History of Astronomy

1. History of Astronomy Pre-AP student notes

2. Aristotle • 384 BC – 322 BC • Thought the universe was stable, and always will be • Believed in a geocentric universe, where the planets revolved around the Earth • Believed the entire universe was the same as the solar system, did not understand the vastness of space

3. Aristarchus • 310-230 BC • Was the first astronomer to believe in a heliocentric (sun centered) universe • Nobody believed him  • First attempt at measuring the distance from the Earth to the moon, and accurately did so • His ideas of a heliocentric universe went unsupported by astronomers and the general public Aristarchus’ moon measurement calculations

4. Ptolmey 90-168 (AD) • A Roman astronomer who wrote in Greek • Began writing a star catalog, naming and describing the stars in the night sky • Huge supporter of geocentric models of the universe • At the time, astronomers were confused about why the planets wandered the sky in unpredictable patterns (unlike stars.) • Ptolmey explained the structure of the universe in terms of epicycles, complicated planetary movements in circles as they orbit the Earth • This geocentric model of the universe was accepted for many, many years

5. Copernicus: Ancient Greek astronomer • First to explain why Earth has seasons • Copernicus took the guesswork out of understanding the universe, and was the first to apply evidence and the scientific method • The church banned Copernicus’ books and saw him as a troublemaker, causing upset in the way we believed our Earth was central to the universe • During his lifetime, he was never given recognition for his heliocentric model of the universe • 100 years after he died, his model was accepted and people changed their way of thinking from geocentric (Earth centered) to heliocentric (sun centered.) • This huge change in people’s perceptions (that the Earth is not central or all that important in the grand scheme of things) is now referred to as the Copernican Revolution

6. Galileo Galilei • Came 100 years after Copernicus • Made his own telescope and improved it • First to point the telescope at the sky • Discovered Jupiter’s 4 largest moons: Io, Europa, Ganymede, and Calisto… these are called the “Galilean moons” now • Popularized the Copernican view of a heliocentric universe • Wrote his books in Italian (instead of Latin) so everyone can read them, not just the scholars • The church warned him to stop writing books about how the universe was heliocentric… his books were banned and Galileo was put on house arrest (This didn’t stop him.) • Finally after the threat of being burned at the stake (like Giordino Bruno was,) Galileo stopped writing. • Galileo’s finest accomplishments: • 1. Discovered Galilean moons of Jupiter • 2. Discovered mountains and valleys on the moon • 3. perfected the telescope , first to point it skyward • 4. Discovered that Venus has phases, like the moon does • 5. Observed sunspots on the sun (NEVER look at the sun.)

7. Tycho Brahe • Wealthy Danish nobleman who loved to party with friends • Built his own observatory in his house • Was the best naked eye astronomer who has ever lived, made accurate measurements of the distances of planets and stars with no telescope • Very eccentric – had a pet moose that he let roam his house (it was fed some beer at one of his parties and then fell down the stairs and died) • 1573 witnessed a supernova and was the only astronomer at the time to maintain that it was a distant phenomenon… others believed it was something occurring as nearby as the moon. Tycho used parallax to demonstrate to others that it was not a local occurance. • Tycho and one of his friends had a duel to settle an argument, Tycho lost his nose (hint: don’t duel in the dark) and had a set of false noses he wore in public… some of them gold, silver, and copper. • Tycho remains to this day the best naked eye astronomer that ever lived • Large lunar crater named after him • Came up with his own model of the universe and was convinced that Copernicus was wrong… died of kidney disease while trying to prove his model was better than Copernicus • Enlisted the help of his assistant, Johannes Kepler, to try to prove his model was more correct than Copernicus’ model of the universe.

8. Johannes Kepler • Friends with Galileo • Was Tycho Brahe’s assistant • Tycho had told him “Let not me have lived in vain” because he wanted Kepler to prove the Copernican model wrong, and convince everyone that the Brahe model was right. It wasn’t. • Kepler, a physicist at heart, came up with 3 laws of planetary motion from studying Brahe’s measurements… • Planets do not follow circular orbits, but instead follow elliptical orbits… most are barely elliptical but Pluto was the most elliptical • A line joining a planet and the Sun sweeps out equal areas during equal intervals of time… this means as a planet is nearing the sun, it speeds up and whips around the sun then slows again • 3.The square of the orbital period of a planet is directly proportional to the cube of the semi-major axis of its orbit. (p squared = a cubed)

9. Women in Astronomy – Harvard’s computers • Williamina Fleming was a housekeeper for a Harvard professor named Edward Pickering, who became angry at his assistant and told him “My maid could do a better job than you.” Sympathetic to the cause of women’s rights…. Pickering hired Williamina to study the spectral classes of stars. She classified stars according to spectral type, A through Q. • Antonia Maury: worked on classifying stars and creating a master stellar catalog. • Henrietta Leavitt: worked to study the magnitude of stars (brightness) and was the first to study Cephied variable stars (stars that do not maintain their brightness, and instead repeat a cycle of bright and dim.) • Annie Jump Cannon: rearranged the classification system of stars to reflect temperature…. Created the OBAFGKM system. • Caroline Herschel (sister to William Herschel, discussed on the next slide, an astronomer who discovered 2 moons of Saturn, and discovered infared radiation and built many telescopes) discovered 8 comets and 3 nebulae, and helped William build the world’s best telescopes of the time

10. William Herschel – German astronomer 1738-1822 Herschel became most famous for the discovery of the planet Uranus in addition to two of its major moons, Titania and Oberon. He also discovered two moons of Saturn and infrared radiation. Finally, Herschel is less known for the twenty-four symphonies that he composed. Herschel was a telescope builder, building many telescopes in his lifetime. His 40 foot telescope with a 49.5 inch mirror (which he cut and polished himself) helped him discover a moon of Saturn the first night he used it. Herschel’s 40 foot telescope

11. Edwin Hubble • Edwin Powell Hubble (November 20, 1889 – September 28,1953) was an American astronomer who profoundly changed understanding of the universe by confirming the existence of galaxies other than our own, the Milky Way. • He also discovered that the degree of "Doppler shift" (specifically "redshift") observed in the light spectra from other galaxies increased in proportion to a particular galaxy's distance from Earth. This relationship became known as Hubble's law, and helped establish that the universe is expanding.