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165
Ptolemy
Ptolemy was a Greek astronomer who lived between 85-165 A.D. He put together his own ideas with those of Aristotle and Hipparchus and formed the geocentric theory. This theory states that the Earth was at the center of the universe and all other heavenly bodies circled it, a model which held for 1400 years until the time of Copernicus. Ptolemy is also famous for his work in geography. He was the first person to use longitude and latitude lines to identify places on the face of the Earth. -
276
Eratosthenes
Eratosthenes was a Greek scientist who lived from 276 to 194 B.C. He studied astronomy, geography, and math. Eratosthenes is famous for making the first good measurement of the size of the Earth. He spent most of his life in the city of Alexandria in Egypt. Eratosthenes was born in 276 B.C. in the city of Cyrene which is in the modern-day country of Libya. He studied for a few years in Athens. Later, he studied and spent most of his adult life in Alexandria. He died in Alexandria in 194 B.C. -
Feb 19, 1473
Nicholas Copernicus
Nicholas Copernicus was a Polish astronomer who lived between 1473-1543. Before his time, people believed in the Ptolemaic model of the solar system, which maintained that the Earth was the center of the universe. Copernicus changed this belief when he introduced the heliocentric model, centered around the sun. He claimed that all the planets, including Earth, moved in orbits around the sun, and showed how this new system could accurately calculate the positions of the planets. -
Dec 14, 1546
Tycho Brahe
Tycho Brahe was a Danish astronomer who lived between 1546-1601. For over twenty years, he made very accurate observations of the night sky, all without the aid of a telescope, which had not yet been invented. Tycho also built the world's first observatory and kept a star catalogue with over 1000 stars. Tycho's records were used by Johan Kepler to describe the orbits of planets around the sun and disprove the Ptolemaic theory. -
Feb 15, 1564
Galileo Galilei
Galileo Galilei was an Italian astronomer and physicist who lived between 1564-1642. He challenged Aristotle's proposition that heavenly bodies were divine and therefore perfect and blemish-free. Galileo was the first person to use a telescope to look at the heavens. He discovered sunspots, and craters and peaks in the moon. Galileo's work offended the Roman Catholic Church and he was sentenced to house arrest for the later years of his life. Today, he is remembered as a martyr for scientific. -
Dec 27, 1571
Johan Kepler
Johan Kepler was a German astronomer who lived between 1571-1630. He introduced three important laws of planetary motion and helped the Copernican model of the solar system gain general acceptance. Kepler inherited Tycho Brahe's observational data on Mars following Brahe's death and showed, mathematically, that Mars followed an elliptical orbit. This new revelation contradicted the age old belief that heavenly bodies all moved in perfect circles. -
Isaac Newton
Isaac Newton was an English scientist and mathematician who lived between 1642-1727. He had one of the most brilliant minds the world has ever known. Legend has it that seeing an apple fall gave Newton the idea that gravity, the force which keeps us bound to the Earth, also controls the motion of planets and stars. -
Edwin Hubble
Edwin Hubble was an American astronomer who lived between 1889-1953. His observations of galaxies helped him develop the idea of an expanding universe, which forms the basis of modern cosmology, the study of the origin of the universe. He also discovered a relationship between a galaxy's speed and its distance. Hubble's studies were interrupted by service in both World Wars. -
Hipparchus
Hipparchus was a Greek astronomer who lived between 190-120 B.C. He created the first accurate star map and kept a catalogue of over 850 stars with their relative brightnesses. He also developed the system of epicycles (where everything in space moved in perfect circles) for the planets that both agreed with observation, and preserved the Earth-centered universe of Aristotle. Hipparchus compiled a table of chord lengths, similar to modern trigonometry tables. -
Aristotle
Aristotle was a Greek philosopher.He was one of the greatest thinkers of the world and his written works encompassed all major areas of thought. Aristotle mistakenly believed that the Earth was at the center of the universe and made up of only four elements: earth, water, air, and fire. He also thought that celestial bodies such as the sun, moon, and stars, were perfect and divine, and made of a fifth element called ether.
