When the Danish astronomer Tycho Brahe died on October 24, 1601, seven years before the invention of the telescope, it was not the end of his contribution to astronomy. Rather, his death opened the door for his student Johannes Kepler to leverage his work to make some of the most crucial discoveries in the history of science.
Brahe led a fascinating life. Kidnapped by his wealthy aunt and uncle to be raised as their own at the age of two, he enjoyed the connections of aristocracy as a young man, including access to the Danish royal court.[1] This helped him secure patronage to work as a mathematician and astronomer, a career he switched to from law after he was captivated by the 1560 solar eclipse and how precisely it had been forecast. He was fiercely dedicated to and defensive of his work, even losing a portion of his nose in a duel over a mathematical disagreement.[2] He longed to produce more accurate predictions of astrologically significant celestial conjunctions (when two or more planets align as viewed from Earth); his interest in the subject was sparked when the established astronomical tables of his day did not accurately predict an alignment of Jupiter and Saturn.[3] To this end, he became obsessive about the accuracy of his astronomical measurements and intensely protective of his data.
Brahe’s big break came in 1572 when, while walking at night, he suddenly noticed what looked like a new star in the constellation Cassiopeia. The appearance of a new star in the night sky in a time when the heavens were regarded as a fixed, perfect realm was shocking, and Brahe only believed his eyes when other astronomers confirmed his discovery. He named it Stella Nova (New Star), although today we know that it was not a “new star” but rather an old one exploding at the end of its life. In modern astronomy, it is known as Tycho’s Supernova.
Brahe’s discovery and subsequent observation of this object not only launched him to celebrity status but also upended his perception of the cosmos. At the time, the Ancient Greek model of the Earth as sitting at the center of a number of concentric, transparent spheres was still dominant, and accordingly, Brahe assumed that his “new star”—which was changing in brightness, not glowing consistently like a typical star—was an atmospheric phenomenon inside the spheres. But over time he observed that its position did not change night to night like nearby objects such as the Moon and planets (which were believed to occupy the inner spheres) but remained fixed like the stars. Likewise, it did not move like other objects then assumed to be atmospheric phenomena, such as comets. This suggested that it was more distant than any of those objects. Although Stella Nova ceased to be visible after eight months, Brahe shortly after had the chance to observe comets, which he calculated made smaller night-to-night movements than the Moon, indicating greater distance. These findings led him to publish a meticulously researched book, De Nova Stella, arguing that the idea that the outer spheres are fixed and unchanging was false and that the heavens were a dynamically changing environment. This was a culturally significant claim, challenging the dominant Christian orthodoxy that the heavens were a perfect, unchanging realm.
Brahe’s newfound fame led him to receive numerous offers of scientific positions around Europe, but King Frederich II of Denmark convinced him to stay in the country by offering to build him a private island retreat with a custom-built observatory. For thirteen years, Brahe worked tirelessly at his new facility and designed new instruments to aid in his observations, including sextants and even a quadrant the size of a house. His measurements of the positions of celestial objects were accurate to within two arc-minutes at a time when six arc-minutes was the typical standard, and some of his measurements have been determined to be as accurate as one-half an arc-minute. (An arc-minute is one sixtieth of one of the 180 degrees from horizon to horizon.) He also meticulously tracked each object’s position throughout its orbit, rather than only at key points as was typical at the time.[4]
With these measurements, he designed a new model of the heavens. Brahe rejected Nicolaus Copernicus’s then-recent and highly controversial heliocentric model that placed the Sun, rather than the Earth, at the center of the cosmos (contrary to Christian belief that the Earth was the center of God’s creation). But Brahe’s detailed observations could not fit the traditional geocentric model, so he created one that attempted to explain his observations while keeping Earth at the center. It featured the Sun orbiting the Earth while all the other planets orbit the Sun in perfectly circular orbits, as in Copernicus’s model.[5] The planets exhibited irregularities in their movements that neither the heliocentric nor geocentric models could account for, but Brahe continued with Aristotle’s assumption that all celestial motion occurred in perfect circles and so designed his model to have the planets tightly orbiting a point that, in turn, orbited the sun. This partially explained these irregularities.[6]
When King Frederich II died in 1588, his son Christian IV took over. The new king withdrew the financial and practical support Brahe had enjoyed, so the astronomer accepted an invitation to work for Holy Roman Emperor Rudolf II in Prague, for whom he later became imperial astronomer and astrologer. It was here that Brahe met Kepler, a student from a poor background whom Brahe regarded as little more than an assistant. Kepler had grander ambitions, and the two often clashed as Kepler tried to gain access to Brahe’s detailed astronomical observations to advance his own theories, which supported the Copernican heliocentric model rather than Brahe’s heliocentric-geocentric hybrid. The two eventually settled on Kepler focusing on the unanswered question of why Mars periodically reversed direction in the sky for short periods, a motion that neither Copernicus’s nor Brahe’s models could explain. But Brahe still denied Kepler access to his wealth of observations of unparalleled accuracy.[7]
In 1601, however, Brahe unexpectedly died of a bladder infection. (Claims that Kepler may have poisoned him based on the presence of mercury in his remains have since been debunked.) Kepler, the most prominent of Brahe’s associates who had remained close to him by that point, took over his imperial role, and the emperor purchased all of Brahe’s equipment and materials from his family. This finally gave Kepler access to Brahe’s measurements, which he spent the next several years employing in his effort to modify the Copernican model to explain the irregularities of planetary motion that Brahe had captured in exquisite detail.[8]
These data were pivotal in Kepler’s discovery of the true reason for Mars’s apparent retrograde motion: Mars’s orbit is not a perfect circle—as Brahe, Copernicus, and virtually everyone since Aristotle had assumed—but rather, an elongated ellipse. This discovery enabled Kepler to formulate two propositions that became, after his lifetime, known as the first two of Kepler’s Three Laws.
All planets move in elliptical orbits around the sun.
If we draw straight lines between a planet and the Sun at two points along its orbit based on how far the planet travels in a given period of time, the area between the two lines will be the same for that time interval at any point along its orbit.
Although Kepler had disproved the idea that celestial bodies move in perfect circles, he had discovered a new kind of perfection in their elliptical orbits: mathematical harmony. No matter how elliptical a planet’s orbit may be, the planet’s speed changes as it traverses the flatter and steeper sides of the orbit in perfect proportion to the angle of its travel.
Kepler, who had once aspired to be a theologian and spent much of his life looking for integrations between different areas of physics and geometry that would point to an overall “harmony” of creation, admired the beauty of this balance of space and motion. Kepler made extensive use of Galileo’s improved version of the telescope (originally invented by Hans Lippershey in 1609). Kepler corresponded with Galileo and made his own observations of Jupiter that supported the latter’s claims that four large objects (moons) were orbiting the planet. This was a shocking claim for many; both men were arguing not only that the Sun—not the Earth—was the center of the cosmos but also that other celestial bodies might also be the centers of other systems of worlds. Galileo would be forced by the Catholic Church to recant his claims, but Kepler, residing in Austria, was freer to develop them. Galileo thanked Kepler for supporting his discoveries, writing to him, “I thank you because you were the first, and almost the only one, to give complete credence to my assertions.”[9]
Kepler continued looking for signs of mathematical proportionality in his observations of the planets, eventually developing what would become known as his third law (the idea of referring to such principles as “laws” was popularized later by Isaac Newton). It states that the square of each planet’s period of revolution around the Sun is directly proportional to the cube of its average distance from the Sun. With his three laws, Kepler had replaced oversimplified circular models of the solar system with a beautiful harmony of mathematical proportions perfectly matching Brahe’s precise observations. The laws Kepler had discovered were the crucial building blocks for identifying the principles of motion that made possible such a beautiful astral dance, which Kepler likened to silent music, noting its similarity to the proportional relationships between harmonizing notes.
Kepler’s ability to further develop his astronomical principles was hampered by numerous personal problems, including his wife’s severe illness, a less favorable relationship with Rudolf II’s successor, Mattias, and accusations of witchcraft made against his mother. He also diverted much of his attention to studies of other subjects, including astrology and alchemy (which had both been interests of Brahe’s as well).
It would be left to Isaac Newton to derive from Kepler’s Laws the laws that govern the motion of bodies in space and thereby on Earth as well. Newton’s first law—that an object at rest remains at rest and an object in motion remains in motion at constant speed and in a straight line unless acted on by an unbalanced force—is fundamental to understanding the motion of objects in space, where the lack of air resistance permits objects to continue moving on a given trajectory indefinitely. Newton realized that the Sun was exerting a force on the planets that pulled their naturally straight trajectories into elliptical orbits; the relationship between their speed and trajectories that Kepler had identified was crucial to this discovery.
Newton formulated three laws, the other two concerning how forces (such as the Sun’s gravity) operate on objects to affect their motion. When Newton said that “if I have seen further, it is by standing on the shoulders of giants,” Kepler and Brahe were two of the giants who gave Newton the foundation upon which to make his discoveries—discoveries that were instrumental in paving the way for the development of mechanical engineering and steam power.[10]
Tycho Brahe’s industrious and detailed observation of the planets, together with Johannes Kepler’s use of those observations to formulate his laws of planetary motion, transformed our understanding of Earth’s place in the universe. Moreover, they provided the building blocks for discovering the fundamental laws of physics and thereby set the stage for the earth-shattering innovations of the Industrial Revolution.
[1] Don Nardo, Tycho Brahe: Pioneer of Astronomy (Minneapolis: Compass Point, 2008), 39.
[2] Emily Staniforth and Nola Taylor Tillman, “Tycho Brahe: Colorful Life, Accomplishments and Bizarre Death,” Space.com, June 21, 2023, https://www.space.com/19623-tycho-brahe-biography.html.
[3] Leah Poffenberger, “November 11, 1572: Tycho Brahe Spots a Supernova,” APS, November 11, 2019, https://www.aps.org/apsnews/2019/11/tycho-brahe-spots-supernova.
[4] The Galileo Project, “Tycho Brahe (1546–1601),” Rice University, https://galileo.library.rice.edu/sci/brahe.html (accessed October 21, 2025).
[5] Technically, in both Copernicus’s and Brahe’s models, the planets orbit an empty point called the “mean sun” that the Sun also orbits closely. This wasn’t far from the truth, as the Sun does revolve around a gravitational barycenter for the whole solar system, although this is deep inside the Sun’s interior owing to the Sun’s vastly larger mass than the rest of the solar system combined.
[6] “Kepler’s New Astronomy: Tycho Brahe’s Hypothesis,” Promethean Updates, February 20, 2016,
[7] Staniforth and Taylor Tillman, “Tycho Brahe: Colorful Life, Accomplishments and Bizarre Death.”
[8] “Johannes Kepler,” Britannica, https://www.britannica.com/science/Keplers-laws-of-planetary-motion (accessed October 21, 2025).
[9] “Johannes Kepler,” Britannica.
[10] “Newton’s Laws of Motion,” NASA, https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/newtons-laws-of-motion (accessed October 21, 2025).