that we know so little of the life of Shakespeare, but our knowledge of him is full and voluminous as compared with what we know of Hipparchus, whose position in science is of the same order of brilliance as that of Shake- 1 An address delivered at a special meeting of the Astronomical Society of the Pacific, at New Haven, Connecticut, on April 11, 1929, at which the Bruce Gold Medal of the Society was presented to Dr. Schlesinger.—Editors. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System ASTRONOMICAL SOCIETY OF THE PACIFIC 213 speare in literature. All that we know with certainty of Hip- par chus is that he was making observations from the island of Rhodes around the year 150 b.c. We do not know when or where he was born, nor when or where he died. Concerning even his work we know practically nothing at first hand. With the exception of one unimportant fragment of his own, our knowledge of what he did comes from the writings of an as- tronomer who lived three centuries later. Of his many contributions to the science, it is only to his success as an observer that we can now refer. Gnomons of various forms, the tall vertical obelisk among them, were the earliest astronomical instruments. By their aid the ancients be- came acquainted with the Sun's apparent motions from hour to hour and from season to season. The only other instrument to be used extensively before the time of Hipparchus was the crude cross-staff, two sticks or strips of metal at right angles to each other, the one containing two sights like those on a modern rifle; the other engraved with a scale of tangents, and movable through the center of the first ; this device can be made to yield the angular separation of any two objects. Hipparchus used, and probably invented, some forms of the armillary sphere and of the astrolabe, an arrangement of three or more circles, capable of giving in the hands of a very skilful observer differ- ences in celestial latitude and celestial longitude between two objects. With instruments of this type Hipparchus was able to set down the positions of the planets and the stars with an aver- age error that was in the neighborhood of four minutes of arc, or the equivalent of about one-eighth the angular diameter of the Sun or the Moon. The rough character of these observations is, I think, sur- prising at first sight, especially when we consider that they were not excelled until seventeen centuries after Hipparchus' time. Surely it ought to be possible to point to a star with an error much less than one-eighth the diameter of the Sun or Moon. I have in my hand a target at which a marksman has fired ten shots from a distance of fifty feet. The ten holes are merged together and on the average they deviate from their common center by less than a tenth of an inch, or about thirty seconds of arc. This © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System 214 PUBLICATIONS OF THE accuracy was attained by means of two ordinary sights, very much like those used by Hipparchus, and separated by about the same distance. We are led by such considerations as this to infer that the accuracy of Hipparchus' observations depended to only a very minor extent upon skill in pointing. The errors in graduating his circles were a more important source of inac- curacy. We must remember that these graduations had all to be made by hand and without any magnifying device whatsoever. On a circle thirty inches in diameter, a minute of arc corre- sponds to about 0.004 inch, and it is therefore not surprising that Hipparchus' observations were not more accurate, nor that they held the record for accuracy for so many centuries. Considering the state of the mechanic arts of the times, it was obvious that the only way in which the accuracy of astro- nomical measurements could then be increased would be by using larger graduated circles. But this was impractical with the types of instruments that were in use, and with the way they had to be used. The armillary sphere, for example, had to have its circles adjusted to certain planes, and these planes were con- stantly changing their apparent positions on account of the rotation of the Earth. For these reasons no considerable im- provement over the accuracy attained by Hipparchus was pos- sible until a new type of instrument was invented; one that remained stationary and that could therefore be made large enough to contain graduated circles of much greater radius. It was Tycho Brahe (1546-1601) who made this important invention. Tycho was the eldest son of a Danish nobleman, and therefore had the choice of a very different career from a scientific one. But his tastes ran so strongly in this direction that against the wishes of his father, and for the most part secretly, he studied mathematics and astronomy at the two uni- versities he attended. In the end he expended the whole of his considerable fortune, inherited from his father and from an uncle, in the construction of great astronomical instruments. These were housed, as all of us know, in palatial establishments on the island of Hveen and later at Prague. Tycho's position as one of the great leaders of our science is conceded by all ; and yet, in my opinion, his contributions have been underrated. © Astronomical Society of the Pacific · Provided by the NASA Astrophysics Data System ASTRONOMICAL SOCIETY OF THE PACIFIC 215 This may be on account of his theory of cosmogony, known as the Tychonic System, which placed the Earth at the center of the motions of the Sun and of the Moon, but made all the other planets revolve about the Sun. This theory has ever since been universally characterized by historians as "unfortunate/' "retro- grade," and the like. But no theory ever fitted all the facts, as they were known at the time, more perfectly than did this one. I wish I had time this evening to defend the Tychonic System from this point of view, but I must not wander too far from my subject. Tycho's most striking instrument was the mural quadrant, a quarter circle no less than nine feet in radius fixed on a north- and-south wall. At its center was a stationary sight and on its circumference a movable one. As an object crossed the meridian the sights were trained upon it, the time of meridian crossing noted, and the position of the movable sight on the circle recorded, thus furnishing information from which the right ascension and the declination could easily be deduced. On the circumference of such a circle one minute of arc corresponds to .032 inch, and this Tycho could subdivide into six parts, each one of which corresponded to 10 seconds. The average error of his b^st observations was about 40 seconds, a very great im- provement over anything that had been done before. There is much evidence to show that Tycho had very defi- nite objects in mind in making all his observations, and there can be little doubt that had he been spared (he died at fifty- five years) he would have examined his planetary observations from the same point of view as actually did his friend and pupil, Kepler, after Tycho's death. Upon these observations Kepler built his three laws of planetary motion, and upon these in turn Newton erected his proof of the law of universal gravi- tation. Less than ten years after Tycho's death came the invention of the telescope and its application to astronomy.