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240 BCE
The measurement of Earth.
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, according to NASA. His measurement of 24,660 miles (39,690 kilometers) was only 211 miles (340 km) off the true measurement. -
126
The orbit of planets.
In ancient Greece, an 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 revolve around Earth. Known as the Ptolemaic system, it remained in place for hundreds of years, though it turned out to be flat wrong. -
964
The first non-Milky Way star observation.
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. -
1508
The first solar model that proposed the planets revolving around the Sun.
In 16th century Poland, astronomer Nicolaus Copernicus (1473–1543) proposed a model of the solar system that involved the Earth revolving around the sun, according to NASA. 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. -
The first optical telescope.
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." -
Saturn's rings and largest Moon.
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 largest moon, Titan. -
Newton's Law
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. -
The rotation of Jupiter and Mars.
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 dubbed Cassini. -
Halley's Comet.
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. -
The speed of light discovery.
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.