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Lesson 1-4
History of Astronomy
Renaissance Astronomy
(1400 A.D. to 1640 A.D.)
Lesson overview
 Copernicus (heliocentric theory)
 Brahe (observations via instruments)
 Kepler (3 laws of motion)
 Galileo (telescopes to observe the solar system)
Introduction
 Renaissance period—four people established
the way we see our solar system:
 Nicolaus Copernicus—challenged earlier theories
 Tycho Brahe—made amazing observations
 Johannes Kepler—built on Brahe’s observations;
created three basic laws
 Galileo Galilei—used a telescope to observe the
Moon, Sun, and three planets
 His observations and Keplar’s math destroyed the earlier
geocentric theories
Nicolaus Copernicus
(1473-1543)
 Polish physician and lawyer,
renaissance humanist and
Catholic cleric
 Had trouble with Ptolemy’s
geocentric (earth-centered) model
 Believed in Aristarchus’
heliocentric (sun-centered) model
 Asserted Earth rotates on its axis
once per day
Heliocentric and retrograde motion?
 Ptolemy used epicycles to
explain retrograde motion
(but got very complicated)
 Heliocentric models explain
retrograde motion as a
natural result of two planets
passing each other as they
revolve counterclockwise
around the Sun
More Copernicus
 Wrote “On Revolutions of Celestial Spheres”
 Calculated with accuracy the relative distances of the
planets to the sun using geometry
 Could not explain lack of “stellar parallax” when
viewing stars
 Copernicus surmised the distances were too great to have
any parallax effects
 People at the time couldn’t imagine such vast distances
 Theory rejected as a model of reality but embraced as
useful for calculations
Tycho Brahe (15461601)
• Build island observatory -- “Uraniborg”
Invented “modern” instruments to observe planets
Accurately measured the locations of the planets
Gathered other data – Mars in particular
Brahe’s Quadrant
Azimuthal Semicircle
For measuring
altitude or angular
distances
Armillary
Sphere
Tycho Brahe (15461601)
 Observed things that suggested heavens were
changeable and complex
 Observed comet traveling through the orbits of several
planets
 Shattered Aristotle’s crystalline-sphere theory
 Proposed different, but still incorrect, geocentric model
 Geo-heliocentric model
 The Sun and the Moon revolved around the Earth
 Everything else revolved around the Sun
 Lacked mathematical know-how to explain the motion of
the planets
 Hired Johannes Kepler
Johannes Kepler (1571-1630)
 Challenged by Brahe to
explain mathematically
Mars’ orbit around the sun
 Used Brahe’s data on Mars
to show the orbit was
elliptical
 Put the sun at one focus
resulting in 3 laws of motion
Kepler’s First Law
 The orbits of the planets are ellipses with the
sun at one focus of the ellipse.
Kepler’s Second Law
 The line joining the planet to the sun sweeps
out equal area in equal times.
 What does this mean? When a planet in its orbit is
closer to the sun, it must speed up to sweep out an
equal area…
Kepler’s Third Law
 The square of the orbital period
(time it takes to complete one orbit)
is directly proportional to the cube
of the mean (average) distance
from the sun to the planet
 What does this mean? A larger orbit requires more time to
make one complete revolution.
 Kepler predicted EXACTLY how much time each planet’s
orbit would take.
Johannes Kepler (1571-1630)
 Brought new emphasis on finding and quantifying
causes of motion
 United geometric or kinematic description of orbits
with their physical, dynamic force that causes it
 Authored fictional account of a space journey, the
Somnium
 Described trip to the Moon
 Inspired other fiction writers, including Jules Verne and
H. G. Wells
Galileo Galilei (1564-1642)
 Refined telescope design to
a state-of-the-art 20X
 Observed several things
about the physics of motion
 Ushered in new era of
space exploration
Galileo’s Discoveries
 Saw imperfections in “perfect bodies” of Aristotle’s
model of the universe
 Mountains and valleys of the Moon
 Sun spots (noticed the sun rotated like the Earth)
 Discovered four moons orbiting Jupiter
 Disproved Aristotle’s assessment that everything revolved about
the Earth
 Noticed the phases of Venus indicating it too revolved
around the sun
 Observed the Milky Way and noted its vast size
 Telescope’s inability to magnify stars confirmed
Copernicus’s guess about their vast distances
Galileo and Gravitation
 Measured time for spheres to
reach bottom of an incline
 Dropped stones from the
Leaning Tower of Pisa
 Weight didn’t affect time to reach
the bottom –both balls had the
same acceleration
 Previously, people thought heavier
objects fall faster
 Theory proven during Apollo 15
using a hammer and feather
Galileo vs the Pope
 His “theories” were at odds with Church “law”
 In 1610, published “Starry Messenger”
 Non-technical style—readable for common class
 Presented as dialogue of the Copernican system
between a wise teacher (him) and an unbeliever he
named Simplicio (the Pope)
 In 1632, published “Dialogue Concerning the Two
Chief World Systems” with the approval of the
Inquisition
 Supposed to be neutral, but wasn’t
 In 1633, placed under house arrest for the rest of
his life (church admitted its mistake in 1992)
Lesson review
 What idea did Copernicus develop and
promote?
 Used planetary parallax to argue for
heliocentric (Sun-centered) solar system
 Measured relative distances of planets from the Sun
 Surmised the vast distances to stars
Lesson review (cont’d)
 How did Tycho Brahe contribute to our
understanding of space?
 Invented new instruments
 Observed and accurately recorded the
locations of the planets over time
 Had a wacky theory of a geo-heliocentric
solar system
Lesson review (cont’d)
 What are Kepler’s three laws?
1. Planets move in elliptical orbits around the sun
2. The closer to the Sun, the faster they move
3. The larger orbit requires longer time to complete
 What did Galileo do?
 Telescope observations led to the downfall of
Aristotle’s geocentric model
 Confirmed vast distances to stars
 One of the first to study gravity