Parallax

By the spring of 1835, Struve's affairs were settling and he could look forward to a resumption of his studies of double stars and the search for parallax. He had married Johanna Bartels, and she expected the birth of their first child in the fall. Johanna, about 28 years old, restored order to the large household. Astronomers who knew her later described her as quiet and pious, but well-loved and respected. Otto described her as ''the refuge of all who needed help and council.''13

Beginning in 1835 and continuing through 1836, Struve resumed his work on double stars, following up on the Catalogus Novus work with measurements of the separations of a promising set of 17 double stars, including the bright star Vega and a nearby companion star. Civil construction had begun on the new observatory at Pulkovo, and Struve was trying to accomplish as much as possible at Dorpat before the inevitable disruption of the move. Bessel, meanwhile, had begun monitoring the position of 61 Cygni in September 1834, using a standard micrometer; he had not yet started using the heliometer for this purpose. He was trying to use two 11th-magnitude stars as references, but these stars were so faint in the heliometer, which had an aperture of only 6 inches, that he found their positions difficult to measure unless atmospheric conditions were outstandingly good. Bad weather made for slow progress on the parallax effort, and in 1835, geodetic field work claimed his attention, too. Thus in 1836, Struve had the opportunity to concentrate on his effort to measure parallax, while Bessel was somewhat diverted both by unrelated work and by difficulties with his approach. Struve's correspondence indicates that he had not yet mentioned to Bessel his intensive effort to find the parallax of Vega.

In January 1837, Struve had some news. He gave a preview of his forthcoming results in a letter to the Permanent Secretary of the Russian Imperial Academy of Sciences, Paul Fuss.

''I trust I have been able to show that observations are now incomparably more exact, and parallax cannot escape us even if it consists of as little as 1/10th of a second of arc,'' he wrote. He added that a special series of observations of the star Vega (otherwise known as Alpha Lyrae) were not yet absolutely decisive, but that calculations gave a parallax of between 0.10 and 0.18 arcseconds. He concluded by explaining his result in units of ''solar distance,'' the Earth-Sun distance: ''If it turns out, as I hope, that further calculations confirm this result, this would constitute the important discovery that Alpha Lyrae is at a distance from the solar system of 1 million solar distances.''14 Such vast distances to the stars had been suspected by some, but Struve's parallax measurements provided welcome hard evidence.

Struve then completed his review of the separations of double stars he had earlier cataloged. In 1837, he published the results, to wide acclaim, as Mensurae Micrometriae. In the introduction to this work Struve laid out his updated thoughts on opportunities for parallax measurements. He considered — incorrectly, as we now know—that the brightest stars are likely to be the nearest, although he admitted that this might not be true in all cases. He noted—perhaps as a result of discussions with Bessel, who had articulated the idea as early as 1812 — that a star's high proper motion probably indicated nearness. This, of course, was Bessel's criterion in selecting 61 Cygni.

The main thrust of Mensurae Micrometriae is Struve's effort to measure stellar distances for stars that he considered the most amenable to the parallax method. He quoted the parallax for Vega as 1/8 of an arcsecond (0.125 arcseconds), with a somewhat large ''probable error'' or range beyond this value of about 1/20 of an arcsecond (0.055 arcseconds). This parallax for Vega compares well with the modern value of 0.123 arcseconds.

Astronomers all over Europe took note of Struve's progress in parallax determinations, the first successful application of Galileo's double-star method. They admired Struve's results despite the large uncertainty associated with the quoted value. James South, the double-star observer in England, remarked to a visitor, ''Struve has reaped the golden harvest among the double stars, and there is little now for me to hope or expect.''15 Indeed, he became so bitter over his own failure in this regard that, in a fit of pique, he destroyed an expensive lens he had ordered for his observations.

Bessel, who had a greater personal fondness for Struve and didn't incline toward fits of pique anyway, praised the book enthusiastically. Struve had advised him of its imminent publication, and of his still inconclusive work on Vega, in a letter dated 25 July 1837.16 A month later, on 26 August, Bessel wrote back to Struve to praise the Mensurae Micrometriae, which he had by then received, and to say that Struve's attempt to measure the parallax of Vega stimulated him to do the same with 61 Cygni and another parallax candidate, Alpha Bootis. He noted, however, that Struve's final parallax results would likely emerge first, since (as Bessel had learned from Struve's letter) Struve had already been collecting data for a year. ''Forever the honor of having tried this method first belongs to you,'' Bessel wrote.17

To Humboldt, Bessel wrote in September that Struve's ''great work'' had given him much pleasure, and that some of Struve's comments had provoked him to scrutinize his own methods again. He did not consider differences between Struve's measurements and his to be pertinent for public comment, but, Bessel wrote, he had satisfied himself that specific disagreements arose either from technical difficulties in the use of the micrometer or from Struve's inability to compensate precisely for the unwanted movement of his telescope due to the clock drive mechanism, whose function was to keep the telescope pointed at the stars as they rose and set.18

In October, Bessel wrote to his mentor Olbers, acknowledging that he saw himself in competition with Struve. ''I think Struve has taken the lead, for he has made an attempt which, though not yet a complete success, nevertheless seems to offer good prospects,'' he wrote.19

Spurred and perhaps worried by Struve's progress, Bessel resumed his effort on 61 Cygni with renewed vigor. As he later wrote to John Herschel, various things, including work on the Earth's gravitational effect on timekeeping devices, had distracted him earlier:

''In the year 1835, researches on the length of the pendulum at Berlin took me away for three months from the Observatory; and when I returned, Halley's comet had made its appearance and claimed all clear nights. In 1836, I was too much occupied with the calculations of the measurement of a degree in this country, and with editing my work on the subject, to be able to prosecute the observations of a Cygni [he meant, 61 Cygni] so uninterruptedly as was necessary, in my opinion, in order that they might afford an unequivocal result. But in 1837 these obstacles were removed, and I, thereupon, resumed the hope that I should be led to the same result which Struve grounded on his observations of Alpha Lyrae, by similar observations of 61 Cygni.''20

During the course of his new observations in the winter of 1837-38, Bessel abandoned work on Alpha Bootis and focused on securing the parallax of 61 Cygni. He stopped using the two 11th-magnitude reference stars he had had difficulty with and found some brighter ones, magnitudes 9 and 10, farther away from 61 Cygni. Now he applied the heliometer to measuring these distances. The comparison stars were about 8 and 12 arcminutes away, far outside the range of a conventional micrometer but well within the range of the heliometer. In March 1838, Bessel described this progress to Struve, adding that Struve would be the first to learn of the results, since they were encouraged by Struve's observations of Vega. Struve responded that measurements of Vega continued to accumulate, but that he would wait to process all his data until November of that

year.21

In October 1838, just before Struve's anticipated new results on Vega, Bessel announced his own results to the editor of the Astronomische Nachrichten and to John Herschel at the Royal Astronomical Society. He found 61 Cygni's annual shift to be about 1/3 of an arcsecond (0.314 arcseconds); the results might be in error by 1/50 of an arcsecond (0.020 arcseconds). The parallax angle for 61 Cygni is larger than that Struve found for Vega, and indicates that Bessel's target was closer, at a distance of about 10 light-years. Bessel's result is not far from the modern value for the parallax of 61 Cygni, 0.287 arcseconds, determined with the use of instruments on board the Hipparcos satellite.

Neither Bessel nor Struve considered the race to measure stellar distances to be over. Both continued to refine their results on the target stars they had chosen. However, textbooks usually cite this parallax determination of Bessel's as the first measurement of the distance to a star. His measurement was more precise: he found the distance to 61 Cygni to be 10.4 light-years, plus or minus 0.6 light-years. Struve, on the other hand, found that the distance to Vega lay in the range 18 to 47 light-years.

Bessel's result appeared in the Astronomische Nachrichten of December 1838. Struve's reaction is not recorded in his correspondence, although he must have noted Bessel's superior result. This year too was a difficult year for Struve, with or without the competition with Bessel. His son Friedrich, the second child with Johanna Bartels, had recently died in infancy. Johanna's father, Struve's close friend and colleague in the mathematics faculty, died. Paul, the couple's third child, was born in 1838, but he did not survive infancy either, leaving only the first child by this marriage, Karl.

Yet again, Struve could not allow himself to be distracted by family misfortunes, on account of the impending move to Pulk-ovo. In the late summer of 1838, he took Otto back to Munich to purchase books for the new library he intended to create, and to inspect the instruments that were nearing completion. The firm of Merz and Mahler, the successor to that of Utzschneider and Fraunhofer, had succeeded in making a lens of 15 inches, significantly bigger than the 10.5 inches that the Czar's commission had originally proposed. During this time, Struve also advised assistants on surveys to determine the levels of the Black and Caspian seas.

Unexpectedly, in January 1839 a newcomer joined the parallax contest, if rather belatedly and timidly. Thomas Henderson had been stationed at the British observatory at the Cape of Good Hope in what is now South Africa. Upon returning to his home in Scotland with his data, he discovered that his observations of the star Alpha Centauri would lend themselves to parallax determinations. Henderson had stumbled on what is in fact the closest star, with a parallax of three-quarters of an arcsecond. He published his result of 1 arcsecond, with an uncertainly of 0.75 arcseconds. The result struck the astronomical community as rather tentative, given that he quoted such a large estimated uncertainty. Henderson's paper must have sounded a kind of warning bell for Struve, however, signaling that other observers deemed parallax measurements within range of their instruments.

Struve gathered as much additional data on Vega as he could at Dorpat, because the precision required for parallax measurements would not allow him to combine data from two different instrumental set-ups at Dorpat and Pulkovo. But he did not have time to analyze his data before the move, which came in May 1839.

Johanna presumably took care of moving the extended — and still growing—family into their new quarters, a two-story apartment connected to the main building. (See figure 5.7 for a nineteenth-century view of Pulkovo.) Between 1839 and 1842, Wilhelm and Johanna had three more children, Anna, Ernst, and Nikolai. Johanna took responsibility for educating the Struve children, too, as there was no school at Pulkovo. The observatory lay too far for the family and visiting astronomers

Figure 5.7 The Pulkovo Observatory at the end of the nineteenth century. Fraunhofer's 15-inch Great Refractor occupied the space under the central dome. (Credit: Institute of Astronomy, Cambridge, UK.)

to have regular contact with St. Petersburg society. They even grew some of their food on the observatory grounds.

The various telescopes, clocks, and accessories that Struve had ordered from Germany and England, including the ''15-inch great refractor'' telescope, arrived in parts in 102 crates a few months after the family moved. Once again, as in Dorpat, Struve could not wait for company representatives to take charge of the assembly. He had taken a keen interest in the details of the instruments' construction—Merz and Mahler's assistants might have called it an overbearing interest, as legend has it that Struve vetted every screw. Armed with this knowledge, he and the new Pulkovo employees erected the new telescopes by themselves in six weeks.

Having assembled Pulkovo's instruments with all possible speed, Struve returned to analyzing his Dorpat data on the parallax of Vega as soon as he could. He put Otto in charge of the ''15-inch great refractor.'' Struve himself did not observe as much at Pulkovo as he had at Dorpat. However, in conjunction with his attempt to refine the parallax measurement on Vega, he made more observations pertaining to the strength of aberration and nutation constants.

His new value for the parallax shift of Vega came out in 1840: an angle of 0.2619 arcseconds, with an uncertainty of 0.0254 arcseconds. With hindsight, it appears that Struve's attempt at refining the value of parallax failed, for his first value was more nearly correct. It is Bessel's 1838 result for 61 Cygni, not Struve's work on Vega, that history has viewed as the crossing of the finish line in the race to measure the distances to the stars by parallax. Even in his day, Struve's colleagues must have felt their confidence shaken by the fact that the new value was almost twice the old. Nevertheless his peers recognized it as a step forward. A small change Struve made in his method of collecting data even prompted Bessel to revise his estimate of the parallax of 61 Cygni.

Bessel's health began to decline about this time, due to cancer. From a professional standpoint, however, he continued to reap the rewards for a life of exceptional industry and accomplishment. In 1841, the Royal Astronomical Society presented him with a Gold Medal for his determination of the parallax of 61 Cygni.

John Herschel, as President of the Royal Astronomical Society, gave the address on the occasion of Bessel's Gold Medal. He alluded to Struve's and Henderson's work also, praising all three results as ''among the fairest flowers of civilization.'' The measurement of stellar distances had seemed a ''great and hitherto impassable barrier to our excursions into the sidereal universe,'' he said, adding that the barrier's being ''almost simultaneously overleaped at three different points'' was ''the greatest and most glorious triumph which practical astronomy has ever witnessed.''22

Certainly the search for parallax had proved to be frustrat-ingly difficult. In the late 1600s, even the parallax of Mars, our neighbor in the solar system, challenged the most sophisticated observers. In the mid-1700s, Bradley, the English astronomer who discovered aberration while searching for parallax, could only conclude that parallax was a smaller effect than many supposed, and that the stars must lie at about 400 000 times the Sun's distance from Earth. About a century of technological progress elapsed after Bradley's laborious attempt to find parallax before Bessel, Struve, and Henderson showed that the nearest stars are truly as remote as Bradley had suspected.

Perhaps because John Herschel took so much care to refer to Struve's and Henderson's accomplishments in his address for Bessel's Gold Medal, Struve made no public complaint about the relatively small fuss made over him. Perhaps it was his affection for Bessel that mollified any bitterness. Bessel died in 1846; two years later, in a pamphlet describing the Pulkovo observatory, Struve remarked without acrimony that Bessel's measurement of the annual parallax of 61 Cygni was ''one of the greatest discoveries of our century.''23

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