Johannes Kepler (1571 – 1630) was a German mathematician and astronomer who was a key figure of the Scientific Revolution. His most famous accomplishment are his three laws of planetary motion which laid the foundation of celestial mechanics. Kepler discovered that planets move in elliptical orbits and at different speeds at different times, according to their distance from the sun. Apart from his contributions to astronomy, Kepler laid the foundation of modern optics. Among other things, he formulated the inverse-square law governing the intensity of light; invented an improved refracting telescope; and correctly explained the working of a human eye. As founder of celestial mechanics and optics, Johannes Kepler is considered one of the most influential scientists in history. Know more about his accomplishments in astronomy, optics and other fields of physics through his 10 major contributions.


#1 He was the first to publish a defense of the heliocentric model of Copernicus

The first major astronomical work of Johannes Kepler, Mysterium Cosmographicum (The Cosmographic Mystery) was published in 1596. The work primarily tried to justify the six known planets and their distances from the sun in terms of the five Platonic solids. Mysterium was the first ever published defense of the Copernican system. Though Copernicus proposed a heliocentric model in his renowned work “De Revolutionibus orbium coelestium”, he resorted to Ptolemaic devices in order to explain the change in planets’ orbital speed, and also continued to use as a point of reference the center of the earth’s orbit rather than that of the sun. Mysterium, despite the flaw of its main thesis, is considered an important work in astronomy as it got rid of the remaining defects of the Ptolemaic theory from the Copernican system.

Heliocentric Model of Copernicus
Heliocentric Model of Copernicus


#2 He discovered that a planet moved at different speeds according to its distance from the sun

Using the accurate astronomical and planetary observations of Danish astronomer Tycho Brahe, Johannes Kepler analyzed the orbit of Mars and discovered that Mars didn’t move in a perfect circle around the sun, as was believed at the time. He found that it was closer to the sun at certain times than at others. Furthermore, it moved faster when it was farther away. Kepler supposed that the motive power radiated by the Sun weakens with distance, causing faster or slower motion as planets move closer or farther from it. In 1602, Kepler deduced what came to be known as his second law: the speed of the planet changes at each moment such that the time between two positions is always proportional to the area swept out on the orbit between these positions.

Second Law of Planetary Motion diagram
Diagram of Kepler’s Second Law of Planetary Motion


#3 Johannes Kepler discovered that planets move in elliptical orbits around the sun

After formulating his second law of planetary motion, Kepler set about calculating the entire orbit of Mars, using the geometrical rate law and assuming an egg-shaped ovoid orbit. In 1605, after around 40 failed attempts, Kepler ultimately found that Mars follows an elliptical path around the sun, an idea he had assumed to be too simple a solution for earlier astronomers to have overlooked. Kepler thus arrived at his first law of planetary motion: that the planets orbit the sun in ellipses, with the sun at one focus. It is interesting to note that Kepler discovered the second law before the first.

First Law of Planetary Motion diagram
Diagram of Kepler’s First Law of Planetary Motion


#4 His work Astronomia nova is considered one of the most important books in astronomy

Kepler published his ten-year-long investigation of the motion of Mars in his most renowned work Astronomia nova (New Astronomy). Apart from including the first two of the three principles known today as Kepler’s laws of planetary motion, it provided strong arguments for heliocentrism. The idea that a planet moved in an elliptical orbit with its speed varying according to its distance from the Sun was completely revolutionary at the time. Kepler also discussed an attractive virtue of earth, similar to magnetism. He considered this attraction as mutual and proportional to the bulk of the bodies. But, he considered gravity to have a limited range and did not know how this force varied with distance. Kepler’s idea thus differed significantly from Newton’s later concept of gravitation. Astronomia nova was a highly influential work of the scientific revolution and it is considered as one of the most significant books in the history of astronomy.


#5 He discovered his third law of planetary motion while writing Harmonices Mundi

Published in 1619, Harmonices Mundi (The Harmony of the World) is a book by Johannes Kepler in which he attempts to find harmony in nature by explaining proportions of the natural world in terms of music. The Harmony of the World is explained in five chapters on: regular polygons; the congruence of figures; music; astrology; and the motions of the planets. Among the harmonies in the book is the third law of planetary motion: that the square of the orbital period of a planet is proportional to the cube of the semi-major axis of its orbit. It was years later, in the 1660s, that the importance of this law was realized by scientists when it was combined with the newly discovered law of centrifugal force. The Harmony also contains the earliest mathematical understanding of two types of regular star polyhedra known as Kepler’s solids, the small stellated dodecahedron and the great stellated dodecahedron.

Third Law of Planetary Motion graph
Graph of Kepler’s Third Law of Planetary Motion


#6 His book Epitome became a highly influential work in astronomy

Epitome Astronomiae Copernicanae (Epitome of Copernican Astronomy) is an influential book by Kepler which was published in three volumes in 1617, 1620 and 1621. The Epitome contained all three laws of planetary motion and attempted to explain heavenly motions through physical causes. It extended the first two laws of planetary motion, which he had previously applied only to Mars, to all the planets as well as the Moon and satellites of Jupiter. Kepler’s laws of planetary motion are part of the foundation of modern astronomy and physics. The Epitome thus had a huge impact on future scientists and is considered one of his most important works. Also, the term “inertia” was introduced for the first time in this book. Kepler defined it only in terms of a resistance to movement and thus it was not exactly the same as its modern interpretation.

Johannes Kepler 1610 Portrait
1610 Portrait of Johannes Kepler


#7 He laid the foundation of modern optics

Johannes Kepler began his investigation of the laws of optics in 1600, and in 1604, his work Astronomiae Pars Optica (The Optical Part of Astronomy) was published. Among other things, the book contains Kepler’s formulation of the law that the intensity of light decreases with the square of distance; description of reflection by flat and curved mirrors; and astronomical implications of optics such as parallax, astronomical refraction and the apparent sizes of heavenly bodies. Astronomiae Pars Optica is considered as the foundation of modern optics. Another influential work in optics by Kepler, Dioptrice, was published in 1611. Among other things, it contained the concepts of real vs. virtual images; upright vs. inverted images; and the effects of focal length on magnification and reduction.

Inverse-square law diagram
Representation of the inverse-square law governing the intensity of light with S as light source


#8 Kepler was the first to correctly explain the working of a human eye

Camera obscura is the natural optical phenomenon in which light rays reverse themselves when they pass through a small aperture resulting in an inverted image on the screen opposite to the opening. It is the principle on which the pinhole camera works. Johannes Kepler was the first to use the term “camera obscura” in his 1604 work Astronomiae Pars Optica; and to discover that it was the principle behind the working of the pinhole camera. Kepler extended his study to the human eye and recognized that an image is reversed by the eye’s lens; and is formed on the retina; and that it is inverted. Thus Kepler laid the foundation for the field of ophthalmologic optics. His discovery later helped explain physical phenomena like central visual acuity, visual field, dark adaptation and errors of refraction.

Human eye image formation diagram
Diagram of image formation in a human eye


#9 He invented the Keplerian telescope, an improved refracting telescope

In his 1604 work Astronomiae Pars Optica, Kepler put forward the first correct explanation as to why convex and concave lenses could correct presbyopia and myopia. Then, in his 1611 work Dioptrice, Kepler set out the theoretical basis of double-convex converging lenses and double-concave diverging lenses; and explained how they combined to produce a Galilean telescope. He went on to describe an improved refracting telescope which would use two convex lenses to produce higher magnification than Galileo’s combination of convex and concave lenses. Keplerian telescope could achieve considerably higher magnifications than Galilean telescope.

Keplerian refracting telescope diagram
Schematic diagram of Keplerian refracting telescope


#10 Johannes Kepler was a key figure of the scientific revolution

In 1627, Johannes Kepler published his Rudolphine Tables, a star catalog and planetary tables based on the observations of Tycho Brahe. For most stars these tables were accurate to within one arc minute, a significant achievement at that time. Rudolphine Tables was the first catalog to include corrective factors for atmospheric refraction; and is considered the best of the pre-telescopic catalogs. Kepler’s novel, titled Somnium (The Dream), was published posthumously in 1634. It described a fantastic trip to the moon; and what lunar astronomy would be like. Famous writer Isaac Asimov referred to Somnium as the first work of science fiction. As founder of celestial mechanics and optics, Johannes Kepler was a key figure of the Scientific Revolution, an age which saw the intellectual transformation of Europe. He laid the foundation for future scientists like Isaac Newton; and is considered among the most influential figures in the history of science.


  1. Lovely but #8 is not accurate. Kepler used the camera obscura as a model to understand the formation of images (pictures) on the retina. He was the first scientist to reason by analogy from a mechanical device (created by human hands) to a phenomenon found in a pure state of nature. This way of reasoning (analogous models) spread throughout the fields of science is still one of the dominant forms of scientific inquiry.


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