Development of Stellar Astronomy

Overview

The advance in astronomy as primarily a progressive improvement in accurate plotting of the stars was reasserted about the middle of the fifteenth century, but within the next 150 years, the discoveries of physical aspects of stars would lead traditional astronomy toward the foundations of astrophysics. The rediscovery of ancient knowledge, called the Renaissance, had advanced in astronomy with improved applications of geometry and trigonometry. Traditional naked-eye sighting instruments progressed in accuracy from the mid-fifteenth century. Astronomers expanded their interest in accurate positional cataloguing by reevaluating ancient appraisal of the stars in terms of relative size and brightness. This led to discoveries of unusual traits in stars; some stars varied in brightness and appeared to pulsate. On rare occasions stars seemed to appear where none had been before. Studying and cataloguing stars by these new features expanded with the introduction and improvement of the telescope through the seventeenth century.

Background

Primitive awe of the celestial pinpoints of sparkling light, the stars, had first prompted study of the heavens as partitioner of time, a natural calendar, and as religion which resolved into ancient astrology, the study of the superior influences of the celestial on every aspect of terrestrial life. Grouping stars into familiar objects such as mythical characters was common to many ancient cultures. Plotting the accurate positions of the stars in the effort to interpret astrological effects on humanity became the basis of astronomy, the study of stars and the heavens as science. Ancient Greek thought passed down earlier conception that the stars were considered a fixed backdrop of the celestial vault that did not change. Aristotle popularized the concept of rigid and individual crystalline spheres holding the sun, planets, and the fixed stars. There was an early tradition in Greek thought that the stars were clouds of fire, fires in general, and even some celestial light shining through pinprick holes in the black cover of the night.

Development of Stellar Astronomy

Overview

Background

Important aspects of Greek astronomy would have been lost or longer in coming to Europe if not for the various medieval Muslim cultures, which preserved and translated Greek science. Yet improving on this secondhand presentation by translating Greek astronomy from the original Greek was taken up by several Europeans in the fifteenth century, among them the German astronomer Georg Peurbach (1423-1461). By the mid-fifteenth century the important mathematical aspect of Greek astronomy, using geometric and trigonometric methods (use of parallax, defined as the apparent change in position of an object when viewed from different points on earth due to orbit and rotation of the earth) to determine star positions, was revitalized in Europe by astronomers, particularly Johannes Regiomontanus (1436-1476), a student and heir to Peurbach's efforts, who developed applications of solving problems by triangulation. Accurate charting of the stars would benefit from both mathematical method and improved sighting instruments (sextants, quadrants, and compass-like tools) and their use. Regiomontanus established a workshop for the construction of astronomical instruments and wrote detailed descriptions of these.

Making a catalogue of the positions of all stars visible to the naked eye was important to the mathematical applications of this data in astronomical almanacs and tables (called ephemerides), calendar making, referencing lunar and planetary motions, astrology, etc. The only comprehensive early star catalogue was that of Greek astronomer Hipparchus (fl. second century b.c.) with about 850 stars, which included spherical coordinates for each. He was the first to designate visual star relative brightness or dimness by magnitude (in his day, first magnitude being the brightest, the sixth being the faintest). This catalogue was preserved and revised significantly by the late Greek astronomer Claudius Ptolemy (fl. 150, d. 180) with additions bringing the total to 1,022 stars in 48 constellations. Star catalogues were also the basis for a Renaissance period of innovation to astronomy, the star atlas, and plot maps of constellations.

Impact

Into the sixteenth century, several astronomers followed Regiomontanus' impetus toward accuracy. Nicolaus Copernicus (1473-1543) himself built his own graduated torquetum (a sort of large three-arm compass), though he was not a regular observer. Gemma Frisius (1508-1555), mathematician of the Netherlands, applied his theory of land triangulation in surveying to astronomy and cartography and made several astronomical sighting instruments, including a long cross-staff with movable sights. Then, in 1572, the stimulus of Mother Nature proved the most efficient of all. What appeared to be a new star in the constellation of Cassiopea shone in early November and had everyone with any instrument attempting to measure the parallax. Among them was Englishman Thomas Digges (1546-1595) who observed the star accurately and wrote trigonometric theorems to find its parallax, which being negligible pointed to a celestial origin.

But the best known observer was the Danish noble Tycho Brahe (1546-1601), beginning to build large and accurate astronomical instruments, who kept a detailed observational log of the new star using his own large sextant (graduated to one minute of angular arc). He measured the angular distances from neighboring stars in the constellation to form the sides of spherical triangles by which he plotted the angular latitude and longitude of the so-called New Star (which was a supernova or exploding star). Significantly, he noted that its position relative to the neighboring stars did not show any measurable parallax, which confirmed that this object was celestial. Thus the celestial sphere was not unchangeable as thought since ancient times and needed to be reappraised. Thinking the star remnant material of the Milky Way, Brahe was about to launch his own campaign of accurate star plotting with a progression of unusually precisely graduated, large instruments using adjustable slits as eyepiece sights to explore that need for celestial reappraisal.

A graphical means of indicating star positions for science and navigation was yet another ancient accomplishment poised for improvement. It was an efficient observational aid, but unless the star positions were actually given coordinates in a useful cartographic projection, they could not be used to locate stars precisely or figure the positions of other stars discovered near them. Ancient peoples including Greek astronomers had made globes and maps of constellations but no Greek examples survived. There had been inaccurate medieval representative depictions, sometimes without showing pertinent stars. The best depiction by the early sixteenth century was the first printed star chart (1515) by German artist Albrecht Durer which depicted two planispheres (for the northern and southern hemispheres) laid on an hourly-graduated zodiac circle (the twelve classical constellations on the ecliptic plane of the earth) with the stars depicted in numerical sequence for each constellation. A stereographic projection of Ptolemy's constellations with their principle stars was done as a single sheet in 1535 by Peter Apian (1495-1552). But the first true atlas in book form was that of Alessandro Piccolomini (1508-1578) in 1540. While he did not use a practical coordinate scale, he depicted the stars as seen from earth in sizes relative to their magnitudes and was the first to use letters to label the prominent stars in constellations, a technique adopted thereafter.

Still standing after 1400 years, the Hipparchus/Ptolemy catalogue was inaccurate and incomplete. With his collection of mammoth sighting instruments for better accuracy and fully aware of how much an accurate star catalogue was needed, Brahe worked from a base of known angular distance coordinates of 22 stars to calculate that of all other naked-eye stars between 1578 and 1591. The coordinates of the resulting 777 stars of this new catalogue (published in 1602) were never in positional error by more than four minutes of arc (amazing eye plotting accuracy). Johannes Kepler (1571-1630), who inherited Brahe's data, was able to bring the later total star count to a less accurate 1,004 (published in the later Rudolfine Tables, 1627). The first star atlas provided with useful spherical coordinates, using Ptolemy's catalogue, was that of Giovanni Paolo Gallucci (1588). Brahe's full catalogue was used in the most complete atlas to the time, the Uranometria (1603), compiled by lawyer turned astronomer Johann Bayer (1572-1625). It also contained the accumulated southern hemisphere stars plotted by Dutch explorer Pieter Keyser as well, for a total of over 2,000 stars, all designated with Greek letters for the first time.

Meanwhile, the supernova of 1572 had generated a keen interest in what seemed to be an inconstant field of fixed stars. Copernicus' heliocentric theory (1543), taking the earth away from the center of the universe, had already generated additional theorizing that the stars could not be fixed. Digges, for instance, the leader of the so-called English Copernicans, believed the stars varied in distance in an infinite space. On August 13, 1596, German clergyman/astronomer David Fabricius (1564-1617) noted a bright star in the constellation of Cetus. Its brightness faded into the next year, so it was thought to be another new star phenomenon. But it was a variable star, the first observed pulsating star (fluctuating in brightness). Bayer observed it in 1603 and designated it by the Greek letter omicron (Omicron Ceti) in his star atlas. There was talk of another new star (P Cygni) in Cygnus in 1600 (observed by one W.J. Blaeu), but this would later be identified as yet another irregular type of star. But in 1604, another new star or supernova did appear in the constellation Ophiuchus (the Serpent Bearer), sighted on October 10-11. Kepler, who had not believed before, saw for himself, believing it celestial but of the same Milky Way material Brahe had labeled the 1572 star. He observed it until it faded in 1606.

Mid-seventeenth century astronomers spent time on observations of the stars, detecting not only more variables but also binary or double stars and multiple star groups. There was the added impetus of the revolutionary refracting telescope (using lenses for magnification) and its power to see more stars. Observational astronomer Giambattista Riccioli (1598-1671) used the telescope to good advantage in studying the solar system and turned it on the stars to discover the first observed double star, Mizar in Ursa Major (1643). Dutch astronomer Johann Phocyclides Holwarda (1618-1651) studied Fabricius's Omicron Ceti in late 1638-39 and realized it varied in luminosity as periodic fluctuations, meaning stars must rotate. French astronomer Ismael Boulliau (1605-1694) would discover this star's actual period in 1667. Holwarda's famous compatriot Christiaan Huygens (1629-1695) telescopically found a grouping of three stars instead of the one star tagged as Theta in Orion. To be called the Trapezium, its fourth star was found by Jean Picard (1620-1682) in 1673. English scientist Robert Hooke (1635-1703) discovered the grouping Gamma Arietis in 1665. Observation in the southern hemisphere revealed the double star systems of Alpha Crucis (1685) and Alpha Centauri (1689). Italian physician Geminiano Montanari (1633-1687) discovered the variable character of Algol (Beta Persei) in 1670, though its brilliant flickering had long determined its name of the Demon or Satan's Head. Famous English astronomer Edmund Halley (1656-1742) discovered the variable Eta Cariae in 1677, and German astronomer Gottfried Kirch (1639-1710) did the same for Chi Cygni in 1687.

One of the most influential observational astronomers of the time was Pole Johannes Hevelius (Jan Heweliusz, or Hewelcke, 1611-1687), who brought innovation to the star catalogue and atlas. A committed observer, he studied Omicron Ceti from 1648 to 1662 and renamed it Mira (the Wonderful). Although Hevelius made and used telescopes, he seemed to take a step back by imitating Brahe's naked-eye sighting instruments, rather than contemporary sighting instruments applying telescopic lenses for sights. But he claimed this preference for measuring stellar positions as just as accurate. With a sophisticated rooftop observatory at his home in Gdansk (Danzig), Hevelius compiled a comprehensive catalogue of 1,564 stars, which included the 341 southern hemisphere stars plotted by Edmund Halley (published with an atlas in 1679). Hevelius' catalogue and accompanying atlas (Uranographia, 1690) provided a wider recognition of Halley's catalogue and its accuracy over Bayer's southern plots compiled crudely by explorers. Hevelius used a graphical projection of the stars as on a celestial globe (rather than from earth as contemporaries did) and added eleven new constellations (seven of which remain in use).

English astronomer John Flamsteed (1647-1719), friend and contributor to Isaac Newton (1642-1727), rounded out the seventeenth century's array of stellar astronomers. Flamsteed, an amateur observational astronomer, was appointed the first astronomer royal at the founding of Greenwich Observatory (1675). Using measuring instruments with telescopic sights, which, contrary to Hevelius's claims, provided the reading of finer measurements, Flamsteed concentrated on observing the moon and the stars with twenty thousand observations between 1676 and 1689, especially using a large sextant accurate to 10 arc seconds. His systematic observations were completed in 1705.

The growing importance of positional astronomy's accuracy to the needs of astronomy and navigation was exemplified in the British furor over the high caliber of Flamsteed's work, which would entail the first Greenwich, star catalogue (nearly 3,000 stars). There was great pressure to publish his catalogue because of its accurate usefulness before it was completed into the early eighteenth century, which did happen against Flamsteed's wishes. His efforts had culminated a 150-year progression of instrumental improvements. But more important was the dimension of new astronomical theory enabled during this time span, lured by heliocentrism and able to systematically dissolve the geocentric cosmos in favor of Newton's mathematico-mechanical one by the late century. This was partially through physical discoveries of the stars. Improving the refracting telescope, joined by the reflecting telescope, pointed to deeper stellar discoveries in a seeming infinite universe but also elevation as a true tool with measuring adaptability. The stars would prove to be tools as well, for those interesting variables were little more than two centuries away from being used as yardsticks to measure the extension of the cosmos itself.

WILLIAM J. MCPEAK