From amateur to professional

The three-year period from 1779 to 1782, culminating with the discovery of the seventh planet of the solar system and events in the wake of that discovery, marked Herschel's transformation from amateur to professional astronomer. His friend William Watson played an important role in helping him make this transformation by introducing him to the philosophical societies in Bath and London. But even before he met Watson on the street outside his house in Bath, Herschel had begun to formulate lofty goals for his telescopic observing program—goals that would have seemed ambitious even to the Astronomer Royal at Greenwich Observatory. Herschel's discovery of Uranus not only made him unexpectedly famous, but set in motion a change in his circumstances that made it possible for him to devote more of his time to the pursuit of his astronomical ambitions.

Bath's Literary and Philosophical Society was just forming and had not yet held its first meeting when Watson encountered Herschel examining the lunar surface through his telescope. Such societies, which became popular in England and the United States in the 1700s, modeled themselves after the Royal Society in London, chartered in 1662. In this pre-Industrial Revolution era the Royal Society welcomed men as disparate in education as the navigator James Cook, who joined the navy without attending university, and the Oxford-educated philosopher and economist Adam Smith. Until 1840, all that was required for membership in the Royal Society was an interest in natural philosophy, however ''amateur'' or self-taught. (The French Academie des Sciences, established in 1666, limited membership to the more mathematically trained scientists from its inception.) In England, the local societies such as Bath's provided an opportunity for scholars, skilled merchants, and craftsmen alike to present their ideas and technological experiments to each other.

Herschel, of course, became one of the Bath society's most active members. He contributed a number of papers on metaphysics and epistemology — the philosophy of ''how we know what we know''—but his reports on astronomical observations elicited the most approbation. The Bath society forwarded papers of particular interest to the Royal Society—which, as a venue for the most learned scientific dialogue and a central repository of information, often served as an advisory body to the monarch. Two of Herschel's first papers to receive the distinction of being forwarded to the Royal Society concerned the variable star Mira Ceti, which brightens and dims with an irregular period of about 11 months, and the lunar mountains.

Watson continued to mentor Herschel after introducing him to the Bath society. He read and commented on Herschel's manuscripts before Herschel submitted them, curbed Herschel's enthusiasm for unfounded speculation and provided feedback on how the papers were received at Royal Society meetings in London. Some of the claims Herschel made about his telescopes in his early papers seemed, indeed, rather fantastic to members of the Royal Society.

In this period Herschel's musical duties began to appear as frankly onerous. He had, by about 1780, set his sights on three main problems, each of which might very well have occupied him full time: the measurement of the distances to the stars through the phenomenon known as parallax; the construction of even bigger and better telescopes; and the discovery of new nebulae like the well-known ''Great Nebula'' in Orion, and other interesting, faint sources. He knew already that his home-made telescopes allowed him to see the stars, planets, and nebulae as few had ever seen them. No wonder Caroline recorded in her journal that the endless round of music lessons, concerts, and rehearsals seemed ''an intolerable waste of time.''23

Parallax is the apparent shift of position of nearby objects when viewed by an observer who is in motion or who observes from different locations. The principle is easily demonstrated with one's own hand and eyes: one holds one's thumb still at arm's length and views it alternately with the right eye open and left eye closed, and vice versa. The thumb appears to ''jump'' back and forth with respect to background objects. The ''jump'' occurs because each eye observes from a slightly different vantage point. The nearer the thumb to one's face, the bigger the jump. Indeed, we unconsciously gauge distances and acquire depth perception, at least qualitatively, through this effect of parallax.

Parallactic shifts or jumps can be used to measure distances. The technique is similar to the surveyor's method Thomas Wright used to measure distances or heights by triangulation. Observations of a target object are made from two stations, separated by a baseline. In the example of the thumb at arm's length, the thumb is the target, about 40 centimeters away, and the eyes are the two stations, separated by a baseline of about 5 centimeters. Mathematically, one can relate the distance to the target to the size of the parallactic shift and the length of the baseline. In the astronomical case, a target star would be observed from two different locations and its position with respect to some ''fixed'' background stars carefully recorded.

If the stars chosen as fixed reference points were not, in fact, more distant than the target star, the technique would not work because these stars would also shift their positions slightly. The search for stellar parallax, which astronomers had sought for more than a century already, thus required a selection of suitable target stars. In the absence of any clues to stellar distances, astronomers generally assumed that the brighter stars were the nearer ones, and focused their efforts on these as targets.

The parallactic shift of nearby objects is such a well-established phenomenon that for hundreds of years the absence of detectable stellar parallax gave some skeptics reason to doubt the Copernican or Sun-centered model of the solar system. As the Earth orbits the Sun, it describes a circle in space. (Actually, it describes an ellipse, but the ellipse is very nearly circular.) Observations of a target star made six months apart—say, in January and June — are separated by some 300 million kilometers [186 million miles], the diameter of Earth's orbit. With such a long baseline, the skeptics asked, shouldn't some of the stars show yearly parallactic shifts? For some time, in fact, astronomers looked for evidence of parallax among the stars as proof that the Earth circles the Sun. By Herschel's time, the Sun-centered model was no longer in doubt, but the search for parallactic shifts continued, as a quest in its own right. In 1760, Nevil Maskelyne, the Astronomer Royal, specifically urged his fellow professional astronomers to intensify their efforts to find parallax.

The problem is that the stars are very far away, and parallax is a very small effect to be sought among thousands of cataloged bright stars. The farther away the star, the smaller the ''jump,'' so the success of the technique depends on being able to record very slight apparent displacements. Pinpointing the locations of many stars in the stellar equivalent of latitude and longitude and re-measuring these coordinates at a later time in the year is time-consuming. Furthermore, the measurements of stars in different parts of the sky might be affected differently by atmospheric conditions, to name just one problem.

In the seventeenth century, Galileo thought of a way to make the search for parallax more efficient. Instead of measuring the positions of bright (and thus presumably nearby) stars, and checking them periodically to see if any had moved back and forth on a yearly timescale, he suggested one should find double stars — that is, two stars next to each other in the sky — and monitor their separation. If one member of the pair is actually near and the other far, the near one will appear to narrow the gap, then increase it again (see figure 4.5).

Herschel appreciated the practicality of Galileo's method. The task of searching for parallax was reduced to a single measurement, the angular separation of two stars, rather than a set of right ascension and declination measurements for each star observed. Galileo's method had never been implemented, even by its originator, however, for it requires a catalog of double stars to use as targets, and a method of measuring the separation of two stars in the telescope field of view.

In 1778, shortly before he met Watson and became involved in the philosophical society, Herschel decided to try his hand at searching for parallax by Galileo's untried method. He would measure the separations of the stars with a micrometer, an instrument for measuring small distances. Micrometers took many different forms in the eighteenth and nineteenth centuries. (A simple one is illustrated in figure 4.6.) In the common wire micrometer, a vertical wire, visible in the eyepiece of the telescope, is fixed; the telescope can be pointed so that one of the two stars in a double is seen to lie on this wire. The position of a second, movable wire can then be adjusted until it matches that of the second star. A scale to the side of the micrometer allows the astronomer to record the linear separation of the wires, which corresponds to the angular separation of the two stars. Herschel also used a different kind of micrometer of his own invention, a lamp micrometer. This was an artificial double star made from two lamps shining through two pinholes in a wooden board. He would observe the real double star with his right eye at the

Figure 4.5 Parallax. Top panel: From the Earth's orbital position around the Sun in June, an observer sees, on the plane of the sky, the nearby star A to the right of the more distant star B. Bottom panel: Six months later, in December, the Earth is at the opposite end of a baseline formed by the diameter of Earth's orbit. (The diameter is twice the Sun-Earth distance, or two Astronomical Units (AU), or 1.5 x 108 km, or 93 million miles.) From this new vantage point, the perspective on the stars is different, and star A appears to have moved closer to star B. Six months later, in June of the following year, the apparent separation of A and B will have increased again. (Credit: Layne Lundstrom.)

Figure 4.5 Parallax. Top panel: From the Earth's orbital position around the Sun in June, an observer sees, on the plane of the sky, the nearby star A to the right of the more distant star B. Bottom panel: Six months later, in December, the Earth is at the opposite end of a baseline formed by the diameter of Earth's orbit. (The diameter is twice the Sun-Earth distance, or two Astronomical Units (AU), or 1.5 x 108 km, or 93 million miles.) From this new vantage point, the perspective on the stars is different, and star A appears to have moved closer to star B. Six months later, in June of the following year, the apparent separation of A and B will have increased again. (Credit: Layne Lundstrom.)

telescope, and the lamp micrometer's artificial double with his left eye. He adjusted the separations of the lamps until the real and artificial doubles matched, and from this set-up he would calculate the angular separation of the real stars.

Herschel did not find parallax immediately among his favorite targets, such as the double star Castor in the constellation Gemini. In fact, he searched for parallax in vain over the course of a 40-year career in astronomy. But the search prompted him to compile catalogs of double stars that might make good target

Fixed wire (Moveable wire

Fixed wire (Moveable wire

Stale

Figure 4.6 A simple type of wire micrometer as used in the late 1700s and early 1800s. The observer positions the telescope so that a fixed wire, which is visible through the eyepiece of the telescope, appears at the same location as a star or other object of interest. The position of a second, moveable wire, can be adjusted so that it appears to rest on the second star. A scale attached to the micrometer indicates the separation of the wires, and hence the angular separation of the stars in the sky. (Credit: Layne Lundstrom.)

Stale

Figure 4.6 A simple type of wire micrometer as used in the late 1700s and early 1800s. The observer positions the telescope so that a fixed wire, which is visible through the eyepiece of the telescope, appears at the same location as a star or other object of interest. The position of a second, moveable wire, can be adjusted so that it appears to rest on the second star. A scale attached to the micrometer indicates the separation of the wires, and hence the angular separation of the stars in the sky. (Credit: Layne Lundstrom.)

objects for future generations of astronomers, who might be able to achieve better precision in their measurements and isolate smaller shifts. As the years went on his double star catalogs became ever longer and more comprehensive, and were recognized as valuable contributions to the field.

The second of Herschel's three main goals, formulated during the period when he was making the transformation from amateur to professional astronomer, brought him more gratification than the search for parallax. He set out to make bigger telescopes, pushing the limits of what could be accomplished with reflectors in the pursuit of faint or nebulous sources of light. Alexander and Caroline provided essential support in this extension of his already complex telescope-making operation.

Previously, Herschel had obtained mirror blanks about 6 inches in diameter from a local craftsman. What was left for

Herschel to do was to grind them to the required figure and polish them. But when his supplier of blank mirror disks could no longer keep up with his requirements, Herschel learned how to cast the blanks himself. First he experimented with trials of different mixtures of metals to see which yielded a mirror surface that was highly reflective but retained its shape over the wide range of temperatures prevailing during the course of a night's observing. Some of the best mirrors were so brittle and sensitive to temperature changes that touching them with a warm hand on a very cold night might shatter them. Herschel found an alloy that pleased him in these respects, although the hardest and most reflective metals tended to tarnish quickly. He found it necessary to keep for each of his telescopes a spare mirror polished and ready to be exchanged with a tarnished one at short notice.

Casting the mirrors proved to be a peculiarly tedious and sometimes dangerous undertaking. Following the advice in Dr. Smith's four-part volume on optics, Herschel melted the copper, tin, and other metals together and poured the molten mixture carefully into a heat-resistant mold to cool. Herschel's recipe for the mold itself called for loam, a soil rich in clay and sand, mixed with dried horse manure. Caroline noted in her memoir that ''an immense quantity'' of the horse dung had to be ''pounded in a mortar and sifted through a fine sieve.'' Alexander and Dr. Watson took their turns at the mortar and pestle.24

The furnace for casting the mirrors shared space in Herschel's flagstone-floor basement with the house's kitchen. It was early in 1781 in this basement room, opening directly onto the garden, that Herschel, his assistants, and Alexander discovered the perils of working with molten metal. They were attempting to cast an exceptionally large mirror, 36 inches in diameter. Caroline wrote, ''[T]he mould, &c., in readiness, a day was set apart for casting, and the metal was in the furnace, but unfortunately it began to leak at the moment when ready for pouring, and both my brothers and the caster with his men were obliged to run out at opposite doors, for the stone flooring (which ought to have been taken up) flew about in all directions, as high as the ceiling. My poor brother fell, exhausted with heat and exertion, on a heap of brickbats.''25

Herschel survived the exploding flagstones episode and eventually succeeded in casting larger mirrors. During this transition period of the late 1770s, however, his largest telescope had an aperture of 12 inches. He referred to this instrument by its length as the ''20 foot telescope,'' and later as his ''former'' or ''small 20 foot'' to distinguish it from a subsequent 20-foot long telescope of 19 inches aperture. His 6.2 inches aperture telescope, which he called his ''7 foot'' telescope, also served him well, for it was more portable. This was the beautiful instrument Watson first saw, a square-section mahogany tube on a wooden stand. The 7-foot telescope was Herschel's favorite instrument for carrying out his reviews or ''sweeps'' of the sky, as he called them.

The search for parallax and the effort to build larger aperture telescopes kept Herschel running from one task to another, and even sometimes giving music students the slip. Yet in 1781, he was spurred to new heights in the third of his main goals, the discovery of new nebulae and other unusual objects. In December of that year Watson gave him a catalog of nebulae prepared by the French astronomer Charles Messier. Messier, a comet-hunter, intended his catalog primarily as a list of comet look-alike objects that comet-hunters could safely ignore. The first item on his list, now known as M1 for ''Messier No. 1'' is the so-called Crab Nebula in Taurus, the aging remnant of a supernova explosion in the year 1054. Many of his items, such as M13, are actually clusters of stars that looked round and ''fuzzy'' to Messier.

Herschel's pride in his telescope's light-collecting aperture and high-power eyepiece magnification is evident in his account of his re-examination of Messier's objects. ''As soon as the first of these volumes [of Messier's] came into my hands,'' he wrote in 1784, ''I applied my former 20-feet reflector of 12 inches aperture to them; and saw, with the greatest pleasure, that most of the nebulae, which I had an opportunity of examining in proper situations, yielded to the force of my light and power, and were resolved into stars.'' He added that in many cases Messier had seen ''only the more luminous part'' of his nebulae.26

As a specific example of the difference between his view and Messier's, he compared their descriptions of the 53rd listed object, the globular cluster M53 (see chapter 2, figure 2.6 for an illustration of a globular cluster). Herschel quoted Messier in French: ''Nebuleuse sans etoiles... ronde et apparente.'' (''Nebula without stars, round and prominent.'') Herschel's own astronomical journal entry for this object ran: ''A cluster of very close stars; one of the most beautiful objects I remember to have seen in the heavens. The cluster appears under the form of a solid ball, consisting of small stars, quite compressed into one blaze of light, with a great number of loose ones surrounding it, and distinctly visible in the general mass.'' For good measure, he appended a hand-drawn illustration of the globular cluster.27

These early observations of Messier's objects, in which the apparent nebulosity ''yielded'' to the force of Herschel's higher resolution and magnification, led him to believe for a long time that virtually all nebulae, no matter how cloudy in appearance, consisted of stars. The most cloudy or faint were simply the most distant. For example, in his enthusiasm for brushing aside earlier accounts of nebulae as consisting of some sort of luminous fluid, he described both M1, the gaseous Crab Nebula, and M3, an indistinctly seen globular cluster of stars, as showing ''a mottled kind of nebulosity, which I shall call resolvable; so that I expect my present telescope [i.e., the second, larger aperture 20-foot, not yet used in the examination of all Messier objects] will, perhaps, render the stars visible of which I suppose them to be composed.''28

Herschel evidently thought of these nebulae as island universes, comparable in nature to our own sidereal system, although clearly of a diversity of shapes and sizes. The contemporary novelist Fanny Burney, daughter of Herschel's friend, the physician Dr. Charles Burney, reported in the 1780s that Herschel told her he had discovered 1500 new universes.

With characteristic enthusiasm and energy, Herschel undertook the search for double stars and nebulae, the casting and endless polishing of mirrors, the construction of wooden stands to support his telescopes, and the reporting of his results in papers for the philosophical societies. The ceaseless round of activity taxed his resources to the utmost, however. His brother and helper Alexander had interests of his own and a musical career that took him to Bristol during the summers when Bath was quiet. Caroline noted in her memoir that these circumstances involved her more closely in her brother's work, though mostly, at this time, as his amanuensis:

''Alex was always very alert, assisting when anything new was going forward, but he wanted perseverance, and never liked to confine himself at home for many hours together. And so it happened that my brother William was obliged to make trial of my abilities in copying for him catalogues, tables, &c., and sometimes whole papers which were lent to him for his perusal. [...] When I found that a hand was sometimes wanted when any particular measures were to be made with the lamp micrometer, &c., or a fire to be kept up, or a dish of coffee necessary during a long night's watching, I undertook with pleasure what others might have thought a hardship,'' she wrote.29

In the spring of 1781, the extraordinary quality of Herschel's telescopes and his unusual assiduity in searching the heavens for new objects converged in allowing him to make a spectacular discovery. On the 13th of March, during the course of his systematic sweeping of the sky, Herschel noted the appearance of an unusual ''nebulous star.'' The star seemed uncommonly large, and he suspected it of being a comet. On the 17th, he noted it had changed place with respect to the background stars, as a comet should. By the 19th he had further confirmed his impression, determining that the object moved in the ecliptic. That is, it traveled through the constellations of the zodiac like most solar system objects.

Other astronomers, including the Astronomer Royal at the Greenwich Observatory, Nevil Maskelyne, confirmed the object's motion. As news spread and astronomers checked their records, it became apparent that the object had been seen before, but had not attracted special attention. Herschel, with his superior optics and well-trained eye, had been the first to notice that this star-like object looked different, and he was therefore the first to track its position over the course of several nights.

Astronomers and mathematicians across Europe set to work calculating the orbit of the comet and comparing its predicted motion to its evolving place among the stars. By May the startling truth was beginning to sink in: the data only made sense if the object orbited the sun at about twice the distance of Saturn, in a nearly circular path more similar to that of a planet than that of a typical comet.

Herschel's ''curious'' object was in fact the first planet to be discovered since the dawn of recorded history. A new wandering star joined the ranks of those five known to the ancient Babylonians and Greeks: Mercury, Venus, Mars, Jupiter, and Saturn.

Strong opinions emerged on all sides about what to call the new planet. Herschel advocated that it be named in honor of

King George III, a fellow Hanoverian. This did not go over well on the continent. As Ireland's nineteenth-century Astronomer Royal, Sir Robert Ball, put it, European astronomers ''considered that the British dominions, on which the sun never sets, were already quite large enough without further extensions to the celestial regions.''30 They in fact proposed the name Herschel, as well as Uranus, and until 1847, the planet went by three different names. The name Uranus at least was in keeping with tradition, because Uranus, in Greek myth, is the father of the character associated with the Roman god Saturn.

The Royal Society promptly elected Herschel a fellow and awarded him its Copely medal for his discovery of the new planet. Soon thereafter George III appointed Herschel as his personal astronomer: not the Astronomer Royal, but the King's Astronomer. A modest life pension freed Herschel from his now onerous music teaching duties. He and Caroline moved closer to Windsor Castle, to be available to entertain members of the court with views of the heavens.

Herschel's skill in the manual art of making telescopes, and his inclination to investigate and debate the nature of things he saw through them, had propelled him from musician-astronomer to full-time astronomer. With the pension from the crown he began filling his time as many modern astronomers do: conducting research, raising money to support his investigations (in Herschel's case, through the sale of telescopes and by applying for grants from the king), and writing up his work as he went along. He trained Caroline as his assistant, and supervised workmen who were constantly refurbishing or maintaining his instruments and the wooden structures for mounting the telescopes.

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