A Short History of Astronomy Ancient (before 500 BC) Egyptians, Babylonians, Mayans, Incas, Chinese Classical Antiquity (500 BC-500 AD) Greeks, Romans: Plato, Aristotle, Ptolemy Middle Ages ( AD) Arabic astronomers Renaissance ( AD) Copernicus Brahe, Kepler, Galilei, Newton

Ancient Astronomy Pyramids, Egypt Stonehenge, England Newgrange, Ireland

The Babylonians Made systematic measurements as early as ~2000 BC By ~ 800–400 BC –State support for the calendar and astrology –Compiled the first star catalogs and began long- term records of planetary motions –Were able to predict lunar and solar eclipses May also have invented astrology

The Greeks Plato (428 BC) Introduces the celestial sphere. The stars are fixed to a sphere that rotates around the Earth introduces prejudice in favor of circles values theory over observation

Alexander the Great ( BC) Much of the knowledge of the world came together as the rule of Alexander spread across Europe, Asia, and Northern Africa. Founded the city of Alexandria Library of Alexandria contained the written works and inventions of many great thinkers and scientists.

The Greeks Aristotle (384–322 BC) –Argued that the planets move on spheres around the Earth (“geocentric” model) –Argues that the earth is spherical based on the shape of its shadow on the moon during lunar eclipses Aristarchus (310–230 BC) –Attempts to measure relative distance and sizes of sun and moon –Proposes, nearly 2000 years before Copernicus, that all planets orbit the Sun, including the Earth (“heliocentric” model)

Using the distance between the Earth and the Moon as a baseline

The Greeks Eratosthenes (ca. 276 BC) –Measures the radius of the earth to about 20%

Eratosthenes (~ 200 B.C.): Calculation of the Earth’s radius Angular distance between Syene and Alexandria: ~ 7 0 Linear distance between Syene and Alexandria: ~ 5,000 stadia  Earth Radius ~ 40,000 stadia (probably ~ 14 % too large) – better than any previous radius estimate.

The Greeks Hipparchus (~190 BC) –His star catalog a standard reference for sixteen centuries! –Introduces coordinates for the celestial sphere: Declination (dec) Right Ascension (RA) (analogous to latitude and longitude, respectively)

Ptolemy (~140 AD) Puts forth a complete geocentric model dominates scientific thought during the Middle Ages Longest lasting (wrong) theory ever: 1000 yrs Major Work: Almagest

Retrograde Motion

Epicycles Ptolemy’s explanation of retrograde motion About 40(!) epicycles necessary to explain all observations  complicated theory

Hypatia of Alexandria (355 or /416 A.D.) Exact dates are unknown. Many records were destroyed. Hypatia was, simply, the last great Alexandrian mathematician and philosopher. Edited the work On the Conics of Apollonius

The Medieval Setting Dominant Church 1000 years of relative stagnation Experimental research greatly reduced To answer a question: “Study the Bible or Aristotle!”

The Renaissance Setting Invention of the print (1450) by Gutenberg  Books widely available! (Think: Manuscripts vs Amazon.com ) End of Middle Age Church Domination Back to the roots (renaissance=rebirth) Study of Arabic astronomers Intellectual movement

Nicolas Copernicus (1473–1543) Rediscovers the heliocentric model of Aristarchus Planets on circles  needs 48(!!) epicycles to explain different speeds of planets Not more accurate than Ptolemy Major Work : De Revolutionibus Orbium Celestium (published posthumously)

The heliocentric Explanation of retrograde planetary motion See also: SkyGaze

The Scientific Method Systematized by Francis Bacon, Descartes and Galileo in the 17 th century Not the only way of knowing, but a very successful one A method to yield conclusions that are independent of the individual Conclusions are based on observation

Tycho Brahe – The Data Taker Key question: Where are things? Catalogued positions of planets in Uraniborg and Prague Working without telescope Data ten times as accurate as before Died at banquet binge drinking Tycho Brahe (1546–1601)

Tycho Brahe collects detailed and accurate (1-2’ accuracy) observations of stellar and planetary positions over a period of 20 years His research costed 5-10% of Danish GNP shows that comets and novas are extralunar contrary to Aristotle Shows that stars can change (Supernova of 1572) Tycho Brahe observing

Johannes Kepler–The Phenomenologist Key question: How are things happening? Major Works: Harmonices Mundi (1619) Rudolphian Tables (1612) Astronomia Nova Dioptrice Johannes Kepler (1571–1630)

Kepler’s Beginnings Astrologer and Mystic Tried to find “music in the skies” Tried to explain distances of the 5 known planets by spheres resting on the 5 mathematical bodies  pre-scientific

Johannes Kepler Manuscript: trying to disentangle The mystery of Mars’ orbit 

Kepler’s First Law The orbits of the planets are ellipses, with the Sun at one focus

Ellipses a = “semimajor axis”; e = “eccentricity”

Conic Sections From Halley’s book (1710)

Kepler’s Second Law An imaginary line connecting the Sun to any planet sweeps out equal areas of the ellipse in equal times

Kepler’s Third Law The square of a planet’s orbital period is proportional to the cube of its orbital semi-major axis: P 2  a 3 a P Planet Orbital Semi-Major AxisOrbital Period Eccentricity P 2 /a 3 Mercury Venus Earth Mars Jupiter Saturn Uranus Neptune Pluto (A.U.)(Earth years)

Galileo Galilei – The Experimentalist Did experiments (falling bodies) rather than studying Aristotle Major Works Siderius Nuntius (1610) Dialogue concerning the Two Chief World Systems (1632) The latter discusses Copernicus vs Ptolemy  ban by Church (1633) –revoked by pope 1992 (1564–1642)

Siderius Nuntius (1610) Dialogo (1632)

Galileo Galilei (1564–1642) Astronomical observations that contradict Aristotle: –Observed mountains on the Moon, suggesting that the Earth is not unique –Sunspots; suggests that celestial bodies are not perfect and can change –Observed four moons of Jupiter; showed that not all bodies orbit Earth –Observed phases of Venus (and correlation of apparent size and phase); evidence that Venus orbits the Sun Also observed –the rings of Saturn –that the Milky Way is made of stars

Phases of Venus Heliocentric (observed) Geocentric (not observed)

Isaac Newton – The Theorist Key question: Why are things happening? Invented calculus and physics while on vacation from college His three Laws of Motion, together with the Law of Universal Gravitation, explain all of Kepler’s Laws (and more!) Isaac Newton (1642–1727)

Major Works: Principia (1687) [Full title: Philosophiae naturalis principia mathematica] Opticks [sic!] (1704) Major findings: – Three axioms of motion –Universal gravity

Law of Universal Gravitation Force = G M earth M man / R 2 M Earth M man R

Orbital Motion

Cannon “Thought Experiment” ets/newt/newtmtn.htmlhttp:// ets/newt/newtmtn.html