Aristotle (384-322 BC) | A Greek philosopher, student of Plato and teacher of Alexander the Great. He wrote on many subjects, including philosophy, physics, metaphysics, poetry, theater, music, logic, rhetoric, politics, government, ethics, biology, and zoology. All this represents only about a third of his works that have survived to modern times. Among his many contributions, he argued that the sun, moon, and earth were all round. He gave good reasons for this conclusion, including the evidence that during a lunar eclipse, the shadow of the earth on the moon is always part of a circle, no matter the height of the moon in the sky or the time of year in which the eclipse should occur. |

Aristarchus of Samos (310-230 BC) | A Greek astronomer and mathematician. He was perhaps the first to use a heliocentric model of the solar system, in which the earth rotates on its own axis and revolves around the sun once a year. This system differed from others in that he put the sun rather than the earth at the center. This idea was later revived by Copernicus, Kepler, Galileo, and later Newton. |

Archimedes of Syracuse (287-212 BC) | A Greek mathematician, physicist, engineer, inventor, and more. He worked on hydrostatics, statics, and gave a mathematical analysis of the law of the lever. He invented many machines, some of which were for war and others used in peace. He developed methods very close to modern calculus, gave very accurate figures for the value of pi, and derived theorems showing the ratio of volume of a sphere to that of the smallest cylinder that could contain it. He is famous for shouting "Eureka!" (I found it) after figuring out how to tell if gold has been diluted with another metal. |

Eratosthenes (276-195 BC) | A Greek mathematician, astronomer and much more. Calculated the circumference of the Earth, yielding an accurate estimate. His estimate was far more accurate than the one Columbus used, which was far too small. |

Hipparchus (150-120 BC) | A Greek astronomer, geographer, and more. Made star charts with catalog of brightness levels; developed a mathematical model of motions of sun and moon. Developed trigonometry. Calculated eclipses of sun and moon and calculated the precession of the axis of rotation of the earth. He used a geocentric model in which the sun, moon, and stars revolve around the earth, which stood still in the center. |

Ptolemy, aka, Claudius Ptolemaeus (90-168 AD) | A Greek astronomer who lived in Egypt during Roman times; he was a geographer, a mathematician, and more. He wrote the book now known as Almagest, "the Greatest," which included and improved upon the ideas of previous writers (especially Hipparchus), and included everything you needed to know about astronomy for the next 1400 years. This book put the earth rather than the sun at the center of the earth system. This work was very important to Arabic and European science history, and was adopted by the church as true. Galileo later got in trouble by writing a book in which this theory was questioned. Ptolemy also had a psychological theory of the moon illusion (fact that the moon looks larger on the horizon than it does overhead). This idea was disputed by Galileo, who thought it was due to atmospheric refraction. |

Christopher Columbus (Colon, 1451-1506) | A Genoese (Italian) navigator and explorer who thought that one could sail from Europe to China by a short trip to the West. His estimate of the size of the earth was far off: his estimate was much smaller than the more accurate estimates that had been known for over a thousand years. The scholars of the time knew Columbus was wrong, but he convinced Isabella of Spain to finance his voyage in 1492. Although he was wrong about the size of the earth, he was lucky in that his ships found land when he expected to land in India. When he brought back some people he met on his voyage, he called them "Indians" and the name stuck, so Native Americans have been called "Indians" even though they were not from India. It was later realized that he landed on a "New World," lands that were not previously known to the people of Southern and Central Europe. For some strange reason, most American college freshmen think that scholars in the time of Columbus thought the world was flat and Columbus argued it was round, apparently showing a basic flaw in our educational system that fails to overcome false information spread by the media. |

Nicolaus Copernicus (Kopernik, 1473-1543) | A Polish mathematician and astronomer (and much more) who revived the theory by Aristarchus that the earth goes round the sun. In his model, the earth spins on its own axis and revolves around the sun, along with the other planets, but the moon revolves around the earth. This model did not require as many "epicycles" and other fudge factors (as did the model of Ptolemy) to fit what was known about the positions of the sun, moon, planets, and stars as seen from earth. |

Johannes Kepler (1571-1630) | A German mathematician and astronomer (and more) who improved the theory by Copernicus that the earth goes round the sun. He used the very accurate observations of planetary positions by the Danish astronomer, Tycho Brahe (1546-1601), to develop and evaluate his model. In his model, the planets travel in elliptical paths around the sun, rather than in uniform circles with epicycles and other adjustments. In addition, the area inside the ellipse that is swept out in any given time interval is equal; this means that the planet travels faster when it is nearer the sun. Furthermore, the time required for a planet to travel around the sun is related to its distance from the sun by the relation that the square of the time for a revolution is proportional to the cube of the average distance from the sun. He also developed a type of telescope and analyzed the optics of the eye. |

Galileo Galilei (1564-1642) | An Italian physicist, mathematician, philosopher, astronomer (and more) who was much influenced by the physics and mathematics of Archimedes (287-212 BC). When he learned of the telescope, he immediately constructed one and used it to examine the moon, stars, and planets. He noticed mountains on the moon, and estimated their altitude. He worked on a principle of inertia and relative motion. He wrote a book on the Copernican heliocentric system versus the Ptolemaic geocentric system. He was forced by the church to recant the theory that the earth goes round the sun, and he spent the rest of his life as a prisoner for having taught otherwise. He thought that the moon illusion was due to atmospheric refraction--due to physics rather than to psychology. |

Isaac Newton (1643-1727) | An English physicist, mathematician, philosopher, astronomer (and more) who was much influenced by the physics of Galileo and Kepler. He developed a set of simple principles (Newton's laws) that could account for sorts of problems involving motion, including Keplers Laws of planetary motion. His principles of inertia, force, mass, and relative motion were later rejected and revised by Albert Einstein (1878-1955). Nevertheless, Newtons laws are still taught in High School and college physics courses. Among his other contributions, he invented a type of reflecting telescope, proposed a particle theory of light and color, and developed the branch of mathematics we call calculus. |