Selection of target stars and instruments

When he wasn't out measuring the Earth or teaching his many classes, Struve liked to sweep the sky with the transit instrument at Dorpat or with a later acquisition, a 3§-inch aperture refracting telescope equipped with a micrometer. As William Herschel had done before him, Struve spent a great part of his early astronomical career looking for double stars. Indeed, Struve consciously took many cues from Herschel, not only cataloging double stars — something few other astronomers devoted so much time to — but also following up on specific observing projects Herschel had begun. For example, Struve confirmed Herschel's contention that the bright star Castor in the constellation Gemini actually consists of a physical binary system, with one star in orbit around the other.

Struve knew that many of the double stars he observed would eventually reveal themselves to be physical binaries, or even triple stars in mutual orbit, and so not suitable for the double-star approach to parallax determinations. Nevertheless, Struve approved of Herschel's plan (actually, Galileo's method) to seek distance measurements among those doubles that were not members of physical binary systems—and especially among those doubles in which one star was much brighter than the other, suggesting that one star was near and the other, far. Struve tried more than one scheme for measuring parallax, but his efforts to compile double star catalogs testify to his belief that double stars would ultimately yield a rich harvest of parallax measurements, and that he might succeed where Herschel had failed.

In 1818, Struve began a multi-year attempt to determine parallax angles for a number of stars near the north celestial pole, including some in Ursa Minor, the ''Little Bear.'' In the same year Bessel published a monumental work, Fundamenta Astronomiae, that would set the stage for his own search for parallax. Fundamenta Astronomiae lists the positions of stars that Bradley had observed in the mid-1700s, corrected for instrumental effects and for distorting effects such as refraction and precession. This work by Bessel allowed later generations of astronomers to compare their observations with Bradley's, giving them a long time span over which to note changes and proper motions of stars. Bessel himself concluded from his review of Bradley's star catalog that parallactic shifts must be less than one arcsecond—a stiff challenge for seekers of parallax.

Bessel had also by then chosen his favorite target for parallax measurements, a star in the constellation Cygnus, known in catalogs as 61 Cygni. He had even published preliminary results in 1815 and 1816. The race was on: for the first time, two astronomers who understood the difficulty of parallax measurements and who perceived the futility of earlier efforts set out to do it right. In these early years of the race, however, both Struve and Bessel concentrated their efforts on laying the groundwork — Struve cataloging double stars and both Struve and Bessel gauging effects that might distort their measurements—knowing that their data on specific stars might have to be corrected later.

The star Bessel chose, 61 Cygni, is actually a binary system about 30 arcseconds wide. Its components, two orange stars of similar brightness that are not hard to see and separate with today's binoculars, orbit each other with a period of several hundred years. Bessel did not rule out 61 Cygni for parallax measurements on account of its binary nature, but treated the binary star system as a whole, and measured its position with respect to a number of widely-spaced background stars.

Bessel cast his lot with 61 Cygni because he had some clues, independent of parallax measurements, that it might be one of the nearer stars. In 1806, Piazzi had drawn the astronomical community's attention to the fact that 61 Cygni, also known as the ''flying star,'' moves very rapidly across the sky, exhibiting a proper motion like the moving stars Edmond Halley had noticed in the early 1700s (see chapter 3). This proper motion is easy to distinguish from the shift due to parallax or aberration: a star with proper motion keeps moving in one direction, instead of wobbling back and forth against a field of background stars as the observer's line of sight changes with the Earth's orbit around the Sun. In the case of 61 Cygni, the proper motion is at the unusually high rate of more than 5 arcseconds a year. Piazzi suggested this might be an indication of the star system's proximity. All stars, he reasoned, might possess proper motion to some degree, and simple geometric arguments indicate that, on average, the nearest stars or star systems would exhibit the highest proper motion. In the same way, cars in an adjacent lane on the highway appear to move quickly, while cars on a distant highway appear to cross one's field of view more slowly.

A second clue that 61 Cygni might prove to be a good target in the quest for parallax lay buried in Bradley's mid-1700s observations of the system. Bradley had determined the approximate period of rotation of the two component stars about their mutual center of gravity. By applying basic principles of physics in the form of the laws of planetary motion (Kepler's third law, in particular), Bessel could deduce the approximate distance between the two stars. A comparison of this linear distance with the apparent separation of the two stars in the sky suggested the pair must be somewhat more than 7 light-years distant, with a parallax somewhat less than half an arcsecond. A small angle, to be sure, but perhaps measurable.

In devoting most of his effort to 61 Cygni, which he thought for independent reasons was relatively close, Bessel cleverly maximized his chances in what might turn out to be a cosmic wild goose chase. Careful astronomers all knew that parallax angles might prove to be too small for even the best observers to measure. Indeed, Bessel's first effort with 61 Cygni gave a negative number for the parallax angle, clearly indicating that his measurements and corrections were not sufficiently refined. Struve, initially, didn't even try to do more than set upper limits on the parallax for his set of polar stars.

In 1820, Struve took a step that would improve his own odds in the search for parallax. On a trip to Munich to order geodetic survey equipment, he looked in on Joseph von Fraunhofer, Europe's pre-eminent glass manufacturer and maker of optical instruments. Fraunhofer's help would be critical to both Struve and Bessel in their efforts to make astronomical observations of very high precision.

Fraunhofer, orphaned at a young age, had worked under harsh conditions as an apprentice mirror-maker and lens-grinder. In 1801, his master's house suddenly collapsed, killing his master's wife, but the young Fraunhofer was pulled alive from the rubble four hours later. The accident brought him to the attention of a wealthy civil servant and entrepreneur, Joseph von Utzschneider, who encouraged Fraunhofer's subsequent technical education and eventually—recognizing Fraunhofer's great talent—formed a partnership with him, the optical firm of Utzschneider and Fraunhofer.

Around 1814, while studying the refraction of light through glass in an effort to improve telescope optics, Fraunhofer made the discovery that he is best known for. He dispersed the Sun's light through prisms and found that the resulting spectrum did not form a continuous rainbow of color, but was crossed by some 500 dark lines. The lines, which are now known as the Fraunhofer lines, proved to be indicators of chemical elements in the solar atmosphere. The discovery of these lines stimulated the development of the science of spectroscopy, which in turn revolutionized astronomy, as we shall see in the next chapter. Fraunhofer, however, was singlemindedly dedicated to refining telescope lenses, and did not inquire in depth into the nature of his solar lines.

At the time of Struve's visit in 1820, Fraunhofer was working on a lens 9.6 inches in diameter (9 ''Paris inches''), engineered to minimize the color distortions or ''chromatic aberration'' that glass lenses tended to produce in the telescopic images of stars. Chromatic aberration is more than a cosmetic problem: images blur when the different colors composing white light focus differently. To diminish the appearance of colored rings around the images of stars, Fraunhofer combined two types of glass, known as flint glass and crown glass, side by side. The chromatic aberrations produced by each lens individually could be made to cancel out in a so-called achromatic lens, producing a visually sharp image.

The challenge for the glass industry was to produce large samples of crown and flint glass without imperfections such as streaks and bubbles. Utzschneider, dissatisfied with the glass available on the market, had set up his own workshop in Benediktbeuern, some 30 miles [50 kilometers] from Munich, to supply his scientific instruments company. There he employed a Swiss artisan, Pierre Louis Guinand, who had developed a technique to make superior glass. Guinand eventually returned to Switzerland, but not before confiding—or being forced to divulge—his trade secrets to Fraunhofer.

A telescope made with Fraunhofer's 9.6-inch achromatic lens would be the largest refracting or lens-based telescope in the world, allowing for the clearest distinction between close double stars and producing crisp, uncolored images. Properly mounted, it would also allow for the most precise position determinations. Of course, in light-gathering power, Herschel's large mirror-based telescopes had the advantage: his 20-foot telescope had an aperture of 12 inches, and the flawed 40-foot telescope had an aperture of 49 inches. However, reflecting telescopes of the eighteenth and early nineteenth centuries produced poor quality images. As Herschel knew from experience, their metal surfaces were more difficult to figure accurately than comparable glass surfaces and they tarnished easily, requiring constant maintenance. Instrument makers such as Utzschneider and Fraunhofer were eager to solve problems of glass production because they knew that refractors promised to form the basis of more maintenance-free, reliable products.

Struve, who was more interested in precision observations than in discovering exceedingly dim sources such as nebulae, took a keen interest in Fraunhofer's prototype large refractor telescope. He resolved to secure it for the observatory at Dorpat if he possibly could. On his way home from Altona, he stopped in Konigsberg and discussed Fraunhofer's innovation with Bessel. As soon as he reached Dorpat he submitted a proposal to the university's rector, offering to sacrifice some of his other research expenditures to make room for the telescope in the university's budget. The proposal wended its way up to the chancellor of the university, and Struve, to his elation, received a positive response.

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