The Kepler Code

Johannes Kepler (1571-1630) played a crucial role in the development of modern astronomy. He embraced and improved Copernicus’s (sun-centered) system of the universe, discovered three essential laws of planetary motion, contributed to the development of the telescope, and even wrote what some consider to be an early work of science fiction. The publication of "Tabulae Rudolphinae" in 1627 was the crowning achievement of his career. As you advance through the exhibit we'll focus on the many different astronomical tools and ideas depicted.

The frontispiece (opening illustration) of Tabulae Rudolphinae — meaning “Rudolphine Tables”, in honor of Emperor Rudolph II — represents the temple of Urania, the muse of astronomy. It has the shape of a dodecagon, with 12 sides referring to the 12 signs of the zodiac. As we move from the back of the temple to the front, the columns get more elaborate, representing the development of astronomy over the centuries.

An eagle symbolizing the Holy Roman empire flies over the temple and releases coins from its beak. This refers to the support given by the Holy Roman Emperor Rudolph II and his successors to support the production of this book.

Urania, the muse of astronomy, is depicted at the top of the temple’s dome presenting a crown of laurels to the imperial eagle. This detail stresses that this book was a triumph of astronomical science. The female figures surrounding Urania represent scientific fields relating to Kepler’s various works and contributions throughout his career.

Physica lucis et umbrarum represents the physics of light and shadow and refers to Kepler’s works in optics. The muse holds a sphere that casts a shadow cone. The sphere and the shadow cone together also resemble a comet, which is possibly a reference to the fact that Kepler observed several comets.

Optica holds a telescope. Kepler took a special interest in this new instrument, which first emerged in 1608. He explained how light behaves when it passes through a system of lenses and came up with a new telescope design that proved particularly suitable for astronomical observing.

Logarithmica refers to Kepler’s use of logarithms (a mathematical function) in the production of the Rudolphine Tables. Kepler was introduced to logarithms by his fellow mathematician Benjamin Ursinus, after which he developed his own logarithmic theory and tables.

Doctrina triangulorum represents trigonometry. This branch of mathematics was central to classical astronomy, providing tools to predict and explain celestial motions. Trigonometry was also crucial for Kepler’s discovery of the three laws of planetary motion. The first of those laws establishes that planets move in elliptical orbits around the Sun. It is illustrated by the diagram held by Doctrina triangolorum.

Stathmica holds a balance with the Sun at the fulcrum and a planet standing in equilibrium to the right. This refers to Kepler’s second law of planetary motion, according to which a planet moves faster when it travels along the part of its elliptical orbit that is closer to the Sun.

Magnetica is depicted holding a lodestone (a piece of naturally magnetized mineral) and a compass needle that points to the lodestone. Kepler proposed that a magnetic force emanating from the Sun kept the planets in motion around the Sun, but this explanation did not hold.

Moving to the inner space of the temple below, we find in the back a man performing observations using his hand as an instrument. This is meant to represent the ancient Babylonian astronomers who compiled numerous observations and developed techniques to predict planetary motions and lunar eclipses, effectively establishing the foundations of astronomical science.

The column on the right refers to the Greek astronomer Meton, who lived in the 5th century BCE. He identified the so-called Metonic cycle, in which 19 solar years roughly correspond to 235 lunar months. It is represented by a disc with 19 divisions that decorates the column. This cycle was essential for the construction of calendars, a topic in which Kepler also worked.

The column on the far left, alludes to Aratus (315-240 BCE), the author of a poem titled, "Phaenomena," in which the classical Western constellations are described. Phaenomena also explains the imaginary circles used by astronomers to make sense of the celestial motions, which are represented by the instrument hanging from the column, an armillary sphere.

Moving towards the front of the temple, on the left we find Hipparchus, a Greek astronomer who lived in the 2nd century BCE. Hipparchus compiled an important catalogue of stars, which is represented by the book in his right hand. Just above Hipparchus is a celestial globe, a model of the sky that could be used to visualize the positions of stars and other celestial objects and to perform astronomical calculations.

On the opposite side of the temple, Ptolemy (100-170 AD) works at a desk on top of which sits his book Megale Syntaxys, better known by the Arabic title Almagest. In this work, Ptolemy elaborates on the geocentric (Earth-centered) conception of the universe, presenting geometric models and tables to calculate the motions of celestial objects. The Almagest remained an authoritative work until Kepler’s lifetime.

The device hanging from Ptolemy’s column is an astrolabe. This disc-shaped instrument, which is made of several moving parts, is at once a model of the sky, an analog computer, and an observing tool. Ptolemy wrote about the geometrical techniques necessary to project the curved sky onto a flat plane, a crucial step in the construction of astrolabes. The astrolabe developed significantly in the Islamic world and was the most sophisticated scientific instrument during the Middle Ages.

At the front of the temple, Nicolaus Copernicus (1473–1543), sitting, is in conversation with Tycho Brahe (1546-1601). On Copernicus’s lap is a book representing "On the Revolutions of the Heavenly Spheres." This work explains his heliocentric theory, in which the planets (including the Earth) orbit the Sun. Kepler was a keen supporter of this concept.

Two astronomical instruments decorate Copernicus’s column. On top is a cross-staff, which was used to measure the angles between celestial objects and their altitude above the horizon. The inclusion of this cross-shaped instrument might also be an allusion to Copernicus’s career as an official of the Catholic Church. The instrument further down is called a triquetrum and served to measure the altitude of the Sun at noon, as well as the positions of stars.

Tycho Brahe is portrayed wearing fine clothes, which symbolize his status as a nobleman. Tycho was an influential astronomer for whom Kepler worked as an assistant between 1600 and 1601.

On the base of the temple is a map of the Danish Island of Hven, where Tycho maintained astronomical observatories between 1576 and 1597.

Tycho’s observatories were equipped with large versions of instruments such as the sextant (top of the column) and quadrant (right above Tycho). With those instruments, Tycho and his assistants carried out the most precise astronomical observations that were made before the invention of the telescope. Kepler used those observations to discover the laws of planetary motion and to elaborate the tables presented in this book.

Tycho did not accept the heliocentric system of Copernicus. He proposed an alternative model (known as Tychonic or geo-heliocentric system) in which the planets orbit the Sun, but the Sun orbits the Earth. Tycho points up at a representation of his model on the ceiling of the temple and asks: “What about that?”

Next to Tycho’s left arm is a book titled "Astronomiae Instauratae Progymnasmata," in which he describes his geo-heliocentric system. Although Kepler stood for the heliocentric system of Copernicus, he completed and published this book in 1602, after Tycho’s death.

The right side of the temple’s base illustrates the process of printing the Rudolphine Tables in a printing shop located in the German city of Ulm. The double-headed eagle between the two scenes symbolizes the Holy Roman Empire and refers to the privilege officially given to Kepler to be the exclusive publisher of this book.

Kepler himself is depicted in his study, on the left side of the temple’s base. One of Tycho Brahe’s heirs (to his left) points at Kepler with one hand, and at books containing Tycho’s astronomical observations with the other. Kepler had a complex relationship with Tycho and his heirs. They showed reluctance in giving Kepler access to those observations and demanded that he review his original plans for this frontispiece so that Tycho featured more prominently in it.

The four coats of arms above Kepler represent places where he lived and worked: Bohemia, Prague, Upper Austria, and Linz. The banner shows the titles of four seminal books previously published by Kepler on subjects including planetary orbits and motion, optics, and the Copernican system.

Kepler looks at the reader. He is wearing nightwear and his eyeglasses lie on the table, to emphasize that after many days and nights of hard work, he has finally completed Tabulae Rudolphinae.

Kepler seems to occupy a modest position in the frontispiece of Tabulae Rudolphinae compared to the other astronomers included in the temple of Urania. But a mock-up of the the temple’s dome on his desk reaffirms the great importance of his work: Kepler has completed and topped the edifice of astronomy that had been under construction since Antiquity.

Several copies of the Rudolphine Tables are preserved in libraries and institutions around the world, but the Adler Planetarium’s copy is quite special. Kepler signed and dedicated it to his fellow mathematician Benjamin Ursinus, who introduced Kepler to logarithms. Kepler’s dedication at the bottom of the frontispiece reads: “Johannes Kepler presents this to the most eminent Benjamin Ursinus, Mathematical Professor at the University on the Oder, longtime best man to these tables, in his own name and that of the Brahe family."