How Johannes Kepler Revolutionized Astronomy

Historical astronomy Johannes Kepler produced the three laws of planetary motion, the first detailed observation of a supernova, How Johannes Kepler and an accurate star and planetary position catalog. revolutionized astronomy Copy of anonymous 1610 portrait This 17th-century German astronomer explained planetary orbits, discovered light’s inverse-square law, and wrote the first modern book on optics. by Daniel Hudon

ohannes Kepler was one of the key and 7 years after Galileo’s birth — in Weil figures of the Scientific Revolution. der Stadt, a small, mostly Catholic town JTrue, he is not as well-known as on the edge of the Black Forest in south- Galileo or the reclusive genius, Isaac ern Germany. Despite being the oldest of Newton, who built on Kepler’s work. But four children (three siblings died in Kepler — Germany’s mathematical infancy), Kepler was always an underdog. genius — was a major player in astrono- His first chore in life was to overcome an my’s history and set the stage for many unhappy childhood that was plagued by subsequent important discoveries. illnesses both real and imaginary. An early bout with smallpox left Great discoveries The Great Comet of 1577 spread its tail across Kepler with defective vision — one rea- By describing the process of human the sky. This event was an early memory of son he eventually pursued theoretical vision, Kepler wrote the first modern Kepler’s. When he was 5, his mother took him to rather than observational astronomy. book of optics, Astronomiae Pars Optica. view the comet when it shone at its brightest. One notably happy event occurred when Woodcut by Jiri Daschitzsky, 1577 He discovered the inverse square law of he was 5. Kepler’s mother took him to a light — the foundation for measuring all hilltop to view the Great Comet of 1577, modern stellar and galactic distances. His resulted in the first book on crystallogra- whose tail arched across the entire sky. thoughts on the shapes of snowflakes phy, De nive sexangula. Kepler’s first job was as a math teacher After Galileo’s telescopic discoveries in in Graz, Austria. It was there that he 1610, Kepler described how the revolu- embarked on his lifelong quest for har- tionary new instrument worked. He also mony. In a flight of mathematical mysti- introduced improvements, including one cism, he tried to link the known planets that increased the field of view. His to Plato’s five geometrical solids (the design came into wide use by the middle cube, tetrahedron, octahedron, icosahe- of the 17th century. dron, and dodecahedron). By nesting We know Kepler mostly for his three each geometrical solid between the orbits planetary laws, which he derived after a of the planets, he thought he had the Herculean battle with the data of the basis for the known number of the plan- great Danish naked-eye observer, Tycho ets. He was so pleased with this idea, he Brahe. Kepler linked physics and astron- wrote it up as a sort of “theory of every- omy by establishing the idea that the thing” called Cosmographicum Mysterium planets had to move due to unseen (“The Cosmic Mystery”). forces. He thus founded celestial The book impressed the most impres- mechanics and opened the door for New- sive astronomer of the day, Tycho Brahe, ton’s work on gravitation. and the two men began what quickly became a tempestuous collaboration. Personal history Brahe had accumulated 20 years of pre- Kepler was born December 27, 1571 — cise observations and needed someone 28 years after the death of Copernicus with Kepler’s mathematical prowess to make sense of it. Kepler promised he could solve the orbit of Mars within 8 Astronomia Nova, published in 1609, details days. Mars was particularly difficult Johannes Kepler’s work on Mars’ orbit. Kepler based his work on the observations Danish because the observations varied from astronomer Tycho Brahe had collected over 20 Tycho’s calculations more than any of the years. Linda Hall Library of Science, Engineering, and Technology planets. Instead, the task took nearly 8

© 2010 Kalmbach Publishing Co. This material may not be reproduced in any form • 56 Astronomy Januarywithout 09 permission from the publisher. www.Astronomy.com www.Astronomy.com 57 years and more than 900 pages of calcu- Kepler searches for Earths lations before he wrapped it up. To harmonize observations with cal- Are there other Earths? This ques- culations, Kepler first determined Earth’s tion looms large in the search for motion as a planet. Think of understand- extrasolar planets. With the launch of the Kepler Mission, ing the motion of an airplane circling an astronomers hope to know the airport as seen from another airplane, answers soon. and you have some sense of the enormity This dedicated space tele- of his task. “I have spent so much pains scope will monitor the on it,” he wrote a friend, “that I could brightnesses of 100,000 have died 10 times.” nearby stars, searching By insisting that his theoretical orbits for planetary transits. A agreed to within the errors of Brahe’s transit occurs when data, Kepler created one of the linchpins a planet passes in of the scientific method. In the process, NASA’s Kepler front of its host star, mission flies the he swept the astronomical house free of causing its light to first spacecraft dim. Johannes its clutter of circles and epicycles and designed to find Kepler first pre- Earth-sized plan- gave the world two of his three laws of dicted such an ets. It measures planetary motion: Planets orbit the Sun event for Mercury in ellipses, and a line drawn from the Sun the light-drop when a planet and Venus almost to the planet sweeps out equal areas in crosses in front 400 years ago. equal times. The wheels of the Scientific of its host star. But the transit Revolution were turning, and the ancient The craft will sur- method has chal- notion of uniform circular motion, which vey 100,000 stars. lenges. Viewed Jon Lomberg, NASA even Copernicus held onto, was aban- from Earth, planets Kepler plotted the position of “his” nova (marked by an “N” near bottom-center) accurately against have only a small doned for good. the stars of the constellation Ophiuchus. Linda Hall Library of Science, Engineering, and Technology chance of crossing in front of their host Kepler’s nova stars. During a transit, earthlike planets will cause the host stars’ brightnesses to In October 1604, a brilliant new star dip by only 1⁄10,000. Of course, such a dip blazed forth in the evening sky. Kepler could be caused by a star spot or the observed the star and wrote about it in star’s own brightness variation. So astron- his 1606 work, De Stella Nova. As he omers must monitor stars precisely for observed the star, he noted it did not several years to catch recurring transits. show any parallax. The Kepler Mission’s main instrument Kepler used the five geometrical solids of Parallax is an apparent shift seen as an is a 37.4-inch (0.95 meter) photometer the philosopher Plato to determine the number that has a particularly large field of view of planets in our solar system. This illustration is object is viewed from two locations. A — 105 square degrees. That area is from his Mysterium Cosmographicum (1596). nearby object shows a large parallax, equivalent to two “scoops” of the Big Linda Hall Library of Science, Engineering, and Technology while a distant object shows small paral- Dipper. If other Earths are common, then lax. He concluded the object lay within Kepler will detect hundreds. the sphere of the fixed stars. Kepler built his planetary laws on Johannes Kepler’s last work, Somnium The supernova remnant from the “new star” data acquired by Tycho Brahe. It’s fitting This conclusion put Kepler at odds (“The Dream”) was a pioneering science Kepler observed in 1604 continues to expand that this statue in Prague pays homage with the church. Indeed, how could such fiction novel about a man’s voyage to the Kepler’s De Stella Nova, published in 1606, today. Three of NASA’s space-based observato- to both astronomers. David Koch, NASA a phenomenon occur if the heavens were Moon. Finding Earths will be like a dream described a “new star” (what astronomers now ries — Hubble, Spitzer, and Chandra — collabo- immutable? Such a “new star” showed come true for many. — D. H. call a supernova) he observed in October 1604. rated to make this image. NASA/ESA/R. Sankrit and W. Linda Hall Library of Science, Engineering, and Technology Blair (Johns Hopkins University) the sky does, indeed, change. Kepler’s Laws of Planetary Motion Triumph through adversity he discovered the third of his planetary my’s most accurate resource for decades. time and again he was forced to take to Planets orbit the Sun in elliptical orbits with the Sun at one focus. (This law destroyed Working while suffering religious perse- laws while revising his book, The Har- Astronomers used them to predict the the roads to flee religious turmoil 1 the notion of circular orbits.) cution and financial hardship, Kepler was mony of the World. He considered this first observed transits of Mercury and brought on by the Counter Reformation part mystic and part modern scientist. law — that the cube of a planet’s distance Venus in 1631 and 1639, respectively. or to seek secure employment. He died of A line drawn from a planet to the Sun sweeps out equal areas in equal times. (This law Despite constant battles with illness is proportional to the square of its orbital Observations of these events helped a fever November 15, 1630, while search- 2 shows why planets [and other objects, like comets] move faster when they lie closer to the Sun than when farther away.) and fever, and tragedies that claimed the period — one of his greatest achieve- solidify acceptance of the heliocentric ing for another job. Though his grave was lives of his first wife and several of his ments. It now forms the basis of the solar model and Kepler’s laws. lost in the Thirty Years War, the epitaph The square of a planet’s orbital period, P, is proportional to the cube of its semi-major children, Kepler kept up his quest for system’s structure. Kepler rarely called attention to his he wrote for himself remains: 3 axis (solar distance), a. If the period is in Earth years and the semi-major axis is in astro- harmony. In 1618, his spirit soared when In 1627, he completed The Rudolphine accomplishments — he was the first sci- I measured the skies, now the shadows I nomical units, then P2=a3. (Astronomers use this law to determine an extrasolar planet’s dis- Tables, a culmination of his life’s work, by entist to advocate freely sharing data — measure, tance from its host star. All they need to know is its period.) — D. H. Daniel Hudon is a natural science lecturer in combining Brahe’s data with his plan- and wanted nothing more than to devote Skybound was the mind, earthbound the core curriculum at Boston University. etary laws. The tables remained astrono- his life to philosophical speculation. But the body rests.