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A Brief History ofAstronomy

Ancient Civilizations

• Astronomical ruins– Mayans– Aztec– European (Stonehenge)– Native Americans (Chaco Canyon)– Arabs– Greeks

• Probably all civilizations watched the sky &we have remains of only a few

Ancient Greek Astronomers

• Pythagorus (550 BCE)– relationship between numbers and nature– mathematics

• Plato (350 BCE)– Geocentric universe, stars revolve around earth– circular motion with uniform, constant speed– planets - wandering stars, more problematic

• each planet on its own sphere• Mars - retrograde motion• problem for 2000 years!!!

Ancient Greek Astronomy• Models were generally wrong

because they were based on wrong“first principles”, believed to be“obvious” and not questioned:

1. Geocentric Universe: Earth at theCenter of the Universe.

2. “Perfect Heavens”: Motions of allcelestial bodies described by motionsinvolving objects of “perfect” shape, i.e.,spheres or circles.

Ancient Greek AstronomersAristotle

(384 – 322 B.C.), majorauthority of philosophy until thelate middle ages: Universe can

be divided in 2 parts:1. Imperfect, changeable Earth,

• earth, water, air, fire• earth is heaviest, so at

center

2. Perfect Heavens (describedby spheres)aether or quintessence

Aristotle’smodel55 spheres!

Moon, Mercury, Venus, Sun,Mars, Jupiter, Saturn, fixed stars,

Prime mover

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Fundamental argument for the geocentric universe:

•Motion of Earth should result in an observable parallax, whichwas not seen.

•Parallax is too small to be seen with naked eye

Issues of Ancient Astronomy (II)The retrograde (westward) motion of the planets

The “solution”: Epicycles!

Planets also vary inbrightness

Hipparchus (150 BCE)

• careful catalog ofstars

•placed planets ofepicyles ingeocentric model

•Hipparchussystem did notmatch

Introduced by Ptolemy (ca. A.D. 140)

The Ptolemaic system was considered the “standardmodel” of the universe until the Copernican Revolution.

Ptolemaic System

• added “equants” - points about whichangular velocity is constant

• earth is not center• violated perfect circular motion• predicted planetary motions well

The Copernican Revolution

Nicolaus Copernicus (1473 – 1543):

heliocentric universe (sun in the center)

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Copernicus’ new (and correct) explanationfor retrograde motion of the planets:

•This made Ptolemy’s epicycles unnecessary.

•Not perfect predictor of planet motions

Retrograde(westward)motion of a

planet occurswhen Earthpasses the

planet.

Aristarchus of Samos (310-230 BCE)

• first reference to heliocentric universe• amazing geometer• measured

– circumference of earth– relative size of earth and moon from lunar eclipse– relative size of moon and sun from solar eclipse

• knew sun was much bigger than earth• presumably this led him to heliocentric model

• we should see parallax of stars if we are orbiting the sunbut don’t– Aristarchus reasoned that stars were enormously far away

• sun is a star!

Tycho Brahe (1546 – 1601)

Use of high-precision instruments for precise astronomicalobservations, meticulously reported in tables.

Later used by Kepler to develop laws of planetary motion.

A Quadrant designed by Brahe

•Used to preciselymeasure an object’sangular distance abovethe horizon

•Detected a new star(supernova)

•heavens changed!

Johannes Kepler (1571 – 1630)Used the precise

observational tables ofTycho Brahe to study

planetary motionmathematically.

1.Circular motion and

• Planets move around the sun on elliptical paths,with non-uniform velocities.

Found a consistentdescription by

abandoning both:

2.Uniform motion.

Kepler’s Laws ofPlanetary Motion

1.The orbits of the planets are ellipses with thesun at one focus.

c

Eccentricity e = c/a

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Eccentricities of planetary orbitsOrbits of planets are virtually indistinguishable from circles:

Earth: e = 0.0167Most extreme example:

Pluto: e = 0.248

But their centersmay be

significantly offsetfrom the sun.

3.A planet’s orbital period (P) squared isproportional to its average distance from thesun (a) cubed:

Py2 = aAU

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2.A line from a planet to the sun sweeps overequal areas in equal intervals of time.

(Py = period in years; aAU = distance in AU)

Galileo Galilei(1594 – 1642)

•Introduced the modern view ofscience:

•Transition from a faith-based“science” to an observation-based science.

•Greatly improved on the newlyinvented telescope technology.

•Galileo did NOT invent thetelescope!

• Was the first to meticulously reporttelescope observations of the sky tosupport the Copernican model of theuniverse.

Major discoveriesof Galileo (I):• Moons of Jupiter (4 Galilean moons)

• Rings of Saturn

(What he really saw)

Major discoveries of Galileo (II):

• sunspots (proving that the sun is not perfect!)

• Phases of Venus (including “full Venus”), provingthat Venus orbits the sun, not Earth!

Major discoveries of Galileo (III):

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Historical Overview Isaac Newton (1643 - 1727)

• Building on the results of Galileo and Kepler

Major achievements:1. Invented calculus as a necessary tool to solve

mathematical problems related to motion

• Adding physics interpretations to the mathematicaldescriptions of astronomy by Copernicus, Galileo and Kepler

2. Discovered the three laws of motion3. Discovered the universal law of mutual gravitation

The Universal Law of Gravity

• Any two bodies are attracting eachother through gravitation, with a forceproportional to the product of theirmasses and inversely proportional tothe square of their distance:

F = - G Mmr2

(G is the gravitational constant.)

Orbital Motion (II)In order to stay on a closed

orbit, an object has to be withina certain range of velocities:

Too slow : Object falls backdown to Earth

Too fast : Object escapesthe Earth’s gravity