Four hundred years ago this year, two events marked what scientists and historians today regard as the birth of modern astronomy. The first of them, the beginning of Galileo's telescopic observations, has been immortalized by playwrights and authors and widely publicized as the cornerstone anniversary for the International Year of Astronomy. Through his looking glass, the Italian astronomer saw the mountains and valleys of the moon, the satellites of Jupiter, and sunspots—observations that would play a huge role in discrediting the prevailing, church-endorsed view of an Earth-centered cosmos.



The second event is not as well known, but is arguably equally important. It was the publication of Johannes Kepler's Astronomia Nova (The New Astronomy) in 1609, a treatise in which the German astronomer introduced the first two of his laws describing planetary motion.



The first law states that the planets travel in elliptical orbits around the sun and describes the sun's position as the focal point in that ellipse. The second law states that an imaginary line connecting a planet to the sun will sweep out a region of equal size in a given time period, wherever in the orbit that time period falls.



Kepler would later go on to introduce yet another law, this one relating the dimensions of an orbit to the time it takes to complete that orbit. He also made fundamental contributions to optics, working out how images are formed by pinhole cameras, a telescope and the human eye as well as developed the principles for corrective lenses for near- and farsightedness. He coined the terms "orbit" and "satellite" and explained how ocean tides are caused by the moon.



"He was an astronomer's astronomer," says Owen Gingerich, a professor emeritus of astronomy and science history at the Harvard–Smithsonian Center for Astrophysics.



Kepler was born in the town of Weil der Stadt in modern-day Baden–Württemberg, Germany, in 1571. His parents were not very well off: His father was a mercenary and his mother a housewife who would later be accused of witchery. Kepler was introspective and sickly as a child but he excelled in mathematics. He decided to join the clergy and won a scholarship to the University of Tübingen, where he was first acquainted with the work of Polish astronomer Nicholaus Copernicus.



In 1609 the accepted view was that Earth was stationary at the center of the universe and the moon, sun and planets moved around it. The stars lay beyond, encircling Earth in a sphere. This view of the heavens had originated with the Greeks, and it was formalized as an astronomical system by Claudius Ptolemy in the second century A.D. Ptolemaic astronomy was not simple—to model the motion of the planets, it made use of a complicated system of circles and epicycles—but it had been accepted as truth for almost a millennium and a half.



In the middle of the 16th century Copernicus had put forward an alternate, heliocentric system in which the sun was the center of the universe, with Earth and the other planets encircling it. Copernicus's treatise on heliocentrism, De Revolutionibus Orbium Coelestium (On the Revolutions of Heavenly Spheres), was published in 1543. Kepler discovered it as a student in Tübingen and was much taken by Copernicus's views.



But most people were not similarly enamored with the concept of a heliocentric universe. First of all, Copernicus's ideas were not widely disseminated, as they ran counter to the teachings of the Roman Catholic Church (although 73 years would pass before De Revolutionibus was listed as a forbidden work by the Church). It was only in universities and their surroundings that they found a following. Secondly, even for those who heard of it, Copernicus's heliocentric astronomy was by some measures hardly more accurate than Ptolemaic astronomy.



The Great Martian Catastrophe

As Gingerich explains, the Ptolemaic system predicted positions of Mars approximately every 32 years that were in error by about 5 degrees in longitude for a short time. Copernicus's system wasn't much better: it was off by about 4 degrees longitudinally. Gingerich calls this "the great Martian catastrophe"—a problem that observers such as Danish astronomer Tycho Brahe knew about, but that Kepler would solve.



After Tübingen Kepler worked as a mathematics teacher in Graz (in modern-day Austria), where he continued his interest in astronomy. It was in Graz that he wrote the Mysterium Cosmographicum (The Cosmographic Mystery), published in 1596, in support of Copernicus. He sent copies of his book to leading astronomers, including Brahe—the greatest observational astronomer of the day. Brahe and Kepler started a correspondence in which they talked about Copernicanism and other astronomical issues. By then, Kepler had realized the need for raw data—observations that would help him understand the underlying laws of nature.



In 1600, as a consequence of the religious and political unrest during the Protestant Reformation, Kepler lost his job at Graz. He made his way to Prague, where Brahe was the court astronomer to Emperor Rudolph II. Prague was where Kepler would spend some of his most productive years. Brahe died suddenly in 1601, and Kepler succeeded him as court astronomer. In addition to his royal duties, Kepler tried to resolve the motion of Mars. He found that his initial model, which assumed that Mars revolved around the sun in a circular orbit, failed to match his predecessor's observations. He reluctantly altered the orbit and made it more egg-shaped.



"There is a myth that Kepler, fitting a curve through Tycho Brahe's records of Mars, discovered that planetary orbits are elliptical," Gingerich says. "The fact is that Tycho's observations showed that the orbit was not a circle, but the choice of an ellipse was largely theoretical."



It was one of those intellectual leaps that would change the course of science. Kepler found that not only did an elliptical orbit with the sun at one focus explain the movement of Mars, but also of the other planets. In fact, as Gingerich points out, Kepler realized the momentous nature of his discovery. In the Astronomia Nova, the typeface suddenly becomes larger to account for the significance as Kepler explains the motion of Mars and puts forward his first two planetary laws. (The third would come later.)



But the underlying physical reason for the planetary motion eluded Kepler, who thought a sort of magnetism was responsible. That puzzle would have to wait for another revolutionary thinker, Isaac Newton, whose law of gravity appeared on the scientific stage and explained orbital behavior eight decades later.