Famous astronomers

See some of the most famous astronomers and physicists throughout history, from humanity's earliest observations of celestial events to today's investigations of deep sky objects that hold the secrets of the universe.

FIRST STOP: Ptolemy

Claudius Ptolemy

In the 2nd century, Ptolemy published the Almagest, a comprehensive treatise on the movements of the stars and planets. It expanded Hipparchus’ geometric model of celestial motions, utilizing epicycles and eccentric circles in a geocentric theory which placed the Earth at the center of the solar sys tem. This Ptolemaic system presented tables of information allowing convenient predictions of planetary locations. Ptolemy also catalogued 48 constellations, the names of which are still in use at present.

Ptolemy’s writings stood as authoritative for more than twelve hundred years. However, his model, which was incorrect, later fell out of use as the heliocentric view of the solar system came into being.

Few details about Ptolemy’s life survived to the present day.

NEXT: Copernicus

Nicolaus Copernicus

Copernicus, of Poland, felt the Ptolemaic view of the planets traveling in circular orbits around the Earth was over-complicated with many smaller circles, epicycles, needed to explain the intermittent retrograde motion of the planets (in which they appear to move in the opposite direction of the the stars). Copernicus published his book, De Revolutionibus Orbium Coelestium ("On the Revolutions of the Heavenly Spheres") when he was 70 and lay dying.

His ideas took almost a hundred years to gain credence, but Galileo's 1632 assertions that the Earth orbited the sun built upon the Polish astronomer's work, cementing the Copernican revolution. [Read more about Copernicus.]

NEXT: Kepler

Johannes Kepler

Kepler deduced that the planets do not travel in perfect circles around the Sun, as Copernicus had thought, but rather possess elliptical orbits, with the sun at one of the foci. This insight formed his first planetary law, which he published in 1609 with the second law which stated that planets do not travel at the same rate throughout their orbits.

Kepler's third law, published a decade later, posited that the relationship between the length of two planets' orbits is related to their distances from the sun. Though he made other contributions to mathematics and optics, Kepler's three laws made him a giant of astronomy. [Read more about Kepler.]

NEXT: Galileo

Galileo Galilei

Galileo, born in Pisa, Italy, made numerous scientific discoveries. He famously proved that all falling bodies fall at the same rate, regardless of mass. Further he developed the first pendulum clock.

Galileo experimented with and refined telescopes (though he did not invent them, as is often incorrectly thought). He is perhaps best known for discovering the four most massive moons of Jupiter, now known as the Galilean moons.

Based on his telescope research, Galileo supported the Copernican heliocentric model of the solar system, publishing his arguments in "Dialogue Concerning the Two Chief World Systems, " during 1632. The ruling Catholic church forced Galileo to recant these theories, and was kept under house arrest for the remaining nine years of his life. Today his legacy lived on in the Galileo spacecraft which probed Jupiter. [Read more about Galileo.]

NEXT: Isaac Newton

Isaac Newton

The well-known Newtonian laws of motion are: 1) an object at rest tends to stay at rest and an object in uniform motion tends to stay in uniform motion unless acted upon by a net external force. 2) The net force on an object is equal to the rate of change of its linear momentum in an inertial reference frame, or if a body is accelerating, there a force is acting on it. 3) For every action there is an equal and opposite action.

In a story that has long since gone into the public consciousness, Newton supposedly found inspiration for his theory of gravitation upon seeing an apple fall from a tree. From this he conjectured that gravity's pull could extend outwards from the earth, even as far as the moon and further.

Newton's achievements have been celebrated in many ways, with statues and poems. Notably the unit for force was named for him, the newton (N). [Read more about Newton.]

NEXT: Christiaan Huygens

Christiaan Huygens

Elsewhere in his research, Huygens proposed a wave theory of light, which was disputed by Newton, who preferred the particle theory. The modern theory of light combines the two into a wave-particle duality model.

NEXT: Giovanni Cassini

Giovanni Cassini

NEXT: Charles Messier

Charles Messier

NEXT: Albert Einstein

Albert Einstein

Einstein’s first revolutionary innovation came in the form of his special theory of relativity which states that the laws of physics are the same everywhere in the universe. Further, the speed of light is a constant. A great deal of modern physics revolves around these ideas. Einstein determined that the faster an object moves, the more massive it becomes, giving rise to his famous equation: E=mc^2, where E is energy, m is mass and c is the speed of light.

Further expanding those ideas, Einstein developed general relativity, which states space and time curve near a massive object, distorting the fabric of space-time. he published the general theory of relativity in 1916.

Einstein received the 1921 Nobel Prize in physics, among many other awards and honors. His distinctive appearance, in particular his flowing hair, made an indelible impression upon world society, serving as a template for eccentric scientists and geniuses in fiction. [Read more about Einstein.]

NEXT: Carl Sagan

Carl Sagan

However, Sagan’s real contribution to astronomy was as a educator and popularizer of science. He published numerous articles and books, including “Cosmos, ” which became a television show viewed by a billion people in sixty countries. As host of the show, he even spawned his own catchphrase — the oft-parodied “billions and billions” — based on his distinctive inflection, though he never uttered that phrase during the show. Sagan also penned the science-fiction novel “Contact, later adapted into a motion picture starring Jodie Foster. Many tributes and memorials were dedicated to Sagan following his death, illustrating how deeply his persona pervaded the cultural landscape. [Read more about Sagan.]

NEXT: Stephen Hawking

Stephen Hawking

Hawking's primary discovery stated that since the universe began (at the Big Bang), it must come to an end. Hawking demonstrated (with Roger Penrose) that since Einstein’s general theory of relativity suggested that space and time began at the birth of the universe, and ends within black holes. This results unites general relativity and quantum theory. Further, Hawking predicted that black holes do emit radiation, called Hawking radiation.

Hawking wrote about these and other discoveries in several books, including the best-seller A Brief History of Time. His wheelchair-bound appearance and his speech-synthesized voice (he is now completely paralyzed) are familiar to the public from appearances on Star Trek: The Next Generation, The Simpsons, Futurama, and The Big Bang Theory. [Read more about Hawking.]