Observing and Defining Comets

Overview

By the middle of the fifteenth century, improvements in the accuracy of astronomical observing instruments and the use of geometrical mathematics presaged a closer look at the strange fiery visitors known as comets and a new era in astronomy beyond dependence on a mix of Greek traditions and instrumental inaccuracy. The sixteenth century marked a landmark period of fortuitous comet appearances, affording a level of observation that pushed professional opinion toward the celestial origin of comets, rather than an ancient terrestrial one. Into the seventeenth century, the appearance of comets and accurate observational data prompted research into past comet records and comparisons which along with the inspiration of the elliptical orbital nature of the solar system and new mechanical theory brought the recognition that comets were celestial, had orbits, and were periodic.

Background

Comets, the transient astronomical phenomena that remain long enough to inspire awe, curiosity, and even fear had remained a puzzle for centuries. Although prior to the fifteenth century there had been diverse opinion on the phenomenon and the state of physical nature, the comprehensiveness of Aristotle's (384-322 b.c.) interpretation dominated the late Middle Ages, particularly in his interpretation of the celestial and terrestrial realms based on a logic of observation and the Greek tradition that all terrestrial matter was composed of four basic elements. These were fire, air, water, and earth, in order of density, light to heavy. Celestial space was formed of a perfect, incorruptible substance called aether. The elemental order fit the logic that the heavy Earth was at rest at the center of the universe with relative circumjacent regions or spheres of the other elements, represented by the oceans, the atmosphere, and a region of fire below the Moon. Beyond that lay the celestial spheres: the Moon, Sun, and planets each within a solid crystalline orb propounded by Aristotle to affirm the unchanging nature of the celestial which spun around the terrestrial realm in concentric circular orbits (defined as a perfect, unending motion) enveloped by a vast firmament of fixed stars. So in what realm did comets fit?

Observing and Defining Comets

Overview

Background

The heavens were considered superior to and the ultimate influence on the terrestrial sphere. This was the basis of astrology from which astronomy developed. The influence also came into play with phenomena within the earthly sphere. Any hot, dry, or fiery byproducts on Earth rose as "exhalations" to the sphere of fire and kindled into meteors, aurora, the Milky Way, and comets, essentially all meteorological phenomena, and all moved by the friction of and influences of celestial motion. Because these were changeable phenomena (even the Milky Way seemed to change through the course of the year) and seemed close to Earth, they were taken as terrestrial in origin. Although there was medieval debate about the possibility of the Milky Way being celestial, comets were accepted as terrestrial phenomena. And since they were engineered from heat and dryness, they were taken as portents of adverse weather conditions and impending bad fortune.

Yet as the century progressed toward the sixteenth century, some thinkers increased the height of the spheres of air and fire, even qualifying a less perfect celestial space, where the four elements might invade to account for growing suspicions that so called fiery impressions, like comets, were celestial. Comets had been observed for centuries but not systematically with naked-eye instruments as with the stars. These instruments were angle-measuring instruments (quadrants, sextants, etc.) meant to plot the locations of the stars and strange patches of dim light seen from Earth before the optical telescope at the end of the sixteenth century. Evidently, the first concerted effort at accurate instrumental observation of cometary paths was by the Florentine mathematician/astronomer Paolo Toscanelli dal Pozzo (1397-1482). He has left detailed manuscript observations of the comets of 1433, 1449-1450, 1456 (this was Halley's Comet), the two comets of 1457 (May and June thru August), and 1472.

By this period, efforts at making more accurate instruments reflected the growing desire for more accurate astronomical measurement, using revitalized geometric and trigonometric techniques of the ancient Greeks. Among early proponents of the effort was German astronomer Johann Regiomontanus (1436-1476). He suggested that as with celestial objects, measurement by parallactic geometry be used to determine a comet's distance from Earth. Parallax is defined as the apparent change in position of an object when viewed from different points on Earth due to orbit and rotation of Earth. Celestial objects were so far away that there was little or no parallax. The Moon's parallax is about 1°, so a comet's should be more if it were below the Moon or less above it. Regiomontanus, who developed more accurate instruments for his observatory at Nuremberg, began careful astronomical observations which included plotting the path of the comet of January 1472. Though his parallax measurements were inaccurate and thus inconclusive, he had pointed the way to determining the origin of comets.

Impact

In the first half of the sixteenth century, comets appeared to the naked eye in 1500, 1517, 1531-1532, 1533, 1538, 1539, 1547, 1556, and 1558, all believed to be terrestrial until revised opinions brought the question of terrestrial or celestial origin to focus. Surprisingly, it was from astrology that published doubts first appeared, namely that the predicted droughts and other natural disasters of several of these comets did not occur which made their terrestrial origins and effects suspect. Netherlander mathematician Gemma Frisius (1508-1555) applied applications of trigonometry to astronomy, noting that comets had a proper motion against the background stars in his observations of those of 1533, 1538, and 1539. He along with Italian physician Girolamo Fracastoro (1483-1553) and German astronomer and cartographer Peter Apian (Bienewitz, 1495-1552) were the first to note upon observing the comet of 1531-1532 (again, Halley's) that its tail pointed away from the Sun. Apian wrote a tract and illustrated this. They saw this influence of the Sun as contrary to a strictly sublunar object, as Aristotle had defined the comet.

Several thinkers tentatively suggested comets were perhaps some sort of celestial reflection. One of the most stimulating reappraisals of the makeup and origins of comets came from the royal professor of mathematics at Paris, Jean Pena (d. 1558), who became intrigued by the fact that a comet's tail pointed away from the Sun. Perhaps recalling the celestial crystalline sphere idea, in his work on geometrical optics (1557) he suggested comets were made of a transparent crystal-like material through which the Sun's rays could be "refracted" causing an internal fire and the tail. He went on to say that optical measurements proved some comets were above the sphere of the Moon (in celestial space), and he joined a growing number of thinkers, among others Girolamo Cardano (1501-1576), who were denying the existence of a sphere of fire to spawn terrestrial comets.

The 1570s brought momentous celestial events and a crises to the supposed perfection of the celestial sphere. Seemingly, a new star (this was a super nova, an exploding star) appeared in 1572 where no star had been recorded before—impossible in the defined unchanging celestial space. The sight prompted an astronomical career for Danish noble named Tycho Brahe (1546-1601), who had begun building large, accurately graduated astronomical instruments. Because this new star slowly faded, many thinkers convinced themselves that it was only an unusual comet. Yet, accurate measurements, particularly Brahe's, showed the object to be celestial. With royal Danish patronage Brahe was able to build and perfect several large, precision instruments set up on the island of Hven off Denmark at his extensive observatory.

When a brilliant comet appeared in 1577, it was keenly observed by many astronomers, none more than Brahe, who recorded it as being far in celestial space and heading for the Sun from a detailed tracking of its total visible path. Other astronomers' detailed observations, particularly those of German Michael Maestlin (1550-1631), prompted the same conclusion. Brahe also decided from his data that comets moved in circular orbits and bolstered this with his observations of comets in 1580 and 1585, and others in 1590, 1593, and 1596. Maestlin's friend Helisaeus Roeslin (fl. c. 1578), offering a more extensive sphere of fire as explaining the comet's celestial aspect, anticipated that comets moved in regular orbits with poles and axes (1578). Brahe's findings, literally, shattered the theory of solid crystal celestial spheres, since the comet intersected some of these supposed spheres. Still there were attempts at qualification, suggesting comets were: below the Moon but in a lesser celestial region; celestial but falling into the upper atmosphere; both celestial and terrestrial (with the elements also found in space); and unexplained miracles.

The strong evidence leaning toward the celestial origin of comets prompted all the more careful observation by astronomers into the seventeenth century. Brahe's assistant and heir to his observational data Johannes Kepler (1571-1630) observed a comet in 1607 (Halley once more), three in 1618, and several others. Though not an exacting observer, he wrote a book on his comet observations (1619), noting that the lack of parallax shift indicated they were celestial. Yet, since he preferred the continued conception of solid orbital spheres separating celestial bodies, he theorized comets moved in straight lines. Some theorized movement in parabolic arcs. Interestingly, the great Italian physicist Galilei Galileo (1564-1642) still agreed that comets were atmospheric in origin but optical phenomenon, not real, so could not be measured legitimately by parallax. René Descartes (1596-1650) argued philosophically that comets were celestial bodies traveling among many solar systems!

The three comets that appeared in 1618 were all the more significant for the new tool used to observe them—the telescope. Though the early century Dutch invention is usually astronomically associated with Galileo, it was not he but a Swiss mathematician/astronomer Johann Baptist Cysat (1586-1657) who first turned the instrument toward the comets of 1618, describing the head and tail—cometary substance still a mystery—with a published work (1619). The path of these comets, as that before in 1607 and those to follow to just after mid century (1652, 1664, 1665), remained a point of contention, because each path was only partially observed and recorded. Through the period, the path of a comet's travel became a matter of defining the comet by two interpretations, as either of permanent or of transitory character. The former defined a celestial object moving in some closed or circular orbit, close to Brahe's assessment. This stance was taken by Adrien Auzout (1622-1691, who built very long telescopes), Giovanni Alfonso Borelli (1608-1679), Giovanni Domenico Cassini (1625-1712), and Pierre Petit (before 1623-c.1677). The second interpretation defined the comet as a transitory object, perhaps of some sub-celestial material, moving in a uniform rectilinear path, the view of Gaileo, Kepler, Polish astronomer Johannes Helvelius (1611-87) who observed four comets, and seminal Dutch scientist Christiaan Huygens (1629-1695).

But in 1680 a comet appeared with a trajectory which allowed its path of travel to be observed before and after perihelion (closest approach as it orbits around the Sun), and the outcome did not fit the theory of rectilinear motion. The celestial origin of comets was now becoming widely accepted. One of the most comprehensive collections of data on this comet was that of English astronomer John Flamsteed (1646-1719). His famed compatriot physicist/mathematician Isaac Newton (1642-1727) took that data and applied his evolving mechanical principles of motion and deduced that comets were matter and would be attracted to the Sun as the planets. As also with the planets, the comet's rectilinear inertia would be drawn to the central pull of the Sun, so that the comet's path would be a conic section, a parabolic orbit which he calculated from three position observations.

It was left to astronomer Edmond Halley (1656-1742) to most effectively apply Newton's mathematical technique. He collected the observational records of 24 comets between 1337 and 1698 and used the parabolic formula of comet orbit to find three with similar trajectories (that of 1531-1532, 1607, and the recent comet of 1682 which he had carefully observed). He also noted the comet of 1456 but that there was a lack of accurate observations on it—not aware of Toscanelli's data. The trajectories were very similar, and since they all were between 75 to 76 years apart, he theorized that this was the same comet and that its periodic character indicated a return in 1758. The comet did return, and became known as Halley's Comet.

The sixteenth century debate over the origins of comets as terrestrial or celestial had helped to redefine the makeup and extension of Earth's atmosphere and provided a first resolution of the boundaries between atmosphere and celestial space. Aristotelian chemical conceptions of elemental constituents of terrestrial and celestial regions had been significantly challenged and ultimately dismissed within the context of the cometary question. So to, the progressive opportunities to apply and record accurate measurement of cometary trajectories, bolstered proof of set laws of celestial motion, first set down by Kepler, departing from the complexities and frustrations of the stubbornly upheld Ptolemaic system. Thus by the beginning of the eighteenth century, comets were commonly accepted to be celestial bodies, moving in defined orbits which could be periodic like the planets, and obeying the mathematics of the newly defined physical laws of Newton and his contemporaries.

WILLIAM J. MCPEAK