Throughout human history, scientists have struggled to understand what they see in the night sky. Famous astronomers — many of them great scientists who mastered many fields — explained the heavens with varying degrees of accuracy. Over the centuries, a geocentric view of the universe — with Earth at the center of everything — gave way to the proper understanding we have today of an expanding universe in which our galaxy is but one of billions. On this list are some of the most famous scientists from the early days of astronomy through the modern era, and a summary of some of their achievements.
When most people believed the world was flat, the notable Greek mathematician, astronomer and geographer Eratosthenes (276–195 B.C.) used the sun to measure the size of the round Earth. His measurement of 24,660 miles (39,690 kilometers) was only 211 miles (340 km) off the true measurement.
In ancient Greece, astronomer and mathematician Claudius Ptolemy (A.D. 90–168) set up a model of the solar system in which the sun, stars, and other planets revolved around Earth. Known as the Ptolemaic system, it remained in place for hundreds of years, though it turned out to be flat wrong. According to NASA, "Ptolemy represents the epitome of knowledge of Grecian astronomy."
Persian astronomer Abd al-Rahman al-Sufi (903–986), known as Azophi to Westerners, made the first known observation of a group of stars outside of the Milky Way, the Andromeda galaxy.
In 16th century Poland, astronomer Nicolaus Copernicus (1473–1543) proposed a model of the solar system that involved the Earth revolving around the sun. The model wasn't completely correct, as astronomers of the time struggled with the backwards path Mars sometimes took, but it eventually changed the way many scientists viewed the solar system.
Using detailed measurements of the path of planets kept by Danish astronomer Tycho Brahe, Johannes Kepler (1571–1630) determined that planets traveled around the sun not in circles but in ellipses. In so doing, he calculated three laws involving the motions of planets that astronomers still use in calculations today. However, closed minds put Kepler's work at risk.
"The era in which Kepler lived was one of tremendous upheaval and change," said Dan Lewis, curator of the history of science and technology at the Huntington Library in San Marino, Calif. "Religious leaders were reluctant to relinquish their ideas about the heavens. Talk by astronomers of a sky filled with objects moving in non-circular orbits and other phenomena that went against an Earth-centric model threatened their beliefs. As a result, Kepler and his first wife, Barbara, created a code with which to write letters to each other so that their correspondence would not put them at risk of persecution."
Born in Italy, Galileo Galilei (1564–1642) is often credited with the creation of the optical telescope, though in truth he improved on existing models. According to the Rice University's Galileo Project, "Galileo made his first telescope in 1609, modeled after telescopes produced in other parts of Europe that could magnify objects three times. He created a telescope later that same year that could magnify objects twenty times."
The astronomer (also mathematician, physicist and philosopher) turned the new observational tool toward the heavens, where he discovered the four primary moons of Jupiter (now known as the Galilean moons), as well as the rings of Saturn. Though a model of the Earth circling the sun was first proposed by Copernicus, it took some time before it became widely accepted. Galileo is most widely known for defending the idea several years after Kepler had already calculated the path of planets, and Galileo wound up under house arrest at the end of his lifetime because of it.
Italian astronomer Giovanni Cassini (1625–1712) measured how long it took the planets Jupiter and Mars to rotate. He also discovered four moons of Saturn and the gap in the planet's rings. When NASA launched a satellite to orbit Saturn and its moons in 1997, it was fittingly dubbed Cassini.
Dutch scientist Christiaan Huygens (1629–1695) proposed the earliest theory about the nature of light, a phenomenon that puzzled scientists for hundreds of years. His improvements on the telescope allowed him to make the first observations of Saturn's rings and to discover its moon, Titan.
English astronomer Sir Isaac Newton (1643–1727) is most famous for his work on forces, specifically gravity. Building on the work of those who had gone before him — he is quoted as saying, "If I have seen further, it is by standing upon the shoulders of giants" — he calculated three laws describing the motion of forces between objects, known today as Newton's laws.
Edmond Halley (1656–1742) was the British scientist who reviewed historical comet sightings and proposed that the comet that had appeared in 1456, 1531, 1607, and 1682 were all the same, and would return in 1758. Although he died before its return, he was proven correct, and the comet was named in his honor.
French astronomer Charles Messier (1730–1817) composed a database of objects known at the time as "nebulae," which included 103 objects at its final publication, though additional objects were added based on his personal notes. Many of these objects are often listed with their catalog name, such as the Andromeda Galaxy, known as M31. Messier also discovered 13 comets over the course of his lifetime.
British astronomer William Herschel (1738–1822) cataloged over 2,500 deep sky objects. He also discovered Uranus and its two brightest moons, two of Saturn's moons, and the Martian ice caps. William trained his sister, Caroline Herschel (1750–1848), in astronomy, and she became the first woman to discover a comet, identifying several over the course of her lifetime.
Henrietta Swann Leavitt (1868–1921) was one of several women working as a human "computer" at Harvard College, identifying images of variable stars on photographic plates. She discovered that the brightness of a special flashing star known as a Cepheid variable was related to how often it pulsed. This relationship allowed astronomers to calculate the distances of stars and galaxies, the size of the Milky Way, and the expansion of the universe.
In the early 20th century, German physicist Albert Einstein (1879–1955) became one of the most famous scientists ever after proposing a new way of looking at the universe that went beyond current understanding. Einstein suggested that the laws of physics are the same throughout the universe, that the speed of light in a vacuum is constant, and that space and time are linked in an entity known as space-time, which is distorted by gravity.
In a lecture given in 1966, fellow scientist Robert Oppenheimer said, "Einstein was a physicist, a natural philosopher, the greatest of our time."
At the same time Einstein was expanding man's view of the universe, American astronomer Edwin Hubble (1899–1953) calculated that a small blob in the sky existed outside of the Milky Way. Prior to his observations, the discussion over the size of the universe was divided as to whether or not only a single galaxy existed. Hubble went on to determine that the universe itself was expanding, a calculation which later came to be known as Hubble's law. Hubble's observations of the various galaxies allowed him to create a standard system of classification still used today.
American astronomer Harlow Shapley (1885–1972) calculated the size of the Milky Way galaxy and general location of its center. He argued that the objects known as "nebulae" lay within the galaxy, rather than outside of it, and incorrectly disagreed with Hubble's observations that the universe boasted galaxies other than the Milky Way.
Frank Drake (born 1930) is one of the pioneers in the search for extraterrestrial intelligence. He was one of the founders of the Search for Extraterrestrial Intelligence (SETI) and devised the Drake equation, a mathematical equation used to estimate the number of extraterrestrial civilizations in the Milky Way galaxy able to be detected.
American astronomer Carl Sagan (1934–1996) may not have been a great scientist in comparison to some on this list, but he is one of the most famous astronomers. Sagan not only made important scientific studies in the fields of planetary science, he also managed to popularize astronomy more than any other individual. His charismatic teaching and boundless energy influenced people around the world as he broke down complicated subjects in a way that interested television viewers even as he educated them. Sagan founded the Planetary Society, a nonprofit organization devoted to advancing space science and exploration.
American astronomer William K. Hartmann (born 1939) put forth the most widely accepted theory on the formation of the moon in 1975. He proposed that, after a collision with a large body scooped, debris from the Earth coalesced into the moon.
Stephen Hawking (born 1942) has made many significant insights into the field of cosmology. He proposed that, as the universe has a beginning, it will likely also end. He also suggested that it has no boundary or border. Despite being viewed as one of the most brilliant minds since Einstein, many of Hawking's books and lectures are steered toward the general public as he seeks to educate people about the universe they live in.